Unlimited hot water with 4 bathrooms - is it possible?

[John Carroll]

8 hours ago, Indy said:

 

It's a complete new build - so there is nothing to replace. It's a new installation. In our current place - we have a Worcestor Bosch Greenstar 37CDi. Does a commendable job though the pressure is a bit lacking for my liking. 

 

A 37kW combi will give a flowrate of 15.2LPM at @dT of 35C, you said in your first post that the pressure was a bit low, by which I take it you mean flowrate, this may have nothing to do with the boiler output but the actual flowrate itself is too low, in which case the boiler will just modulate down in output to match the demand. If two could shower at the same time at a flowrate of 7.6LPM each, would this be acceptable? the Navien will give 11.0LPM each if 2 showers in service and theoretically 7.3LPM if 3 showers in service, practically speaking I would think its unrealistic to expect 3 showers in service together so maybe go for a well known boiler make of ~ 40/50kW rather than the Navien which might only have a limited pool of experienced installers for servicing etc.

[marshian]

1 hour ago, Indy said:

I have not lost interest, but find it really hard to argue against a group for merely mentioning that I don't want to have an ASHP. 

 

I have been following all the comments in the thread and really like the suggestion of getting an oversized boiler - especially the Navien NCB700 which seems to be able to offer 22lpm for 3 simultaneous showers. This would potentially meet our demand - we're unlikely to have 3 bathrooms in use at the same time but it's very likely that 2 showers plus the kitchen tap are being simultaneously used and this fits the bill.

 

The only concern I would have here is the pressure when 2 showers are being used together, so will speak to my builder about putting in a small cylinder plus this combi boiler and see what he recommends. 

 

Our use case is more focussed on DHW rather than CH anyway. The property is a new build built to BR standards, triple glazed and UFH all around, so well insulated and  shouldn't need too much in terms of CH. 

 

 

Down side (from a quick look at the specs) whilst it does modulate down to 3.5 kW the min flow temp for CH is 40 Deg (max is 90 Deg)

 

With UFH throughout seems a shame to have to blend the temp down rather than get a boiler that can run at temps low enough for UFH but I'm pretty sure that'll just me me that's concerned about that

[Oz07]

45 minutes ago, Indy said:

 

It's a complete new build - so there is nothing to replace. It's a new installation. In our current place - we have a Worcestor Bosch Greenstar 37CDi. Does a commendable job though the pressure is a bit lacking for my liking. 

Have you put your supply pipe in yet? What size have you specced? Here you can have 25 or 32 i think. I had the bigger one on last place for better flow rate but think the meter pipe is the same size 

[SteamyTea]

Not read all the above, been way too busy at work (major refurbishment of restaurant).

 

Has anyone mentioned in-line supplementary electric water heaters?

They only start when the incoming water gets too cool, and as they are generally around 10 kW in power, cost about £3/hour to run.

Get 3 clean kids at an extra quid each.

[Indy]

21 minutes ago, Oz07 said:

Have you put your supply pipe in yet? What size have you specced? Here you can have 25 or 32 i think. I had the bigger one on last place for better flow rate but think the meter pipe is the same size 

Not yet but it will be 32mm - mentioned it to the builder and he agreed. He puts in the larger diameter as standard though the incoming won't change. 

[Indy]

3 minutes ago, SteamyTea said:

Not read all the above, been way too busy at work (major refurbishment of restaurant).

 

Has anyone mentioned in-line supplementary electric water heaters?

They only start when the incoming water gets too cool, and as they are generally around 10 kW in power, cost about £3/hour to run.

Get 3 clean kids at an extra quid each.

 

Are these a standalone item or something like an immersion heater in the cylinder, which kicks in if the temperature inside drops too much? Looks interesting and a space saving solution. Extra cost is manageable as it won't be getting triggered all of the time but the for 2-3 instances in a month when we do need it - could be useful to have. 

[-rick-]

7 minutes ago, Indy said:

Are these a standalone item or something like an immersion heater in the cylinder, which kicks in if the temperature inside drops too much? Looks interesting and a space saving solution. Extra cost is manageable as it won't be getting triggered all of the time but the for 2-3 instances in a month when we do need it - could be useful to have. 

 

They are basically electric showers with better controls but installed elsewhere.

 

https://www.flexiheatuk.com/product/electric-instantaneous-water-heaters/

[SteamyTea]

38 minutes ago, Indy said:

Are these a standalone item or something like an immersion heater in the cylinder

No, they are, as @-rick- says, just instantaneous electric water heaters.

They are fitted after the DHW cylinder and with the more sophisticated models, modulate to keep a constant temperature and flow at the outlet.

The problem with using immersion heating in the cylinder is that there will be times when you may need to heat a depleted 300 or 400 litre cylinder up by say 20°C, rather than just the water that is needed i.e 60 litres over 20 minutes.  This affect the total power requirements needed.

[-rick-]

1 hour ago, Indy said:

Extra cost is manageable as it won't be getting triggered all of the time but the for 2-3 instances in a month when we do need it - could be useful to have. 

 

If you are thinking about something like this then definitely also look into waste water heat recovery. Might not work well for your floorplan but if it does I doubt it would cost significantly more to install than an electric booster and could reduce the need for heat input by 40-60%.

 

Edit: I had second thoughts about this and went to check the specs. These things have a limited flow rate range so they lose efficiency if shared between showers they won't work as well, and installing one per shower gets expensive so it's not as clear cut as I initially thought.

Edited October 3 by -rick-

[Nickfromwales]

8 hours ago, John Carroll said:

 

A 37kW combi will give a flowrate of 15.2LPM at @dT of 35C, you said in your first post that the pressure was a bit low, by which I take it you mean flowrate, this may have nothing to do with the boiler output but the actual flowrate itself is too low, in which case the boiler will just modulate down in output to match the demand. If two could shower at the same time at a flowrate of 7.6LPM each, would this be acceptable? the Navien will give 11.0LPM each if 2 showers in service and theoretically 7.3LPM if 3 showers in service, practically speaking I would think its unrealistic to expect 3 showers in service together so maybe go for a well known boiler make of ~ 40/50kW rather than the Navien which might only have a limited pool of experienced installers for servicing etc.

There are large instantaneous gas water heaters available, eg not boilers. Just zero chance of PV > DHW without some kind of cylinder.

[SimonD]

I have to say I think this is just one of those very weird threads we get here on Buildhub where sense seems to be thrown out of the window. If it were a heating question, everyone would be posting to say you have to start off with a heat loss calc. Or the very least some design drawings to provide a starting point, but the basic questions about DHW design are completely missing on this one.

 

We don't know anything about pressure/flow rates to the house - regardless of size of main, what are typical static and dynamic pressures where the op is planning to build. Second is whether the requirements for infinite hot water is even really what the op wants or actually needs (I've been here too and the questions were resolved through careful whole system design taking into account flow volume restrictions we have got until I can dig a trench and re-lay our mains supply to the road at least). I realised I didn't need infinite hot water.

 

The assumptions we often make about both hot water and heating lead to over-sizing, which I think is what is going on here. e.g. 

22 hours ago, Indy said:

for 2-3 instances in a month when we do need it

 

So first we need to ask more fundamental questions about use and behaviour (and also how transient that behaviour might be and not make long term costly decisions about a temporary, short term problem). And then look at the variety of ways to deal with those, which probably don't need to involve chucking in the biggest pieces of kit that we can find.

 

Again and again I find the mention of flow regulators and system pipework sizing missing in hot water management discussion. For example, plenty of calculations about delivery of hot water at certain temperatures at certain flow rates, e.g. 30lpm, bit this misses that a lot, if not all drench showers, for example, provide about 16lpm. But it's very easy to buy models that have interchangeable flow regulators that massively enhance perceived availability of hot water but actually massively reduce hot water use. I too have teenagers who disappear into the shower with their bluetooth speakers, put on a rave and appear what seems to be like a few days later. Not once have I ever run out of hot water or had a complaint by someone we haven't got sufficient hot water. All we've got is 19kW going into a 210l thermal store. 

 

Edited October 4 by SimonD

[Nickfromwales]

1 hour ago, SimonD said:

I have to say I think this is just one of those very weird threads we get here on Buildhub where sense seems to be thrown out of the window. If it were a heating question, everyone would be posting to say you have to start off with a heat loss calc. Or the very least some design drawings to provide a starting point, but the basic questions about DHW design are completely missing on this one.

 

We don't know anything about pressure/flow rates to the house - regardless of size of main, what are typical static and dynamic pressures where the op is planning to build. Second is whether the requirements for infinite hot water is even really what the op wants or actually needs (I've been here too and the questions were resolved through careful whole system design taking into account flow volume restrictions we have got until I can dig a trench and re-lay our mains supply to the road at least). I realised I didn't need infinite hot water.

 

The assumptions we often make about both hot water and heating lead to over-sizing, which I think is what is going on here. e.g. 

 

So first we need to ask more fundamental questions about use and behaviour (and also how transient that behaviour might be and not make long term costly decisions about a temporary, short term problem). And then look at the variety of ways to deal with those, which probably don't need to involve chucking in the biggest pieces of kit that we can find.

 

Again and again I find the mention of flow regulators and system pipework sizing missing in hot water management discussion. For example, plenty of calculations about delivery of hot water at certain temperatures at certain flow rates, e.g. 30lpm, bit this misses that a lot, if not all drench showers, for example, provide about 16lpm. But it's very easy to buy models that have interchangeable flow regulators that massively enhance perceived availability of hot water but actually massively reduce hot water use. I too have teenagers who disappear into the shower with their bluetooth speakers, put on a rave and appear what seems to be like a few days later. Not once have I ever run out of hot water or had a complaint by someone we haven't got sufficient hot water. All we've got is 19kW going into a 210l thermal store. 

 

Agreed. But easy(er) to say if you’re someone using a TS as a hot water heater exchanger?

 

The pic I shared used one, and they never ‘ran out’ of DHW, so case in point.

 

They had more chance of running out of dynamic flow, than DHW, so I directed a TS capable of still outputting space heating whilst the boiler went full wallop doing max demand DHW without either being compromised.

 

I put a lot of thought into that, and chose a quality gas boiler capable of <4kw when Lilly modulating, or a huge >37kw of heat energy output whenever it was necessary to ‘recharge’ the TS.

 

I don’t think it was grotesquely oversized personally, and the client and his family were fully consulted before any prices were arrived at and kit purchased, with costs know, and expectations understood and met (actually exceeded).

 

Some good words about perceived flow rates and what shower flow rate would (should) be acceptable, and that can bring a lot of the capacity related issues back down to earth.

 

I had that chat and was told to ‘go large’, with the client then making their own fully informed choices.

 

If they’d have insisted on a HP, I’d have walked away, or I’d have had a contract loaded with caveats to protect me for when disappointment kicked in.

 

Sense prevails if the persons involved  are sensible, imo.

[Indy]

Ok, so let me restate the problem statement - which is very much in the vein of how do I make sure we never run out of DHW rather than unlimited hot water.

 

Measured the flow rate of the tap installed for the builders using a weir cup (Arctic Hayes 888000 U Flow Water Gauge for Water Flow Measurement : Amazon.co.uk: DIY & Tools)  and it easily saturated the 22lpm mark - so that means we should have excellent incoming water pressure (unless there's another way to check). 

 

This is the setup of what we're going to have:

• 5kW Solar PV array + 10kWH of battery
• Gas Boiler (plus gas fire and gas hob, but that doesn't matter too much)
• UFH on both floors - which will be designed to run at lower temps for a future swap out to ASHP (say in 5 years or so)
• Assuming unvented cylinder - 400l 
• 3 full bathrooms with showers + 1 guest WC

 

Talking about the issue with the missus, she thinks I'm overthinking and overengineering the solution for a problem that will occur at max once a month - when we're all in a rush to get ready. Most days, the kids shower in the evening and both of us do it in the morning and even then it's staggered.

 

ChatGPT presented us with a solution which is very much in the vein of talk to a plumber that designs a priority DHW solution. I like the idea of having something like an immersion heater in the cylinder which will allow the hot water to regenerate in 10-15m, rather than waiting an hour for the cylinder to replenish. 

 

 

[SteamyTea]

3 hours ago, Indy said:

I like the idea of having something like an immersion heater in the cylinder which will allow the hot water to regenerate in 10-15m, rather than waiting an hour for the cylinder to replenish. 

You can get cylinders modified from the usual standard setup.

Maybe 2, 3 kW elements in the top third, that would really heat up the water fast.

Then 1, 3 kW in the base to take advantage of excess PV generation i.e after batteries are charged. Can also be mains powered.

[John Carroll]

Thats quite interesting when you think about it, but it depends on where the coil is positioned in the cylinder, see below.

 

My own 150L vented cylinder has 3 very tightly wound microbore solar coils connected in parallel but only take up about 23L ("height") in the bottom of the cylinder, from 11L to 33L from the cyl bottom the 0,65m2 coil has a total "height"  of 55L, the bottom coil connection is 44L from the cyl bottom and the top coil connection is 99L from the cyl bottom,  there is also a top mounted electric immersion element that heats ~ 28L, three PT1000 probes, 38L, 73L & 123l from the cyl bottom, I've obviously often observed three different cyl temperatures but its almost uncanny how stratification works, if say the bottom solar probe is 20C, the middle probe is 40C and the top probe is 60C when the sun comes up, coil& immersion both off, then the bottom solar probe will start rising but the middle probe will stay exactly at 40C (& the top probe at 60C) until the bottom probe reaches 40C, then the middle&bottom probes will both start rising exactly together until thety both reach 60C, then the middle, bottom & top probes will rise at exactly the same temperature (to within 1C) until  the whole cylinder reaches ~ 80C, I have a TMV on the HW outlet.

 

So, back to reality, if say, the whole cylinder is at 55C and the boiler cuts in to heat the 106L then no matter how cold the coil  might be initially, there will still be almost 50L of water available at exactly 55C, by which time, even if having a shower which with a oil or gas fired boiler is literally a few minutes the coil will be up to its normal temperature and the bottom 100L will probably only have cooled by ~ 5C or so to 50C, no big deal?. 

 

Mixergy, with their PHEX have I think two different secondary return stratigies, with gas firing with very high flow temperatures they return is at the top of the cylinder and thats why I suggest it might be worth looking at this for a "endless" HW supply  because even if the cylinder is totally exhausted, a say, 25kW system boiler will still give a DHW flowrate via the PHEX of 11.95LPM (continuously like a combi) at 40C from a cold mains of 10C.

Using a ASHP then to keep a reasonable COP the cylinder heating has to have a much lower dT while reheating so the return is back into the bottom of the cylinder, heating from bottom up.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

My HW Cylinder.pdf

[John Carroll]

13 hours ago, Indy said:

Ok, so let me restate the problem statement - which is very much in the vein of how do I make sure we never run out of DHW rather than unlimited hot water.

 

Measured the flow rate of the tap installed for the builders using a weir cup (Arctic Hayes 888000 U Flow Water Gauge for Water Flow Measurement : Amazon.co.uk: DIY & Tools)  and it easily saturated the 22lpm mark - so that means we should have excellent incoming water pressure (unless there's another way to check). 

 

This is the setup of what we're going to have:

• 5kW Solar PV array + 10kWH of battery
• Gas Boiler (plus gas fire and gas hob, but that doesn't matter too much)
• UFH on both floors - which will be designed to run at lower temps for a future swap out to ASHP (say in 5 years or so)
• Assuming unvented cylinder - 400l 
• 3 full bathrooms with showers + 1 guest WC

 

Talking about the issue with the missus, she thinks I'm overthinking and overengineering the solution for a problem that will occur at max once a month - when we're all in a rush to get ready. Most days, the kids shower in the evening and both of us do it in the morning and even then it's staggered.

 

ChatGPT presented us with a solution which is very much in the vein of talk to a plumber that designs a priority DHW solution. I like the idea of having something like an immersion heater in the cylinder which will allow the hot water to regenerate in 10-15m, rather than waiting an hour for the cylinder to replenish. 

 

 

 

Whatever system you go for, It might be prudent to consider installing a say 10kW electric shower which may not be too popular with the children but will still flow ~ 5LPM, not too far away from the showering experience of 6LPM I am now increasingly getting in Hotels on my frequent trips abroad with their Eco showers, purely in the interest of saving the planet of course. The stand alone shower would also be a good back up if/when the boiler breaks down even if you have heating elements in the cylinder.

[Nickfromwales]

1 hour ago, John Carroll said:

 

Whatever system you go for, It might be prudent to consider installing a say 10kW electric shower which may not be too popular with the children but will still flow ~ 5LPM, not too far away from the showering experience of 6LPM I am now increasingly getting in Hotels on my frequent trips abroad with their Eco showers, purely in the interest of saving the planet of course. The stand alone shower would also be a good back up if/when the boiler breaks down even if you have heating elements in the cylinder.

Electric showers are horrible, and why would you go to all the expense of installing a system with a high flow UVC, then top it up for very little money or free (ASHP or PV) and then get into a shower, using grid electric on demand, that is the same flow rate as an OAP peeing? 

 

Just “no”.

 

Flow rate and temp changes which go up and down like a fiddlers elbow, if you’re not the only one in residence, as these are affected massively if anyone else so much as gets close to opening a tap whilst you’re singing and swinging.

 

2x 3kw immersions, or more if a huge tank, is ample insurance.

 

Yuk 🤢

[JohnMo]

16 hours ago, Indy said:

ChatGPT presented us

I like using ChatGTP but you have to integrate if that is really the best solution, and at the end, ask if it's the simplest solution, most often it isn't. It's fine for having a conversation and bouncing ideas about, but easily goes of on tangents and can get numbers very wrong and therefore give a wrong solution. Test and test again. When you have a solution that looks ok. Post that solution in a new conversation and ask if it's the best solution to do xyz. You may get a completely different answer.

 

It needs to know lots of details, to get a good solution.

[John Carroll]

Maybe the OP should decide what is his/family's acceptable showering flowrate is, then decide if he requires 3 x showering simultaneously or, IMO, a more realistic 2, then, that will surely go a long way to determine the cylinder capacity , etc.

[SimonD]

On 04/10/2025 at 13:54, Nickfromwales said:

Agreed. But easy(er) to say if you’re someone using a TS as a hot water heater exchanger?

 

Yeah, that's pretty much how I'm using it, but with the difference that I've lowered my storage temp so that cylinder temp at the sensor level is at 60C with a 4 degree hysterisis, and this gives me an indicated temperature of 70-75C 1 third the way down from the top of the cylinder. When I tried to have the water stored like a proper TS, the boiler would just run for far too long on reheat.

 

I'll be able to tell you how this compares to a heatpump install with UVC sized to MCS specs with a storage temp of 50C very shortly as I'm just installing our heatpump at the moment. So we'll go from 19kW input into the TS to a max 6.5kW input to the heatpump cylinder. I've already warned my wife that it might be a bit different going forwards and we'll have to experiment with the DHW strategy...

 

[Indy]

14 hours ago, John Carroll said:

Maybe the OP should decide what is his/family's acceptable showering flowrate is, then decide if he requires 3 x showering simultaneously or, IMO, a more realistic 2, then, that will surely go a long way to determine the cylinder capacity , etc.

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). 

[Nickfromwales]

2 hours ago, SimonD said:

I've already warned my wife

Liar. 
 

She has warned you  

 

🤣

[Nickfromwales]

2 hours ago, 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). 

Unlimited hot water you said! 
 

Are you beginning to get cold feet?

 

At least your head will be hot, with all that continuous hot water hitting it from the shower head   🚿 🥵 

[JohnMo]

10 hours ago, SimonD said:

storage temp of 50C

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.

[SimonD]

8 hours ago, Nickfromwales said:

Liar. 
 

She has warned you  

 

🤣

Hehe, well, the TS is staying in the shed for a while, just in case. And if necessary I'll have to put Octopus intelligent go to full use.. 🙂