Linking Two Unvented Cylinders

[Dragoon]

Hi Dragoon. I'm not sure whether AW is online, but in case he isn't, this is what he is suggesting.

On the heating side, a combi is just like any other boiler - it has a flow and return which can be used as an S plan (preferred) or Y plan (if you must) to heat a cylinder as well as heating the radiators.

In addition to that, it has the instanteous hot water system, albeit with a limited flow rate, as you have identified.

So if you mate a combi with an unvented cylinder, you can have stored water available for some outlets and instantaneously heated water for others. Exactly how you divide them up is up to you, but normally outlets that use a lot of water, relatively rarely (say a bath) should be fed by stored hot water, whereas outlets that frequently use just a few litres (say kitchen sink) would be better on the instant.

The limiting factor (assuming that the cylinder is unvented) is the capacity of the incoming cold main, but yours sounds like it has plenty of flow at least.

Thanks Ray, so would you suggest that in my case the shower and the sinks etc be fed by the unvented cylinder, while the large bath and the bath basin be fed by the instantaneously heated side? That sounds pretty good to me, as a small drop in flow rate at the bath shouldn't make too much of a difference.

 

[Dragoon]

Well you would have superb insulation with a decent thermal store, so you would be just topping up temp each day. It works very well and have never had any problems in the past.

Hi Cardiff Plumber. That sounds good actually. But would that be the same for an unvented cylinder i.e.. that it would just be topping up temp each day?
If I lived anywhere near you (Cardiff) I might seriously consider it, but as I've said before it's hard to find people who have a good working knowledge of Thermal Stores.

 

[chris watkins]

Sounds like a good idea don't it, however your bath would be cold by the time it is full & you are ready to get in.
Your bath 400+ Litres I believe, combi flow rate lets say it's a 28kW @ 12 L/M @35deg C rise = 33mins to fill it i.e. cold & a long wait.

 

[Dragoon]

Sounds like a good idea don't it, however your bath would be cold by the time it is full & you are ready to get in.
Your bath 400+ Litres I believe, combi flow rate lets say it's a 28kW @ 12 L/M @35deg C rise = 33mins to fill it i.e. cold & a long wait.

Hi Chris
Yeah, that's true. Do you think that what Cardiff Plumber says is true for unvented i.e. that I would just be topping up temperature each day so it wouldn't be too expensive?

 

[Dragoon]

Sounds like a good idea don't it, however your bath would be cold by the time it is full & you are ready to get in.
Your bath 400+ Litres I believe, combi flow rate lets say it's a 28kW @ 12 L/M @35deg C rise = 33mins to fill it i.e. cold & a long wait.

However, as you said before, filling a bath would be 60(hot):40(cold) so for a 400ltr bath I would need 240 ltrs hot water which, at 12 ltrs/min would take 20 mins.
Is 20 mins okay to fill a bath? I don't really have baths often so I wouldn't know. This is really just for the girlfriend!

 

[Ray Stafford]

Thanks Ray, so would you suggest that in my case the shower and the sinks etc be fed by the unvented cylinder, while the large bath and the bath basin be fed by the instantaneously heated side? That sounds pretty good to me, as a small drop in flow rate at the bath shouldn't make too much of a difference.

Nope. Other way round.

Infrequent use but lots of water = bath. Take that off the cylinder.

Frequent use but middling amounts of water = shower/basin/sink. Take them from the combi.

Maybe also think about the most likely clashes, and make sure they are on separate supplies

 

[Nostrum]

How about a smaller bath? :hurray:

I'd stick with a 250-300 litre cylinder. Wire the system to prioritise hot water so when your running a bath the boiler is diverted to hot water only.

Also remember to subtract the water displaced by people being in the bath. (Depending what you were going to use a big bath for :rolleyes2

The heat losses between the 300 and the 180 is less than a kW a day. Gas is approx 4.5p per kW so (ignoring boiler efficiency) and assuming it all goes to waste (ie not airing clothes etc) your looking at less than £20 a year. The additional cost in complex installation would far outweigh the saving.

 

[Dragoon]

Nope. Other way round.

Infrequent use but lots of water = bath. Take that off the cylinder.

Frequent use but middling amounts of water = shower/basin/sink. Take them from the combi.

Maybe also think about the most likely clashes, and make sure they are on separate supplies

So the cylinder would be heated up every day, but not used until Bath day while the Combi would be in use everyday for the rest of the outlets?

Wouldn’t it be better to have it the other way around where the cylinder can be used like a ‘normal’ unvented system (where the cylinder is heated up everyday), and then on Bath day we can have the extra hot water “straight from the tap” without any storage?

Or am I missing the point? (which wouldn’t be the first time I can assure you!)

 

[Dragoon]

How about a smaller bath? :hurray:

I'd stick with a 250-300 litre cylinder. Wire the system to prioritise hot water so when your running a bath the boiler is diverted to hot water only.

Also remember to subtract the water displaced by people being in the bath. (Depending what you were going to use a big bath for :rolleyes2

The heat losses between the 300 and the 180 is less than a kW a day. Gas is approx 4.5p per kW so (ignoring boiler efficiency) and assuming it all goes to waste (ie not airing clothes etc) your looking at less than £20 a year. The additional cost in complex installation would far outweigh the saving.

Hi Nostrum.
You’re right! I completely forgot about the displaced water! Also, it would actually be worth it if the rolling eyes were applicable! But she likes lighting candles and reading her books in the bath.

And it has been bugging me if I’m being overly concerned about the extra expense. When you say that the heat losses between the 300 and the 180 is less than a kW a day what does that mean?

 

[Nostrum]

The energy losses of a good unvented cylinder are 1.5 - 2.5 kWh's per day. Ie naff all. You are worrying over nothing, or pennies per day.

:welcome:

 

[chris watkins]

However, as you said before, filling a bath would be 60(hot):40(cold) so for a 400ltr bath I would need 240 ltrs hot water which, at 12 ltrs/min would take 20 mins.
Is 20 mins okay to fill a bath? I don't really have baths often so I wouldn't know. This is really just for the girlfriend!

The 60/40 bit only works if hot is around 60 - 65 & cold is around 10deg C the blend would give hot water at around 40. With a combi (instantaneous heater) the power is fixed (say 28kW) & the flow rate can be adjusted to give the require outlet temperature. So increase the flow & outlet temp goes down, slow it & it increase (it is said to be proportionate).
Most manufacturers state the flow rate with a 35deg rise, so if incoming mains water is at 10 deg C the outlet temp would be 45 with a flow rate of 12L/M.

45 is little more than hand hot so by the time it gets to said bath & heats up the material, sits around for half hour it all equals a cold bath.

 

[Gray0689]

Thermal store for your situation is not a good idea
They are ok for a solid fuel link up just about IMHO

 

[AWheating]

400ltr bath, minus the volume of the two people enjoying each others company.

may even increase the water temps........

 

[Dragoon]

The energy losses of a good unvented cylinder are 1.5 - 2.5 kWh's per day. Ie naff all. You are worrying over nothing, or pennies per day.

:welcome:

Hi Nostrum.

Thanks for putting some sort of a figure on the losses for me. If it only amounts to just pennies a day, it’s not really worth compromising on flow rates.

Also the system will be housed in a dedicated small boiler room, which I intend to also use as a largish airing cupboard. And any heat losses from that will go to heat the hall so really the losses will be almost negligible.

Oh and thanks for the welcome!

 

[Best]

You could also make sure the pipes are well insulated at the unvented unit - especially the hot supply pipe leaving the top of the cylinder. If that pipe drops downwards, it will prevent heat gravitating up the pipe. Small savings, but still helps.
Also get rid of any women from your house. :smile:

 

[Dragoon]

The 60/40 bit only works if hot is around 60 - 65 & cold is around 10deg C the blend would give hot water at around 40. With a combi (instantaneous heater) the power is fixed (say 28kW) & the flow rate can be adjusted to give the require outlet temperature. So increase the flow & outlet temp goes down, slow it & it increase (it is said to be proportionate).
Most manufacturers state the flow rate with a 35deg rise, so if incoming mains water is at 10 deg C the outlet temp would be 45 with a flow rate of 12L/M.

45 is little more than hand hot so by the time it gets to said bath & heats up the material, sits around for half hour it all equals a cold bath.

Hi Chris.

Thanks for that, I never did understand the 35deg rise as quoted by boiler companies, now I do. So that’s why Ray suggested that the cylinder should feed the bath and the combi should feed the shower and the rest.

So do you think I should just stick with a system boiler with an unvented cylinder? As Nostrum suggested that I’m worrying over nothing and that the energy losses will not really amount to that much, just pennies a day?

 

[Dragoon]

You could also make sure the pipes are well insulated at the unvented unit - especially the hot supply pipe leaving the top of the cylinder. If that pipe drops downwards, it will prevent heat gravitating up the pipe. Small savings, but still helps.
Also get rid of any women from your house. :smile:

The place is a bungalow on just the ground floor, so the pipe leaving the top of the cylinder will probably have to drop downward at some point.

Also I've tried doing without the women before. . . . . . but it's not good for the ol' eye sight. . . . if you know what I mean!:chinese:

 

[Dragoon]

400ltr bath, minus the volume of the two people enjoying each others company.

may even increase the water temps........

Well that was the idea behind the large bath, but judging from previous attempts, the increase in the water temp was quickly counteracted by the coldness of a shoulder which she offered, before going back to reading her book! :disappointed:

 

[Dragoon]

Thermal store for your situation is not a good idea
They are ok for a solid fuel link up just about IMHO

Hi Gray.

I actually like the idea of the way Thermal Stores work, it's almost like a large powerful Combi. However I don't know anyone who has one installed, and from the net the picture I get is that they can be temperamental, so some people really hate them while others rave about them. And like you said I don't think it's a good idea for my situation.

 

[SimonG]

Theres nothing temperamental, special or complicated with a thermal store. Without having multiple inputs though it will cost you an arm and a leg maintaining sufficient temperature in the thermal store for your requirements.

 

[Dragoon]

Theres nothing temperamental, special or complicated with a thermal store. Without having multiple inputs though it will cost you an arm and a leg maintaining sufficient temperature in the thermal store for your requirements.

I did my research on thermal stores a couple of years ago, but I remember vaguely something about issues with the mixing with cold water after the hot water's come through from the heat exchanger. Also heat exchanger problems as well, but the ol' memory's not too good so couldn't say for certain. But I do remember being impressed by them!

 

[Best]

Stainless Steel unvented cylinder = hot water up to as much as 65 degrees at usually 3bar pressure & full flow of your pipe work. Nothing really competes with that - & a powerful whole house pump costs a lot & is mechanical, so it won't last forever & it makes a noise.

 

[SimonG]

I did my research on thermal stores a couple of years ago, but I remember vaguely something about issues with the mixing with cold water after the hot water's come through from the heat exchanger. Also heat exchanger problems as well, but the ol' memory's not too good so couldn't say for certain. But I do remember being impressed by them!

Horses for courses, but I dont think this is your horse

 

[Dragoon]

Stainless Steel unvented cylinder = hot water up to as much as 65 degrees at usually 3bar pressure & full flow of your pipe work. Nothing really competes with that - & a powerful whole house pump costs a lot & is mechanical, so it won't last forever & it makes a noise.

Hi Best.

Yeah thats what the original idea, and I think I'm slowly coming back to that again. It's tried and tested! And it works! And all plumbers are familiar with it! And it's hot enough not to be dampened by a cold shoulder! :yes:

 

[Micksta]

Im Probably to late but have you thought about a Worcester Bosch high flow 440, works on the same principle as a Combi but slightly different.