Rad sizing

[chris watkins]

Well Peter where do you want me to start? As heating engineers whether domestic or commercial & I really do not see any difference between the two, (I have done both) they require exactly the same levels of knowledge, understanding & professionalism.
Systems in the past have being sized correctly to match the heat losses & then at least 10% has been added to allow a capacity to heat the rooms up from cold, known as intermittent heating (& not to hold a steady stat). Not having the benefit of the details of your house & system it is difficult to comment but I bet you leave your heating on longer than most when it gets cold & that's not a bad thing but you should know the why's.
Re the condensing please have a look at my earlier post, it is not the differential that is important but the returning water temp back to the boiler. (less than 53deg C)
I agree with you about the modern controls but it is not just the wireless stats you can get the same results if you exchanged your old mechanical / electrical room stat for an electronic one & they only cost a couple of £ more than the normal Honeywell ones.
All the manufactures use the same types of heat exchangers with small waterways (I would not install one without protecting it with a filter on the return before it) they are all trying to come up with boilers which will adjust there heat outputs automatically right the way down as low as possible to match the required heat output with the demand thus maintaining a cool return water temp & a boiler that is up to 13% more efficient that exactly the same one that not condensing. 13% saving!!!! in any other industry if I could offer that, people would be biting my arm off.

 

[petercj]

Well Peter where do you want me to start? As heating engineers whether domestic or commercial & I really do not see any difference between the two, (I have done both) they require exactly the same levels of knowledge, understanding & professionalism.
Systems in the past have being sized correctly to match the heat losses & then at least 10% has been added to allow a capacity to heat the rooms up from cold, known as intermittent heating (& not to hold a steady stat). Not having the benefit of the details of your house & system it is difficult to comment but I bet you leave your heating on longer than most when it gets cold & that's not a bad thing but you should know the why's.
Re the condensing please have a look at my earlier post, it is not the differential that is important but the returning water temp back to the boiler. (less than 53deg C)
I agree with you about the modern controls but it is not just the wireless stats you can get the same results if you exchanged your old mechanical / electrical room stat for an electronic one & they only cost a couple of £ more than the normal Honeywell ones.
All the manufactures use the same types of heat exchangers with small waterways (I would not install one without protecting it with a filter on the return before it) they are all trying to come up with boilers which will adjust there heat outputs automatically right the way down as low as possible to match the required heat output with the demand thus maintaining a cool return water temp & a boiler that is up to 13% more efficient that exactly the same one that not condensing. 13% saving!!!! in any other industry if I could offer that, people would be biting my arm off.

I appreciate that I should have mentioned the size of my house for my post to make any sense - thought of that after posting!

4 bedrooms, two bathrooms, 25' lounge, dining room, 30' kitchen, utility room, large hall with two staircases. It has been updated insulation-wise to the latest spec.

I know you're going to tell me that the boiler is too small chris, but in practice it works.

Previously, the room stat was in the hall, near the front door - totally bonkers.

The mobility of the wireless stat has allowed me to manage the heating in a very different way, and no, the boiler's not running a lot. My gas bill for last winter was 30% down on the previous year - which was somewhat colder, but then I was on a price fixed rate for the previous years - so I was well pleased.

The simple basis of it is that the stat is in the lounge for waking hours, and in the bedroom at nights. Because of the high level of insulation I can switch the double radiator in the lounge off at bedtime, and the temperature drop during the night is only a couple of degrees. The hall and stairs are kept at a background temperature. I have lowered the temperature in the kitchen because it's only used for cooking, which raises the temperature anyway, plus the kitchen retains heat pretty well too. The other thing I did was to fit a door closer on the lounge door.

I was tempted to buy timers for the TRV's, but thought better of it, as they don't come cheap, and I'm cautious about their life span.

Have to disagree about staying with a fixed room stat, i.e. having a mobile stat makes a world of difference and opens up a range of options that are not possible with the fixed variety.

The Valliant handbook states that there needs to be a 20 degree differential between F & R for the boiler to be in condensing mode.

If the temperature in the house was allowed to drop right down in a cold winter, then the boiler would have a right sweat on (it's like a steam train when it gets going on a cold day as it is), but it never does.

The other choice in the Valliant range was 28 KW, and I thought long and hard about going for the bigger model, but having had the 18.5 for five years, I have no regrets.

But in theory it's the wrong choice.

 

[chris watkins]

Hi Pete
I am not going to tell you it is too small, its of an output that suits you & as most people know the harder a boiler works the more efficient it is likely to be (you told me "about twice in the last 5 years, which was slight rad-lag when heating the system from cold". If I sold a customer a system then they might well come back & say that I had miss sold it, if it didn't heat the house in the winter. Thats not to say it should be oversized either but because I have done the calc's I can show what is happening or not.
The Valliant MI's do state 20deg C temp diff as an indication of the flow rate through the boiler to carry the heat away, it has nothing to do with the boiler being in or out of condensing mode, this is basic science not rocket, please have a look on google.
I understand what you are saying about the wireless stat & if you are using it like that & it is working for you, great, but most people would not be so diligent in moving it around & this is not how it is designed to be used. The TRV's on each of the rads control the individual room temperatures by adjusting the amount of water going to each one, with the exception of one rad (normally the hall) which does not have one fitted but this is where the room stat is installed, so long as this rad is correctly sized it should give a fair representation of the rest of the dwelling & shuts the heating off or on giving overall control along with the programer. So you can see that if the room stat is in the same room as a TRV they could well be competing with each other.
So all in all right choice of boiler when you know the theory.

 

[petercj]

Hi Pete
I am not going to tell you it is too small, its of an output that suits you & as most people know the harder a boiler works the more efficient it is likely to be (you told me "about twice in the last 5 years, which was slight rad-lag when heating the system from cold". If I sold a customer a system then they might well come back & say that I had miss sold it, if it didn't heat the house in the winter. Thats not to say it should be oversized either but because I have done the calc's I can show what is happening or not.
The Valliant MI's do state 20deg C temp diff as an indication of the flow rate through the boiler to carry the heat away, it has nothing to do with the boiler being in or out of condensing mode, this is basic science not rocket, please have a look on google.
I understand what you are saying about the wireless stat & if you are using it like that & it is working for you, great, but most people would not be so diligent in moving it around & this is not how it is designed to be used. The TRV's on each of the rads control the individual room temperatures by adjusting the amount of water going to each one, with the exception of one rad (normally the hall) which does not have one fitted but this is where the room stat is installed, so long as this rad is correctly sized it should give a fair representation of the rest of the dwelling & shuts the heating off or on giving overall control along with the programer. So you can see that if the room stat is in the same room as a TRV they could well be competing with each other.
So all in all right choice of boiler when you know the theory.

Completely agree with your first paragraph chris: if I was thinking of selling this place, and it was time for a boiler change, I would fit the bigger model. Someone else could easily spend 3 times as much on gas as I do heating this place, and a bigger boiler would help them along the way. As it is, I begrudge BG every ruddy penny I pay to them. I've just change supplier after 8 years with BG, and I had to chase them to get them to pay me the money I was in credit with them.

Sizing might be an exact science, but human behaviour definitely does not conform to such predictable rules.

I don't know if you watch Grand Designs on Channel 4, but there was a guy on there last year who was heavilly into energy conservation, and he based his heating on recycling the air in the building rather than opening windows, so preserving body heat, cooking heat, etc, as well as the input from the heating system, and the results were surprising.

Not that I go that far - lol - but the ten pounds I spent on a door closer, plus the wireless stat, was definitely money well spent. Seems strange to think that something so simple can make such a difference.

 

[chris watkins]

Not that I go that far - lol - but the ten pounds I spent on a door closer, plus the wireless stat, was definitely money well spent. Seems strange to think that something so simple can make such a difference.

Simple is most certainly the best, that's for sure, have you met the wife !!!

 

[chris watkins]

If anybody whats to find out more about heat loss, design, controls or just about anything else about this subject, I can highly recommend this book- Domestic Heating Design Guide 2007 ISBN = 1 903287 40 5. You may have to get it through CIPHE or a few others but it has taught me even thing I know (well almost)

 

[Nostrum]

Totally agree the Chris Watkins on all aspects.


People get hung up about doing a few calculations that take a few minutes a room and give you an accurate, approved and logical reasoning for sizing the radiators, something that will make you look much more professional, experienced and can provide you with something to fall back on if you need it.

Radiators in 90% of UK households are well oversized, partly because of this ft2x5 business thats been going on for years, and partly because houses were not insulated like they are today.

I newish house can have a 450x400 single panel in a single room, something that you'd find in most people airing cupboards but works just fine.

Petercj, your 18kw boiler sounds like it could be a touch oversized according to your information and levels of insulation. I certainly wouldn't put a bigger boiler in until you sized it correctly. You should go through the calc and you may be very surprised of the outcome.

Bigger rad's will mean an oversized boiler if your adding up the emitters to size the boiler, more expense, less space and potentially increased installation difficulty. It's not so much of a problem for gas boilers as they modulate (except for the gas supply requirements) but if you oversized a pellet boiler or similar you could be looking at thousands more.

Also moving your thermostat round really doesn't make sense, the hall is usually the best place as it is subject to the most air changes per hour, if you put it in your front room with the gas fire on etc the heating will be off for hours regardless of how warm the other rooms are, but that's another matter!

Come on lads, we have got to earn the 'engineer' in heating engineer some how, other wise the term 'pipe monkey' becomes a little more accurate!

P.S no offence intended to anyone

 

[chris watkins]

People get hung up about doing a few calculations that take a few minutes a room and give you an accurate, approved and logical reasoning for sizing the radiators, something that will make you look much more professional, experienced and can provide you with something to fall back on if you need it.

Radiators in 90% of UK households are well oversized, partly because of this ft2x5 business thats been going on for years, and partly because houses were not insulated like they are today.

I newish house can have a 450x400 single panel in a single room, something that you'd find in most people airing cupboards but works just fine.

Come on lads, we have got to earn the 'engineer' in heating engineer some how, other wise the term 'pipe monkey' becomes a little more accurate!

P.S no offence intended to anyone

Well said Sambotc, could not have put it better myself, how could they take offence it is a requirement for installing new systems as well as replacing a single rad that it has to be sized correctly, it is not difficult to do or learn, it does not take a long time to do, either, so there are no excuses if you are going to call yourself a professional Plumbing / Heating ENGINEER.

P.S. We have modulating boilers that will not work correctly because they are oversized there is a limit as to how low they will go because the boiler controls are designed for the Kw rating stated, if it is connected to a much smaller system it will not work correctly.

 

[SimonG]

No offence taken, and as a professional plumber and heating engineer I will continue to use the method that is proven and works for myself.

 

[Nostrum]

Yes I think we have talked about the minimum rated output problems before. My own house is a typical example. Thermostatic shower so wanted a good flow rate, ecotec 831 combi rated down to 8kw on the boiler but min output is 6.7kw I think. My house heat loss (mid terrace 2 bed 1970's) is about 5kw at minus 3 so once the house is up to temp it's bouncing of the bottom limit all night. Not much I can do about it as there's no room for a cylinder and the boilers don't yet modulate low enough.

In reality, it works and who's to know any better, but potentially there are savings to be made and ultimately emissions could be lowered (save a few polar bears)

Didn't you say the boiler manufacturers were looking into twin gas valve designs to combat this problem?

 

[Nostrum]

No offence taken, and as a professional plumber and heating engineer I will continue to use the method that is proven and works for myself.

You could also get yourselves a 4.8 v8 Chevy ( proven, tried and tested method that works) to carry your tools around in, but you wouldn't !.....? :hand:

 

[SimonG]

and you could cut the crap

 

[petercj]

Big room = Big radiator ... Medium room = Medium radiator ... Small room = small radiator

Big house = big boiler ... Small house = small boiler (ref: Vaillant eco pro max range)

Over-sizing unlikely to lead to come backs, whereas under-sizing might.

Job done!:yes:

 

[Nostrum]

and you could cut the crap

Sure thing pipe monkey



Seriously though sometimes you have to accept that things change, nothing about what you were doing before being wrong, just a different way otherwise we would still be burning coal.

 

[SimonG]

I refer you to post #62

 

[SimonG]

Anyway what's supposed to have changed?

All the old stone buildings with no insulation?

 

[Nostrum]

Double glazing, loft insulation, fitted carpets, less ventilation but more importantly an accurate method for figuring heat loss using a logical process rather than a formula written on the back of a *** packet in 1950.

Bottom line is each to their own, I don't care how big a radiator you specify as it won't be going in my house.

I'm surprised that some of the things people are advised against on here could be seen as trivial (whether or not to put jointing compound or PTFE on an olive) yet sizing a radiator and ultimately a full heating system in the correct way is frowned upon.

 

[SimonG]

I'm not frowning upon anything and if I had to sit down and design a heating system for scratch for money then calculations it would be. All I'm saying is that I can go and do a quck calculation (in my head) and define the rad or rads there and then. All based on experience and what I see.

I used to think this was magical because my boss used to do this when I was an apprentice. He would have me calculating the U value of all the surfaces using r values and define the heat loss down to the nearest btu. My calcs and his estimate would be within 50btu's either way.

All I'm saying is that it works for me and rarely get it wrong.

But to be called unprofessional because that's the way I do it is not on.

 

[Nostrum]

With all due respect simonG I did specifically say that no offence was intended. I'm not doubting or questioning your experience or ability in the trade, I've no right too, I don't know you and I've no interest to do so either. (doubt you)

The example above about being 50 btu's out cannot possibly be correct seeing as a simple change in wall construction from solid to insulated cavity could cause a greater difference and the ft2x5 rule of thumb can't allow for construction material.

Its pretty obvious not just from your replies but from the general response that the method me and Chris Watkins are familiar with isn't common practice amongst the majority on here and most definitely not with the majority of engineers I have met or worked with over the years in my area. That's not surprising as most systems aren't installed with economy in mind. If it gets hot enough it works tends to be the general approach, and it will because the rule of thumb and mears calculator All oversize by quite a way.

Its almost like saying we should put a 35mm gas supply in for every new boiler because it will always work.

All I will say is that times are changing and new technologies and restraints call for more accurate methods in sizing emitters, heat pumps being a prime example. If required there are spreadsheets available for free that make the whole process take about 2 mins per room, a little more if the construction material is non standard.

I'm happy to agree to disagree, and if you genuinely thought my comments were out of order, I apologise as it was intended to be a bit tongue in cheek and certainly not meant to cause offence.

 

[SimonG]

We agree to disagree then.

 

[chris watkins]

All I'm saying is that it works for me and rarely get it wrong.

But to be called unprofessional because that's the way I do it is not on.

How do you know that you don't get it wrong then Simon ? if you don't do a calc involving measuring the room & windows, checking the construction of the building etc how would you know? You, & I agree most other plumbers do guess-emate to size a rad & that includes no corrections to the rad table outputs which are based on a temp difference of 50deg C.
Do you know what I believe Sambotc, I think the hostile reaction we receive ever time this gets brought up (not just on this forum) has more to do with the fact that a lot of plumbers & so call heating engineers do not know how to do this, have no interest in finding out how it is done (even when it is reasonable easy & won't cost them either), they just keep on telling anybody who will listen that they have the 'power' to predict what the heat loss & rad size will be, based upon years of experience dating back to when BTU's where used & their plumber handed on this sacred knowledge. Even though as you say time s have moved on, designs, boilers, heat emitters, insulation etc etc have all moved on !!!!!!!!

 

[Nostrum]

It will filter through I think in the next few years as the courses start to include it. As i said above ive no interest if people don't want to know about the changes. In fact I see it as a bonus if I have to quote against a company who pulls numbers out of a hat or if I can sit down and work out the specific requirement on site in minutes and explain the process to the client.

 

[chris watkins]

one of the same Sam, one of the same.

 

[Mike Jackson]

When I quote a job I always specify the room temperatures that I've allowed but not the rad sizes. I tell the customer that they can have a copy of the calculations including the rad sizes when the job is accepted. If I'm in a rush when doing the quote I can guesstimate the rad sizes needed and do the proper calcs if i get the job. I normally find my guesstimate is larger than the required size.

 

[stevetheplumber]

ive always been a 5 per sqft man and also own a mears wheel for larger than the normal 3 bed house
its all very well having the wonderfull program for calcs if you are on new build work where a u value of each surface is probably an email to the desinger away
how do you work out the u value of a retro fit into a single flat in a block of flats built by persons unknown which is clad in some sort of finish you cant id ?where you dont know if the flat above and below may be heated or not and theres no access to loft space to check insulation levels ?