Please criticize my plan

I am new to the UK and just bought a house in Norwich. It has a vented cylinder and vented CH that is suffering from pretty severe corrosion, some of the rads are rusted through, so I would like to replace it with a sealed system. The rads are all plumbed in 10mm, so I would like to replace that too.

Here is a plan of what I would like install. Any (constructive) criticism of the design is most welcome. The control consists of an old school cylinder stat, thermostatic valves, and smart pumps.


I am also looking for a G3 around Norwich to install/commission/notify the cylinder. I will do all the messy stuff, lifting floorboards, getting all the pipes to the garage, hanging the expansion tank, providing power. If you can supply all the kit for a decent price, that would be great too, I have no access to local wholesale plumbing suppliers.

Finally, if anyone knows of a good deal on traditional cast iron rads and nice looking TRVs to match, please let me know. For example like this:
Traditional Victorian 4 Column 760mm Cast Iron Radiator - https://www.castironradiatorcentre.co.uk/products.asp?code=CDC-760&name=traditional-victorian-4-column-760mm-br-cast-iron-radiator

Beware Norwich I understand has hard water, if I am correct you might well future proof the system.
For what they cost replace all the rads (diagram shows existing rads). centralheatking

Without knowing the heating loads, I cannot comment as to how the system would perform. I would suggest that you look at the hot water side as to how that will perform in cold weather when the heating is at a high demand level.

Obviously the overall efficiency of the system is going to be lower than a more conventional set up.

Crazy set up, reinventing the wheel for no good reason.
A lot going on when an unvented cylinder would do the job or a thermal store. But would only recommend thermal store if you had multiple heat inputs.

Complexity for no good reason just means there is more to go wrong.

More expensive to run.

Also nobody will get materials at cost for you, as an example I charge cost plus 30% on all materials I supply.

Also I wouldn't want you to do any pipework, suppose it leaks? You paying for return visits whilst you put your work right?

But each to their own.

SimonG said:

Crazy set up, reinventing the wheel for no good reason.
A lot going on when an unvented cylinder would do the job or a thermal store. But would only recommend thermal store if you had multiple heat inputs.

Complexity for no good reason just means there is more to go wrong.

More expensive to run.

Also nobody will get materials at cost for you, as an example I charge cost plus 30% on all materials I supply.

Also I wouldn't want you to do any pipework, suppose it leaks? You paying for return visits whilst you put your work right?

But each to their own.

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Cost + 30% from someone with good connections to suppliers is going to be better than I can get retail, although online shopping is starting to change that a bit.

IMO, the only thing overly complex about the setup is the plate exchanger for DHW. I want to run the tank at a lower temperature than usual for a thermal store, say 60-65C, and I don't think there are tanks available with big enough coils to support that.

Please elaborate on why you think it would be more expensive to run. I don't see why it would be less efficient than a standard system, provided the tank is well insulated. In some scenarios it will likely be more efficient. Standard systems suffer because there is no buffer between the boiler and the rads. Modulation helps, but IMO this is a better, less complex solution.

If something I fit leaks and it were to stall commissioning or something like that, then yes, I would pay for a return visit.

More expensive based on a higher tank temp to maintain.

Cant see how a tank temp of 60 to 65 will deliver satisfactory dhw performance.

centralheatking said:

Beware Norwich I understand has hard water, if I am correct you might well future proof the system.
For what they cost replace all the rads (diagram shows existing rads). centralheatking

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I plan to install an ion exchange type water softener, the family is already complaining about their hair.

I had though to replace the rads and the piping over time as I renovate the house, room by room. But perhaps you are right, it might be better to do it all at once, so I don't mix the new with the old.
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Brambles said:

Without knowing the heating loads, I cannot comment as to how the system would perform. I would suggest that you look at the hot water side as to how that will perform in cold weather when the heating is at a high demand level.

Obviously the overall efficiency of the system is going to be lower than a more conventional set up.

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The current boiler seems big enough for the house now, although it has not been that cold yet. We are going to build an extension in a couple years time, so we'll likely have to replace it then. With this setup it should be possible to oversize it somewhat, for those times where there is heavy demand.

Care to elaborate why you think it would be less efficient?

You can calculate the theoretical efficiency of your system once you know all the loads and heat loss factors. Generally, to improve efficiency you minimise heat transfer and deliver the heat to point of use as soon as it is created. The concept of a thermal store is to store energy that you would otherwise loose. With a gas fired boiler (even on older one) you have extensive flexibility on control - so you only produce heat when you need it and immediately deliver it to where you need it. To produce heat from a gas fired boiler to store (in a small domestic situation) is just odd.

As a starting point for your proposed system, you will use just under 20kw to heat your store / buffer to 65 degrees ( assuming a 10 degree inflow temperature). Thereafter, around an hour, your system will start to perform as you have designed it.

With respect to the plate heat exchanger for DHW you would normally configure it to take the primary flow from the heat source and the secondary to the buffer vessel. You also don’t normally specify them in Kw. You establish the required flow rate, determine your maximum flow, duration of max flow and your input flow and return temperature requirements. That then determines the gross output of the exchanger against the specific flow criteria. The flow criteria for a heat exchanger to work is absolutely key.

To give you an order of magnitude, in very simple terms, a 35kw gas fired combi uses a 30 to 40kw exchanger (configured to the boiler flow criteria) to deliver DHW. At best, your proposed system is starting with less than 20kw of energy to input into the exchanger, with a falling flow temperature.

Sometimes, when you get a really good idea it is useful to ask yourself why other people are not doing it. I don’t mean that to be rude,
but it is a useful exercise to draw you back to reality.

I don’t want to be Dr Doom and spoil your fun in developing such a system, but a new gas fired combi will be a lot more efficient, probably cheaper to instal and give you a consistent flow of hot water.

Ion exchange was a magic fix but is a temporary water fix at best, the naughty crystals reform very quickly. The only proper fix is a water softener which must also treat the water that is introduced to the ch system otherwise its bye bye heat exchangers sooner rather than later.
Boiler..do a proper heat requirement calculation inc. extension and new rads get a suitable sized boiler
for that and water storage that will cope with all your peak demands at periods of high area demand
...am before school run, pm 17.00..19.00 AND during summer when pressure is low due to over demand. centralheatking

SimonG said:

More expensive based on a higher tank temp to maintain.

Cant see how a tank temp of 60 to 65 will deliver satisfactory dhw performance.

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This is the reason for the 100kW plate exchanger, rather than an internal coil, which will allow the tank to run at a lower temperature.

But you are correct in that one would expect tank losses to be bigger, because the tank is bigger than an unvented would be. For the tank, exchanger, and piping losses, I am planning to insulate them well to reduce this, and to upgrade the insulation in the house in general.

With my current boiler it will very likely cost less to run, as it will stop the boiler cycling in the period after the rads warm up, but before the room thermostat cuts out.
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Brambles said:

You can calculate the theoretical efficiency of your system once you know all the loads and heat loss factors. Generally, to improve efficiency you minimise heat transfer and deliver the heat to point of use as soon as it is created. The concept of a thermal store is to store energy that you would otherwise loose. With a gas fired boiler (even on older one) you have extensive flexibility on control - so you only produce heat when you need it and immediately deliver it to where you need it. To produce heat from a gas fired boiler to store (in a small domestic situation) is just odd.

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By this logic we should use instantaneous gas waters heaters all round, for example combis. The flaw there is that they don't cope well with variable demand, and any efficiency gained from reducing tank losses is often lost. I would bet in a real world situation, unventeds are more efficient than combi's for this reason, unless they have a decent sized tank, and then they become an unvented. Think how many times you turn the hot tap on and off when you are cooking.

Brambles said:

As a starting point for your proposed system, you will use just under 20kw to heat your store / buffer to 65 degrees ( assuming a 10 degree inflow temperature). Thereafter, around an hour, your system will start to perform as you have designed it.

With respect to the plate heat exchanger for DHW you would normally configure it to take the primary flow from the heat source and the secondary to the buffer vessel. You also don’t normally specify them in Kw. You establish the required flow rate, determine your maximum flow, duration of max flow and your input flow and return temperature requirements. That then determines the gross output of the exchanger against the specific flow criteria. The flow criteria for a heat exchanger to work is absolutely key.

To give you an order of magnitude, in very simple terms, a 35kw gas fired combi uses a 30 to 40kw exchanger (configured to the boiler flow criteria) to deliver DHW. At best, your proposed system is starting with less than 20kw of energy to input into the exchanger, with a falling flow temperature.

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All plate exchangers I have seen are rated in kW, and that is for a 40C delta. In this case it will be more like 15C, so I calculated how many watts to heat water from 5C-45C based on 12 litres/minute flow, multiplied by 3, and padded it a little for insurance. Do you think I have made a mistake, and 100kW is too small?

Brambles said:

Sometimes, when you get a really good idea it is useful to ask yourself why other people are not doing it. I don’t mean that to be rude,
but it is a useful exercise to draw you back to reality.

Click to expand...

I believe this is one of the standard ways to set up a thermal store, aside from the balancing valve and smart pump instead of a flow switch, that is indeed an experiment.

Brambles said:

I don’t want to be Dr Doom and spoil your fun in developing such a system, but a new gas fired combi will be a lot more efficient, probably cheaper to install and give you a consistent flow of hot water.

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No worries, I asked for criticism. I have space for a cylinder, so I am not limited to a combi, although it most certainly would be cheaper.

I did install the exact same system in a previous house, except it used an internal coil for DHW rather than a plate exchanger. Until you have lived in a house with a buffered heating system it is hard to understand the comfort you are missing. No cycling of room or radiator temperatures when the thermostat cuts in an out, heat always there, much easier to control individual room temperature, no boiler cycling, etc. It is very pleasant.

A 300litre tank at 60 degrees with 40mm of PUR insulation will loose 7 degrees in 24 hours at 20 degrees ambient. That is around 2.4kwhr. There are more sophisticated ways of determining the actual loss. For heat pumps, I work on a rule of thumb for a 40mm insulated DHW tank in regular daily use @ 20 degree ambient is that 10% of the energy stored is lost. Your losses will be higher (if you store at 65 degrees) - unless of course you increase the tank insulation further.

siricosm said:

This is the reason for the 100kW plate exchanger, rather than an internal coil, which will allow the tank to run at a lower temperature.

But you are correct in that one would expect tank losses to be bigger, because the tank is bigger than an unvented would be. For the tank, exchanger, and piping losses, I am planning to insulate them well to reduce this, and to upgrade the insulation in the house in general.

With my current boiler it will very likely cost less to run, as it will stop the boiler cycling in the period after the rads warm up, but before the room thermostat cuts out.
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By this logic we should use instantaneous gas waters heaters all round, for example combis. The flaw there is that they don't cope well with variable demand, and any efficiency gained from reducing tank losses is often lost. I would bet in a real world situation, unventeds are more efficient than combi's for this reason, unless they have a decent sized tank, and then they become an unvented. Think how many times you turn the hot tap on and off when you are cooking.


All plate exchangers I have seen are rated in kW, and that is for a 40C delta. In this case it will be more like 15C, so I calculated how many watts to heat water from 5C-45C based on 12 litres/minute flow, multiplied by 3, and padded it a little for insurance. Do you think I have made a mistake, and 100kW is too small?


I believe this is one of the standard ways to set up a thermal store, aside from the balancing valve and smart pump instead of a flow switch, that is indeed an experiment.


No worries, I asked for criticism. I have space for a cylinder, so I am not limited to a combi, although it most certainly would be cheaper.

I did install the exact same system in a previous house, except it used an internal coil for DHW rather than a plate exchanger. Until you have lived in a house with a buffered heating system it is hard to understand the comfort you are missing. No cycling of room or radiator temperatures when the thermostat cuts in an out, heat always there, much easier to control individual room temperature, no boiler cycling, etc. It is very pleasant.

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i had the same in my last house with a viessmann 200 combi with weather compensation comfort levels perfect and good hot water

I can see this system working ...my previous comments re hard water and supply still stand...however its a bit like the heating systems in Hotels and Nursing Homes which we have been involved in for many years.
In fact a few sacraficial hex is cheap compared to a replacement boiler was why we wacked them in.
One system was the largest nursing home in Europe...the Wills one on the Downs outside of Brisol near the M5 but the hot water circuit was 2/3 inch galv ...chking

Heat exchanger performance:

In simple terms

From the boiler output you have feeding a 300 litre store at 65 degrees C - a 5 minute shower at 40 degrees and 15 lpm will demand a input into the exchanger of around 42kw/hr. That assumes that the exchanger has an efficiency of 80% ( which is high). The primary flow would be around 30 lpm, assuming that the store is fully charged.
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Just for clarity, an exchanger is generally sized from The primary to The secondary ( in your case 65 degrees to 40 degrees). So if your target temperature is 40 degrees, that would be the balance point. So from your store, the maximum gross amount of energy that is available to be released to create dhw at 40 degrees is around 7kw net. However, to heat the store from 10 degrees to 65 degrees, you have used circa 20kw.

Brambles said:

Heat exchanger performance:

In simple terms

From the boiler output you have feeding a 300 litre store at 65 degrees C - a 5 minute shower at 40 degrees and 15 lpm will demand a input into the exchanger of around 42kw/hr. That assumes that the exchanger has an efficiency of 80% ( which is high). The primary flow would be around 30 lpm, assuming that the store is fully charged.
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Just for clarity, an exchanger is generally sized from The primary to The secondary ( in your case 65 degrees to 40 degrees). So if your target temperature is 40 degrees, that would be the balance point. So from your store, the maximum gross amount of energy that is available to be released to create dhw at 40 degrees is around 7kw net. However, to heat the store from 10 degrees to 65 degrees, you have used circa 20kw.

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I calculated this as follows. Energy required to heat water is 70 watt-minutes / litre / degree. Ie. it takes a 70W heat source 1 minute to heat a litre of water 1 degree C. 12l/min from 10C to 45C gives 70 * 12 * 35 = 29.4kW, which makes sense since a 6l/minute electric shower is around 11kW.

Now triple and add a bit extra and you are at 100kW.

How large an exchanger do you think is required?

I am afraid that is not how you calculate heat transfer across a heat exchanger. You are just using a DIY method for calculating the output - do the next step and calculate the input required. That is where your problem is. The critical element is the time duration over which that the heat needs to be transferred from the HT to the LT circuit. For a decent single shower think in terms of 35kw/hr. That is why flow rates are so important.

The correct method is:

You calculate the heat load, theta and log mean temperature differential (LMTD). Any book on basic thermodynamics will show you the calculation method, or a decent supplier will do the calculations for you. As a design principle you work back from the outlet temperature and peak flow rate ( and time the peak is in operation) through the exchanger, through the store to determine the thermal input required. In your case the thermal input is fixed (your existing boiler) so the only variable you have is the capacity of the store (which in my view would need to be 600 to 800 litres for what you are trying to achieve).

If you doubt me, talk to the key suppliers Alfa Laval, McDonald et al.

If you look in the market place, most small exchangers (the £200 ones) are configured to specific boilers, to be fed from the boiler primary and controlled through the boiler pcb. Exchangers configured for low temperature thermal stores are in the £1k bracket and normally require a separate control loop.

From the schematic you have shown, the exchanger is configured as a fixed flow unit ( which is how commercial units are set up). Most exchangers used in a domestic situation are variable flow - but obviously the control system is then bespoke (see above).

What you are trying to achieve for DHW is what a Combi boiler does, it is not as straightforward to achieve on a small domestic scale as the theory would imply. On larger systems, small hotels and care homes et al the size (volume) of the thermal store makes the conversion to DHW far more straightforward.

I have said enough on this topic now (and will say no more), the system you have outlined, if correctly sized and controlled, will give you a decent heating system. However, it won’t give you a constant (15 lpm) supply of hot water at 40 degrees - at the very best with a properly designed and configured exchange it will give you 7 to 12 minutes followed by a 60 minute recharge period. The latter will obviously impact your heating whilst the store recovers.

All heat exchangers work - the first law of thermodynamics sorted that - but minimising / optimising the time it takes to transfer the heat is the clever bit ( particularly if you like a decent shower).

Brambles said:

I am afraid that is not how you calculate heat transfer across a heat exchanger. You are just using a DIY method for calculating the output - do the next step and calculate the input required. That is where your problem is. The critical element is the time duration over which that the heat needs to be transferred from the HT to the LT circuit. For a decent single shower think in terms of 35kw/hr. That is why flow rates are so important.

The correct method is:

You calculate the heat load, theta and log mean temperature differential (LMTD). Any book on basic thermodynamics will show you the calculation method, or a decent supplier will do the calculations for you. As a design principle you work back from the outlet temperature and peak flow rate ( and time the peak is in operation) through the exchanger, through the store to determine the thermal input required. In your case the thermal input is fixed (your existing boiler) so the only variable you have is the capacity of the store (which in my view would need to be 600 to 800 litres for what you are trying to achieve).

If you doubt me, talk to the key suppliers Alfa Laval, McDonald et al.

If you look in the market place, most small exchangers (the £200 ones) are configured to specific boilers, to be fed from the boiler primary and controlled through the boiler pcb. Exchangers configured for low temperature thermal stores are in the £1k bracket and normally require a separate control loop.

From the schematic you have shown, the exchanger is configured as a fixed flow unit ( which is how commercial units are set up). Most exchangers used in a domestic situation are variable flow - but obviously the control system is then bespoke (see above).

What you are trying to achieve for DHW is what a Combi boiler does, it is not as straightforward to achieve on a small domestic scale as the theory would imply. On larger systems, small hotels and care homes et al the size (volume) of the thermal store makes the conversion to DHW far more straightforward.

I have said enough on this topic now (and will say no more), the system you have outlined, if correctly sized and controlled, will give you a decent heating system. However, it won’t give you a constant (15 lpm) supply of hot water at 40 degrees - at the very best with a properly designed and configured exchange it will give you 7 to 12 minutes followed by a 60 minute recharge period. The latter will obviously impact your heating whilst the store recovers.

All heat exchangers work - the first law of thermodynamics sorted that - but minimising / optimising the time it takes to transfer the heat is the clever bit ( particularly if you like a decent shower).

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Hi Brambles.

Thank you for your post and your interest.

1K for an exchanger is a bit rich for me, I was planning to use something like this:
Stainless Steel Plate Heat Exchanger B3 12A 50 110kW with Insulation Shell 791688774504 | eBay - https://www.ebay.co.uk/itm/Stainless-Steel-Plate-Heat-Exchanger-B3-12A-50-110kW-with-Insulation-Shell/112959831840

I think your numbers for available heat make sense. For a 10 minute shower at 15l/min from 10C to 40C it takes 150x30x4200J = 5.25kWh of energy. I guess it would be the same for a typical bath. Lets suppose the exchanger is large enough to maintain 40C output temperature down to a 50C tank temperature, then a 300l tank at 65C can provide 5.25kWh, without factoring in any additional input from the boiler. In 10 minutes a 17.6kW boiler gives 2.9kWh, leaving a shortfall of 2.3kW, so recovery will be under 10 minutes in the summer. My house has 9kW of radiators, but even on the very coldest days I can't imagine them all operating continuously at T50, so it is probably significantly less than 1/2 the boiler output. But even at 1/2, in the winter we would have a bit under 30 minutes recovery on the coldest days, and something in between most of the time.

Of course if we are using a lot of hot water for other things, or have a bunch of visitors over on a very cold day, we could easily hit a wall, but we are heating with gas, so we can always crank it up to 75 or even 80 to double the stored capacity if we needed it for a while.

We plan to build a small addition (~ 1.2kW) in a couple of years, and then the boiler has to move, so we will replace it, probably with a 35kW unit. At 65C this could give infinite hot water in the summer, and probably also in the winter since our house flow rate is only 12l/min. Of course I will play and turn it down further, to the minimum tank temperature that works for us, to save energy and encourage condensing. I wonder, does anyone sell a weather compensating cylinder stat?

I installed a similar setup in my previous house. The house was a bit bigger in size, with a slightly larger boiler, in a bit colder climate. My 300l OEG tank had an internal corrugated stainless coil of 4.1 sq m, which I estimate would be around 120kW, so a bit larger than the exchanger above.


I know you said you would comment no more on this, but I very much appreciate the thoughts of those that are experts in this area, and I would be interested to hear what you think about the idea to control the primary of the DHW plate exchanger with a thermal balancing valve and a smart pump, rather than a flow switch.

Cheers,
Richard

Richard

I am sorry there is not really any further input I can give. Hopefully, I have given you direction in how to size a heat exchanger for that type of system and the performance issues you may encounter. The next stage for you is to hydraulically design the system from end to end, to meet your delivery requirements.

With respect to the Exchanger you have cited. That is probably a constant flow and temperature (primary) exchanger, used to extract the heat for a flat on a district heating system. That is why the performance curve shows Delta T in and Delta T out being very similar. A more realistic graph for your application would be two curves as a mirror image diverging at you approach the target temperature.

In your case, the exchanger is intermittent flow from a nearly static source delivering a changing (reducing) primary temperature. So delta T in ( primary in - secondary in) and Delta T out ( Primary out - secondary out) have a much wider variation. So the determination of Delta T (representative) requires some work.

Note: To read the graph as presented with your exchanger, secondary in is read from the right and primary in from the left.

In simple terms :

Heat transfer (Q) = Effective Surface Area (S/A)* Delta T (representative).

If you look at the LMTD aspect of the design in a text book, it will be much clearer than I can explain.

With respect to your question on pump control, I suspect that it would be slow to respond - but it is not something that I have ever considered or tried. As always with these type of systems, ensure that there are no conditions where there is potential for a thermo-syphon to establish its self.

One solution that may meet your requirements is a tank in tank solution. They are simple and work well on small systems.

One thought that you may find useful: Under your proposal the CH flow temperature is being set by its ability to deliver a suitable supply of DHW through a plate exchanger. In the UK, we generally try to drive down the CH flow (and return) temperature to improve efficiency ( particularly with condensing boilers). To achieve this the systems are in part separated - which is what weather compensation achieves (the boiler only operates at its set temperature when DHW is called) when CH is called to flow temp is determined by curve that correlates flow to outside temperature. From an energy perspective most households consume more energy for heating than they do to deliver hot water.

With respect to bespoke solutions, it is worth talking it through with Building Control ( or whoever is going to certify) the aspects that need approval, before you implement. It is an issue (uncertified systems) that is now increasingly raising its head when you come to sell the property (in England and Wales).

@Brambles Hi buddy. Can you recommend a book for thermodynamics as you said above that covers this area for plumbing in particular? I've had a look online and finding all sorts but for different areas.

Without reading the whole thread, what are you trying to achieve with this system?

You've only got one heat input so why not keep it simple and go down the route of a Unvented cylinder...

SJB

Thermodynamics for Dummies is the one our apprentices use (I assume they read it!). Don’t be put off by the title - it is a good book.

If you are specifically interested in Heat Exchangers - The Thermopedia web site is free and quite good.

For plate heat exchangers using water - Google LMTD - which is part of the calculation method. If you make up a few peak flow, duration and temperature requirements then try the calculation. Don't be put off by the Greek symbols - you may not get it right first time - but it will give you an understanding of how sensitive the design is pressure drop and flow conditions.

The growth of combi’s in the European and Uk market is all about heat exchanger design, efficiency and cost - to have the ability consistently deliver dhw. The concept ( heat exchanger) is simple, achieving what you want it to deliver is quite complex.

Thanks Brambles, yet again your knowledge and memory impress me.
I will purchase the book you mentioned (it was one I was considering anyway) and I will google LMTD and do some mock ups.

I have been following this one with interest as Norwich is in my area you seem to have your heart set on the type of system to you would like installed siricosm ? . Brambles has given some good sound detailed advice in his replies and alot for you to consider I personally would be guiding you to look at Joule range of products I speak from experience here they are up there with the best manufacturing buffer tanks , unvented cylinders, thermal stores and everything you will require control wise. If you go with traditional cast iron radiators then your heating supply pipework will without a doubt needed replacing as you simply will not achieve the flow rate to heat them I will continue to monitor your posts from experience you will definitely need a water softener in the Norwich area and it's even worse as you get out of the city and in the surrounding countryside. Regards kop

SJB060685 said:

@Brambles Hi buddy. Can you recommend a book for thermodynamics as you said above that covers this area for plumbing in particular? I've had a look online and finding all sorts but for different areas.

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I wouldn't worry about thermodunamics. The only law that has any relevance is the 1st law, and as there is no work involved even that reduces to the statement that heat in = heat out.
How big is the house, no. of rads etc? For a normal sized place your system (3 pumps) looks complicated to me.

fixitflav said:

I wouldn't worry about thermodunamics. The only law that has any relevance is the 1st law, and as there is no work involved even that reduces to the statement that heat in = heat out.
How big is the house, no. of rads etc? For a normal sized place your system (3 pumps) looks complicated to me.

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Thanks fixitflav. Yes I have a basic understanding of the laws of thermodynamics but I feel I should know more lol