Unlimited hot water with 4 bathrooms - is it possible?

[SimonD]

4 hours ago, JohnMo said:

With lower storage temp, depending on shower mixer make, you may have to tweek the settings at the back of face panel to get shower feeling comfortable - basically re commission the shower mixer.

 

Yup, got that one covered. I'd set the blending valve to 50C when I commissioned the TS, but it's a good call for others that decide to change any DHW storage temp!

[Nickfromwales]

4 hours ago, SimonD said:

 

Yup, got that one covered. I'd set the blending valve to 50C when I commissioned the TS, but it's a good call for others that decide to change any DHW storage temp!

Most modern devices will cope at that temp. Try it first, fiddle after, only if absolutely necessary. 

[Dave Jones]

without reading the rest, biggest tank you can fit. let ASHP get it to 52, use an immersion overnight on cheap rate to get it up to 75c+ 

[Nickfromwales]

6 hours ago, Dave Jones said:

without reading the rest

You lazy bar steward.

 

[John Carroll]

On 07/10/2025 at 22:16, Indy said:

Very very rare that 3 showers would be used together simultaneously. 2 is more realistic and I'd like a good flow rate on both of them (15lpm + ideally). 

 

Assuming you opt for a 400L cylinder, when heated to 60C and assuming you would prudently leave say 50L after showering then 350L at 60C (and assuming a mains temp of 10C) will allow almost 19.5 minutes showering time for 2 at 15LPM @ 40C or almost 13LPM if 3 showering together, the recovery time to 60C assuming a average coil output of 20kW will be just over 60 minutes or with a average coil output of 25kW, 40 minutes.

 

Probably a "step too far" for you, but a 20/25kW output PHEX would still give you unlimited HW after the 350L is used up.... a continuous flow rate of 9.56/11.95LPM at 40C.

Edited October 11 by John Carroll

[JohnMo]

2 minutes ago, John Carroll said:

Probably a "step too far" for you, but a 20/25kW output PHEX would still give you unlimited HW after the 350L is used up a continuous flow rate of 9.56/11.95LPM at 40C.

Just use a heat pump cylinder 400L you easy get a 4-5m² coil in. Even a 3m² would give quick recovery. And possibly stay in condensing mode for most the reheat if you set it up well.

 

But if you have room for 400L cylinder 2x 210L slimline heat pump cylinders would give way more flexibility and faster reheats and you would get 6m² coil out the box. 

[marshian]

1 hour ago, JohnMo said:

 

But if you have room for 400L cylinder 2x 210L slimline heat pump cylinders would give way more flexibility and faster reheats and you would get 6m² coil out the box. 

this was why my suggestion of two tank set up - one for kids bathroom and one for the others - could schedule the reheats to be just before requirements and size the tanks accordingly - as a result you don’t end up with one massive tank that takes a while to reheat or requires a serious kW input to do it faster and the boiler can be sized more appropriately for house heating needs (plus each tank if chosen wisely with a heat pump compatible coil wouldn’t need the boiler to ramp up to very high flow temps)

 

OP’s house still in planning stage so finding space for the two smaller tanks can be incorporated into the design 

[Oz07]

On 10/10/2025 at 16:31, Dave Jones said:

without reading the rest, biggest tank you can fit. let ASHP get it to 52, use an immersion overnight on cheap rate to get it up to 75c+ 

how quick can an immersion recharge a tank after most of the water is gone? Or would it be ashp and immersion combined to provide quickest recharge? Is it a how long is a piece of string question?

[Nickfromwales]

1 hour ago, Oz07 said:

how quick can an immersion recharge a tank after most of the water is gone? Or would it be ashp and immersion combined to provide quickest recharge? Is it a how long is a piece of string question?

Depends on how far you let it drop before you catch it and start regenerating. 
 

This is referred to as “recovery” time btw.

If it’s from cold mains from the street cold, there’s a huge amount of lifting that needs doing. If it’s from say 45° and back up, there’s a significant reduction in recovery time.

 

This is why a lot of modern, larger UVC’s have a high and a low pocket for the temp sensors, as well

as a standard socket for the generic cylinder stat. Means you have options to take better control over this in more demanding situations. 

 

[Oz07]

how would it be from 45 and back up, if the tank only empties halfway? Mains water is always going to be cold isn't it. The more you look at it the more you see the appeal of that big combi. Just seems like less cost and less to go wrong. Can it be passed on the SAP though @Indy

[JohnMo]

1 minute ago, Oz07 said:

more you look at it the more you see the appeal of that big combi. Just seems like less cost and less to go wrong

They can be gas guzzlers, depending on how you operate, my wife like putting the hot tap on for a few seconds and then off again, repeated during the whole washing up, hot tap runs just enough time for the boiler to fire up and then tap is closed and it stops again - loads of times. Bit like short cycling.

 

We now have a cylinder, way better.

 

Heating a 210L cylinder using a 6kW heat pump heating from 40 to 50 is generally about 40 mins, can be longer or shorter depending on flow rate you set. CoP varies with time. The same cylinder (3m² coil) heated by gas is about a 1/3 the time running at a max flow temp of 60 degs, but can be really quick if you let the boiler run full load.

[Dave Jones]

On 12/10/2025 at 10:38, Oz07 said:

how quick can an immersion recharge a tank after most of the water is gone? Or would it be ashp and immersion combined to provide quickest recharge? Is it a how long is a piece of string question?

 

my 7kw panasonic takes 30-50 minutes to get a incoming water temp 250L cylinder to 52.