Linking Two Unvented Cylinders

[AWheating]

Unvented cylinder and combi boiler used as a system boiler but the hot water from combi supplying the bath and basin in the bathroom.

 

[Ray Stafford]

Unvented cylinder and combi boiler used as a system boiler but the hot water from combi supplying the bath and basin in the bathroom.

If there is no shortage of incoming pressure and flow (and it sounds like there isn't) then this would be an excellent solution, and very cost effective both to install and to run.

 

[Gray0689]

Hi Gray0689

Could you explain what a solar thermal is please.

Solar thermal
Basically tubes or panels on roof (South facing preferably)
That when sun out will heat hot water through a coil at the bottom of your cylinder

And stay away from thermal stores there not good

 

[Ray Stafford]

And stay away from thermal stores there not good

Thermal stores are excellent if you need to integrate multiple heat sources, particularly hard-to-control or unpredictable sources like solid fuel or renewables.

If there is no need to manage such challenging inputs, then its difficult to make the case for a thermal store in a domestic installation.

 

[BLOD]

You could fit an unvented sized for just the shower & hot taps and fit a combi boiler zoned for heating & cylinder but using the hot water side just for the bath.

 

[SimonG]

I wouldnt be keen using a combi to fill such a large bath. It takes about 25 minutes just to fill a standard bath with our gas combi.

 

[Dragoon]

Unvented cylinder and combi boiler used as a system boiler but the hot water from combi supplying the bath and basin in the bathroom.

Hi AWheating

Could you please explain the above system a little more. So would that be the one boiler (a Combi boiler) and one unvented cylinder? If so, could you explain a little more how it would work?

 

[Ray Stafford]

Hi AWheating

Could you please explain the above system a little more. So would that be the one boiler (a Combi boiler) and one unvented cylinder? If so, could you explain a little more how it would work?

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.

 

[Cardiff Plumber]

Hi Cardiff Plumber.
Wow what place that Lymm Water Tower is!!! I wish mine was anywhere near as good! Impressive work!!

I have considered Thermal Stores before and they sound a good idea. The only problem I have with them is that not many plumbers are familiar with them, so if and when things go wrong with them, I wouldn't be able to just phone the first available plumber to sort the problem.

Also in my particular case, wouldn't it again involve heating a large cylinder of water for the occasional use of the bath?

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.

 

[Dragoon]

Thermal stores are excellent if you need to integrate multiple heat sources, particularly hard-to-control or unpredictable sources like solid fuel or renewables.

If there is no need to manage such challenging inputs, then its difficult to make the case for a thermal store in a domestic installation.

Hi Ray.
Thermal stores sound like a good idea, however I don't have multiple heat sources so it's not really needed in my case. But also I heard that they can be problematic (as with all systems really as they age), and because not many plumbers are familiar with them, that would be my biggest cause of worry with them.

 

[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: