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.