Install a pair of boilers and cylinders

[allthatpebbledash]

Not wanting to jump on to the other post of seeking unlimited hot water for 4 bathrooms, I have a similar question however. Currently at technical planning stages now so trying to gauge possible options.

 

I have a plant room, approx 4.5x5x3m. I have a en-suite bathroom to master bedroom (on FF above plant room) and kitchen/utility is on the same side too of the house as the plant room. I then have 2 more bathrooms (on FF) and a WC (on GF) at the other end of the house. No electric showers.
 

Occupation is 2 adults and 3 children below 10 to begin with. Decade from now, 2 adults and 3 teenagers/young adults. Anticipation is for 2 baths a week and daily showers for 2 adults in the en-suite. In the 2 family bathrooms, anticipating a shower/wash a day from each of the children now and in future.

 

I plan on keeping gas for CH via UFH and DHW. I also have planned a large PV array and battery to be installed.

 

What would be the most efficient way to set this up? From the browsing and reading, does this make much sense.

 

Have 2 cylinders say 300 each, or a single 500/600l? Having 2 would work in this way, one supplies the en-suite and kitchen/utility, and other supplies the family bathrooms and WC. Keeps everyone happy and hot water doesn’t run out when it is needed as adults and children will use at different times, avoids waiting for reheating.

 

If 2 water cylinders, then does it make sense to install a pair of boilers too?

 

Like run a combi for CH and use a system for DHW?

 

Given the solar potential I have, I could divert excess into the cylinders, or use immersion at low price overnight.

[JohnMo]

Only two off us, but we never shower at the same time almost 12 hrs apart generally so ample time to reheat if needed.

 

I would look at water and energy conversation first. Waste water heat recovery for showers and aerated shower heads. Bring down hot water quantities needed.

 

Big two cylinders gives more flexibility. Heating to a high temperature also gives more capacity.

 

Do not do S or Y plan plumbing, do X or W and have priority hot water. This will allow boiler to run balls out for DHW and low and slow for central heating.

 

Not even hotels have unlimited hot water. So be careful for what you actually ask for.

[-rick-]

If you can locate the two cylinders close to point of use you save wasting water warming up the pipes when the tap/shower gets turned on (or installing a pumped loop - which has a maintenance/energy cost). When you do get to the teenage stage the separate cylinders will stop the kids draining the adults hot water and vice versa which removes one reason for arguments.

 

Don't see the need for separate boilers especially if you think you might have a lot of excess solar to keep the tanks topped off with heat. Have the two tanks piped in parallel to the boiler which should minimise recharge time. If you feel like you want two boilers and have money to throw at it, consider a heatpump and a boiler. Heatpump for most heating (run off solar energy when possible) and boiler as top up/insurance. You might end up finding you don't actually need the boiler in the end and even if you do you are using more of your solar energy.

[Temp]

We opted for a 300L tank and a powerful 40kW boiler. This was able to keep up with our two high flow rate showers and similar size family to yours. The tank feeds a secondary loop that keeps the pipes hot in distant parts of the house so no waiting for HW.

 

The problem these days is that such a powerful boiler is too big for the heating load. You can have issues getting it to throttle down and stay in condensing mode.

 

To solve that our tank is actually a thermal store. The boiler heats the store and the store provides both heat/UFH and DHW. 

 

This also solves the problem that oil boilers don't modulate/throttle down. It can run flat out when heating the store.

 

This has worked well for us but the store and associated pumps, heat exchangers and pipes leak a lot of heat into the room it's in so probably not very efficient. It was also hard to find engineers who understood the issues.

[JohnMo]

Something like this may be the answer

Amazon_HXIN.pdf

[allthatpebbledash]

On 05/11/2025 at 14:24, JohnMo said:

Do not do S or Y plan plumbing, do X or W and have priority hot water. This will allow boiler to run balls out for DHW and low and slow for central heating.

Can you elaborate more on what the SY and XW mean? Thanks, not as proficient yet in plumbing.

 

On 05/11/2025 at 15:35, -rick- said:

 

If you can locate the two cylinders close to point of use you save wasting water warming up the pipes when the tap/shower gets turned on (or installing a pumped loop - which has a maintenance/energy cost)

 

I have read on here where someone has commented the secondary loop system would use quite a bit of electricity and in the end they resulted in turning the system on when guests were around and off during family use to reduce the cost of it. I think it might be possible to locate one of the tanks on the other side of the house, but that would mean losing some space either in the office or cloak area. The latter would be a good bet as residual heat would help keep coats shows boots dry.

 

On 05/11/2025 at 15:38, Temp said:

To solve that our tank is actually a thermal store. The boiler heats the store and the store provides both heat/UFH and DHW. 

My architect has mentioned something about making a thermal store using a gas boiler given I don’t want an ASHP. Can you expand on how this works and what its realtime benefits are? Presumably there are some compromises so maybe those too? Thanks.

[JohnMo]

4 minutes ago, allthatpebbledash said:

Can you elaborate more on what the SY and XW mean? Thanks, not as proficient yet in plumbing.

S and Y plan, just use boiler at a single flow temperature, so you loose any gains in efficiency from running a two different flow temps - one for DHW cylinder heating and another for central heating. X and W allow two different flow temperatures and priorities heating DHW.

[Temp]

On 10/11/2025 at 10:38, allthatpebbledash said:My architect has mentioned something about making a thermal store using a gas boiler given I don’t want an ASHP. Can you expand on how this works and what its realtime benefits are? Presumably there are some compromises so maybe those too? Thanks.

 

 

 

Basically it's a tank of hot water that delivers heat to both the heating and the DHW.

 

The room thermostats control a pump that takes heat from the thermal store and send it to the rads/ufh

 

A thermostat on the store controls the boiler telling it when to put heat into the thermal store. 

 

So the store acts like a buffer between the boiler and  heating/rads/UFH allowing them to run semi independently. Bit like a gearbox in a car which matches the engine with it's limited range of rpm to the load/wheels.

 

The main advantage occurs when you have an oil boiler as they always run the burner flat out, they don't modulate/throttle down when the heating load is light. Instead they cycle the flame fully on and fully off.  You can imagine how driving a car with a two position throttle would be less efficient. The thermal store allows the boiler to run flat out for longer but also with longer gaps when the flame is off.

 

Gas boilers benefit less as they can modulate down the flame. Especially models that are fully modulating. Not all can modulate down as far as others

 

The down side of a thermal store is the pumps and heat exchangers needed leak more heat than a regular DHW tank.

 

As you are looking at a gas boiler I'd probably do without a thermal store but choose a fully modulating boiler.

 

 

 

 

 

[JamesPa]

On 10/11/2025 at 10:38, allthatpebbledash said:

I have read on here where someone has commented the secondary loop system would use quite a bit of electricity

Really don't go down the recirculating loop route.  In summer it heats your house continuously when you dont want it to, so you will pay to heat and pay to cool.  In winter it heats the house at high cost when you could be heating it for one third of the cost.  For hotels, fine, for houses no.

 

Instead design out the need by locating dhw storage near points of use.  If necessary use point of use heating for everything other than baths and showers.

 

Unless of course neither money or carbon footprint matters to you and you can get joy by showing off about having instant hot water everywhere.

Edited yesterday at 19:53 by JamesPa

[JohnMo]

31 minutes ago, JamesPa said:

Really don't go down the recirculating loop route.  In summer it heats your house continuously when you dont want it to, so you will pay to heat and pay to cool.  In winter it heats the house at high cost when you could be heating it for one third of the cost.  For hotels, fine, for houses no.

You can when building a house put pipes with in insulation etc. you can also use thermostatic and timer control so little or no impact on house temperature, any time of the year. Ours runs for 2 hours in the morning only and the pump only runs until the return temp hits about 35 degs. This is the impact on cylinder temp over a 3 hour period including two people using the sinks etc.

 

Switching on the tap and having immediate warm water is great, no waiting and endless water draw off.

 

 

 

[JamesPa]

21 minutes ago, JohnMo said:

You can when building a house put pipes with in insulation etc. you can also use thermostatic and timer control so little or no impact on house temperature, any time of the year. Ours runs for 2 hours in the morning only and the pump only runs until return temp hits about 35 degs.

 

 

Fair enough if you make it that sophisticated.  Is that what your average plumber does or do they just have it running continuously? 

 

There is a running 'ashp' thread elsewhere in which it's obvious that the main problem is in fact a circulating loop (IE nothing at all to do with the ashp).

[Andehh]

We've got a circulation loop, and wouldn't be without it. Would have a minute wait for hot water in the kitchen otherwise (large bungalow) .

 

I just Armaflex'd it in 40mm (ish?) thick pipe lagging.

 

We start it going during the cheap off peak rate, to earn the loop up and cylinder off peak... Then let it crack in through the day. 

[JohnMo]

20 minutes ago, JamesPa said:

Fair enough if you make it that sophisticated. 

Just used a pump that has the timer and adjustable thermostat built in to it, nothing complex, got it from eBay for about £125.

 

https://www.completepumpsupplies.co.uk/lowara-ecocirc-pro-15-1-65-u-circulating-pump?srsltid=AfmBOool1Fk6POF4FC1q5gROaJ7sAXKec93riA7uyJc3N1OgDdt_bucDE48

 

The circulation loop is just a single pipe with pump and check valve. The water is just water in an unvented cylinder - in my case heated by ASHP, but could be heated by anything that heats the cylinder. Water is drawn via normal DHW pipes, and a additional pipe takes the water back to cylinder and injects it back in the cylinder.

 

Why is secondary circulation circuit anything to do with an ASHP, it has nothing to do with heat source. It's just part of the DHW distribution system.

[JamesPa]

12 hours ago, JohnMo said:

Why is secondary circulation circuit anything to do with an ASHP, it has nothing to do with heat source. It's just part of the DHW distribution system.

It hasn't.  However in the thread concerned the complainant was blaming his ASHP (because thats what people do) for problems clearly caused by a poorly insulated circulation loop running continuously. 

 

I suppose with a boiler the problems wouldn't be so easily noticed, because the boiler would just carry on chucking heat into the loop.   

Edited 11 hours ago by JamesPa

[allthatpebbledash]

My takeaway here is I don’t really need a secondary loop running. I can fit 2 water tanks, servicing each side of the house where water is needed to reduce the arrival time.

First in the plant area on ground floor below the master en-suite and adjacent to the kitchen and utility area. Second on the first floor in an airing cupboard next to the 2 family bathrooms and above the ground floor WC. Residual heat would help benefit the airing cupboard too.

 

Plant room tank could be larger than the one needed on first floor too.

[JohnMo]

Or just fit a single one in a central location, or install in a remote location and have a timed secondary loop. If installing one get one with a large plate exchange and have better than combi performance for reheat - as I linked too further up thread.

[Nickfromwales]

1 hour ago, allthatpebbledash said:

My takeaway here is I don’t really need a secondary loop running. I can fit 2 water tanks, servicing each side of the house where water is needed to reduce the arrival time.

First in the plant area on ground floor below the master en-suite and adjacent to the kitchen and utility area. Second on the first floor in an airing cupboard next to the 2 family bathrooms and above the ground floor WC. Residual heat would help benefit the airing cupboard too.

 

Plant room tank could be larger than the one needed on first floor too.

I’d go for the single location, wherever, with a larger lower temp cylinder, and a secondary hot return circuit which was insulated very well; this is what I do on most jobs and works a treat. 
 

When you get used to near zero wait time for premium temp DHW to come out of the tap, you’ll not go back to tolerating the wait. 
 

If there’s solar or cheap lecky to use to offset the small uplift in energy costs to run this system then it’s a very nice luxury to have and doesn’t cost a lot.

 

Place the money saved from the 2nd cylinder ( labour / materials etc ) into a glass jar, and use that to pay the energy bill.