4 bathrooms, and making sure that there is enough hot water.

[Big Jimbo]

People on here have been very helpful so far to this out of date old git.

I'm building a house with 4 bathrooms. I need to be sure that i fit a system that will supply hot water to those bathrooms. It's no use saying make sure people don't use them all at the same time.

I would like to think that a decent build, say 3 on the airtightness, and "u" values to 2023 regs, will be better than a house built 3 years ago, and much better than most of the old stock housing that we have in this country. As i am intending to have underfloor heating on the ground floor, i may step up to 200mm insulation, rather than 150mm, above the block and beam floor. I may also stick a couple of extra layers of insulation in the loft, say 400mm to 600mm. (Its a cold not very high loft) both of those are easy, and should not cost a great deal of extra money. I've costed for tripple glazing in the build.

I would expect that the energy required for heating demand would be quite reasonable for the 2750 odd square foot of the property providing that the build and insulation is carried out with a bit of care.

You would think that this would be a good case for an ASHP.

My concern however, is the supply of hot water. My understanding is that stored water supplied by an ashp is, lets say 40 degrees. Where as normally, i would be storing hot water at say 65 degrees. This to me means that at 65 degrees i am using quite a bit of cold water in my showers, and blending it with my stored hot water to bring it down to a comfort level at my shower head. With my stored water at 40 degrees im using less cold water to blend. This to me says i need to have a larger hot water storage tank because i am going to be using more ?

I think i like the idea of having an ASHP but, yesterday, i spoke, at length, to an engineer who is very highly regarded. Teaches. Ownes a company that installs both Gas and ASHP systems. He said that if it was him he would fit a gas system, and make it ASHP ready.

The only other things that perhaps should be taken into consideration is that i have to have an electric car charger (planning condition), and i have set aside a budget of £10k for 10, or 12 400w solar panels that i will install pre roofing. i may or may not add a battery to this. I will also be fitting an MVHR for indoor air quality, and because i will have 7 wet areas, and dont fancy pumping all that heat straight out.

is the fitting of gas boilers after 2025 set in stone ? Might that change ?

Am i a bit mad to even be considering fitting a gas system.

I should note that i may well sell as soon as built, but at most may keep it for 3 years.

As i said i think the heating side can be easily taken care off by both systems. It's the hot water supply that is concerning me.

I know i have sort of already been here, but would appreciate any of your thoughts. Thanks

[Dan F]

16 minutes ago, Big Jimbo said:

My concern however, is the supply of hot water. My understanding is that stored water supplied by an ashp is, lets say 40 degrees. Where as normally, i would be storing hot water at say 65 degrees.

A lot of people store hot water at around 48C, but there is no you can't store water at up to 60C if you have R290 ASHP.  It's a trade-off between COP and the heat losses from UVC (reasons to use lower temp) vs. more hot water for showers (reason to use higher temperature).

 

A 300L tank at 48C will give you 360L of shower water, whereas the same tank at 60C will give you 420L of "shower water" at 40C.  How many showers this gives you, will depend on how long you shower for and your design flow rate, but assuming a 12min shower (@ 12L/min), that's between 2.5 and 3 showers.  An ASHP recovers more slowly though, so (assuming ASHP is shared with heating and you don't oversize it) it will take an hour or more to recover (although there are some approaches using PHEs which try to work around this).  You can also get another 30%+ from your stored water by using WWHRS.

 

I don't think there is really anything you can do with gas that you can't do with ASHP (aside from the very fast reheat time), it is really about being clear on what your requirements are and designing for this.

 

What is your design shower flow rate, how long are showers?  Maximum parallel/consecutive showers?

 

 

 

Edited June 13, 2023 by Dan F

[Big Jimbo]

Thanks Dan for the reply. I'm still waiting for my building reg drawings so i can either do, or get some calcs done. In my world i want all the showers to be usable at the same time. girls, and me tend to like decent showers. 12mins looks good.

[Dan F]

9 minutes ago, Big Jimbo said:

In my world i want all the showers to be usable at the same time. girls, and me tend to like decent showers. 12mins looks good.

All four at the same time?  You'll need to consider that when pipe runs get designed, especially if you want to minimize the effect on the flow rate of other showers.  Probably want a dedicated 22mm run to each shower (depending on length).  I'd avoid a standard approach, which uses a loop around the house, in this case.

 

12L/min, but how long is an average shower?

[Big Jimbo]

Thanks Dan. yes i get the dedicated supply to each shower, rather than legs off. I would say i would like each shower to be say 10mins.

[JohnMo]

 

9 minutes ago, Dan F said:

dedicated 22mm run to each shower

Not sure you really need that, you will have got fed up waiting for the shower run hot by the time water got there. 15mm is fine, maybe a dedicated 15mm to each wet room from a manifold close to the cylinder may be more practical. That what we did and the showers flow plenty.

 

46 minutes ago, Big Jimbo said:

You would think that this would be a good case for an ASHP.

 

I completed my build i.e. had council sign off a few months ago. I fitted a gas combi with preheat cylinder during the build. Since sign off, I have installed an ASHP. The fact is with UFH and well insulated an ASHP is/should be cheaper to run for 98-99% of the year compared to gas. With PV you get to offset some of the running cost. In summer your hot water should be free. Even if your PV is only generating 1kW you are filling the cylinder with 3 to 4 kW by using your ASHP. Will use the ASHP in the summer for hot water and use the nearly new gas one in the winter.

 

You need to shop around for PV your prices are huge. I picked up 12x285W, 2 year old panels yesterday for £800, picked up the nearly new inverter for £150, ground mount frame will be about £300 in materials, plus a load of 6mm2 armoured cable.

[Big Jimbo]

One thing i just thought about is the flow from a shower head. The crappy bungalow that i currently live in has a 10kw electric shower. It was fitted with the shower head that came with the unit. The Mrs always complained that there was not enough power to wash conditioner out of her hair.  The head eventually scaled up and i replaced it with a cheap one from one of the sheds. No more complaints.... I expect that the new shower head has a lower flow rate, and this leads to the perception that it is more powerful. So i could look at having a nice big flow in the master, and less flow in the other bathrooms ?

[Dan F]

2 minutes ago, JohnMo said:

Not sure you really need that, you will have got fed up waiting for the shower run hot by the time water got there. 15mm is fine, maybe a dedicated 15mm to each wet room from a manifold close to the cylinder may be more practical. That what we did and the showers flow plenty.

 

It all depends on the length.  15mm may be fine for 12L/min fairly close to UVC, but if you want 15+L/min and lengths are 15m+ that's a different story.

[Dave Jones]

have exactly the same issue with our current build. 

 

5 beds / 4 baths.

 

Current plan is an accumulator to maintain cold pressure and starting with 2 x 300L tanks with space to add more. I'll keep adding them until we dont run out of hot.

 

Heating the water, will use ASHP to get it 45ish then overnight immersion to get it as hot as safely possible. Solar PV dump when available etc.

[Dan F]

30 minutes ago, Big Jimbo said:

Thanks Dan. yes i get the dedicated supply to each shower, rather than legs off. I would say i would like each shower to be say 10mins.

 

So thats not too dissimilar to our usage.  We have a 300L tank which we heat at night (cheap tariff) to 55C.  We also have WWHRS.  This gives us a total effective amount of hot water available at 40C of about 520L.  At 12L/min, that's 4 x 11-minute showers. In practice though that (considering other hot water usage) and allowing for longer showers that's 3 showers.  In our case though, it is very rare for us to have more than 2 (rarely 3) consecutive/parallel showers.  So, while some days the UVC is only heated between 2-5 am and this lasts us all day, if the UVC goes below 20% during the day then it gets reheated without waiting until 2 am.

 

In your case, I'd look at maybe using a 400L + WWHRS based on the flow rate and showering time you mentioned.  If you want longer higher flow-rate consecutive/parallel showers then you'll likely need multiple tanks like @Dave Jones. I'm not sure a gas boiler would necessarily avoid this.

 

 

[Dan F]

4 minutes ago, Dave Jones said:

Heating the water, will use ASHP to get it 45ish then overnight immersion to get it as hot as safely possible.

 

Why not heat it more with ASHP?  When I know we'll need more hot water (e.g. visitors) I heat ours to 60C with ASHP.  Yes, the COP drops when you get up to 65C flow temperature, but it's still almost twice as efficient as an immersion.

[Dave Jones]

10 minutes ago, Dan F said:

 

Why not heat it more with ASHP?  When I know we'll need more hot water (e.g. visitors) I heat ours to 60C with ASHP.  Yes, the COP drops when you get up to 65C flow temperature, but it's still almost twice as efficient as an immersion.

Plan to have a play and find the sweet spot. Hoping the solar will mean very little is needed from the ASHP apart from deep winter.

[JohnMo]

3 hours ago, Dave Jones said:

Plan to have a play and find the sweet spot. Hoping the solar will mean very little is needed from the ASHP apart from deep winter.

You will need plenty of solar to heat 2 x 300l cylinders to a usable temperature. A third the amount of solar if you used a heat pump while generating solar.

[Pocster]

ASHP , big uvc 

 

My let property has 4 showers - I made certain all could run simultaneously for 10 minutes . Uvc not gas ‘boosted ‘ - purely electric .

If you get issues ( I actually had 5 showers ) you can get a shower that feeds directly off a cold header tank . 

[Dave Jones]

14 minutes ago, JohnMo said:

You will need plenty of solar to heat 2 x 300l cylinders to a usable temperature. A third the amount of solar if you used a heat pump while generating solar.

 

have 9.9kw solar, this time of year the heat pump will be in cooling mode though so not available for dhw.

[Dave Jones]

2 minutes ago, Dave Jones said:

 

have 9.9kw solar, this time of year the heat pump will be in cooling mode though so not available for dhw.

 

maybe there is a double ashp so in cooling mode it can shed the heat into the other pump to blast the dhw.

[JohnMo]

8 minutes ago, Dave Jones said:

 

maybe there is a double ashp so in cooling mode it can shed the heat into the other pump to blast the dhw.

You can get them, uses the refrigeration circuit waste heat to heat the cylinder.

[dpmiller]

16 hours ago, Dave Jones said:

have 9.9kw solar, this time of year the heat pump will be in cooling mode though so not available for dhw.

 Our HP will happily switch itself from cooling to DHW if it needs to. Plus how long do you think you'll need to run it for to take a wee bit of extra heat out of the house?

[JohnMo]

2 hours ago, dpmiller said:

 Our HP will happily switch itself from cooling to DHW if it needs to. Plus how long do you think you'll need to run it for to take a wee bit of extra heat out of the house?

Our does everyday at the moment. Cooling starts at 8am, hot water 9am, cooling goes off at 5pm. It just waits a couple of min after hot water before starting cooling circulation again. It will circulate through UFH until has reached the cooling switch on point, which cooling set point, plus target delta T, plus 2. So for us it will start cooling again at 18 degree return temp.

[Nic]

Maybe a silly question ( I was told once there aren’t any of those🤔)

if you have a mhpv or ashp that is heating the hot water via a tank. If you run out of hot water, say someone has a bath a shower and also running the washing machine etc . How long does it take to heat back up on average? Is there a general time ? Or depending on system . Are there such things as boost overrides ? 
told you it was quite silly 😜 

[dpmiller]

I'm  using a 9kW ASHP on a smallish thermal store, so we don't have a massive amount of *stored* energy until there's plenty of PV diverted to it. Nevertheless, the tank never runs cold and the heat pump will cut in and recover seamlessly. Less than an hour will recover the tank.