Unlimited hot water with 4 bathrooms - is it possible?

[Indy]

A lot of discussion since I last saw but I'll address some of the points.

 

1. Gas fire - was a compromise as I wanted a log burner and the missus wasn't on board with me faffing around finding wood, storing wood outside in the garden and then needing to top up the fire regularly. It's not meant as a heating source, purely ornamental as I like a real fire (rather than electric fire) and will be used all of 6-8 times a year. But the heart wants what it wants. 

 

2. Gas hob vs induction - no argument here. I want gas as I like cooking on gas. Induction may be quicker, sleeker but I've never really liked it when we've stayed over in holiday cottages or at my brother's place. He plans to rip the induction hob out when he does the kitchen (inherited a high end Miele which is pretty swish, but no comparison to a gas hob).

 

3. ASHP vs Gas boiler - all this discussion above really underlines the point why I moved away from ASHP. You need to know an awful lot and understand flow rates, and DHW, and weather compensation, and what temp to heat at, and take the risk of legionella into account. On top of that, the rates we've been quoted for ASHP install are stupid and I'm sure I can find someone to do it at a reasonable price. But life is short, I like convenience, and the gas option comes in at a reasonable price that my builder understands and can supply. With the million other decisions I need to make, spending weeks researching and setting up the perfect ASHP system is something I can do without. 

 

 

[dpmiller]

I'll throw a curveball. 9kw ASHP plus thermal store, thermistor probe halfway down the tank...

 

[-rick-]

26 minutes ago, marshian said:

I think you have a miss-understanding of DHWP....... I think this is partly due to the word "priority" in the description

 

Yes, after John's last post I realised that I wasn't thinking/communicating clearly about this. I'm aware how it works but clearly got thrown off track.

 

I think my point stands that if you set your heat source gas or ashp to run during the shower time then it will extend the amount of hot water available from a tank, priority doesn't even come in to this. The caveat is that it needs to be on and near temp before you start and thinking harder I have experienced the cold slug from the heating system coming on mid shower occasionally in the past so I should have been less surprised by John's experience.

Edited September 30 by -rick-

[Nickfromwales]

5 minutes ago, dpmiller said:

I'll throw a curveball. 9kw ASHP plus thermal store, thermistor probe halfway down the tank...

 

Would still struggle methinks.   
 

It may ‘work’ but it won’t do as the op asks. 

[Nickfromwales]

32 minutes ago, Indy said:

A lot of discussion since I last saw but I'll address some of the points.

 

1. Gas fire - was a compromise as I wanted a log burner and the missus wasn't on board with me faffing around finding wood, storing wood outside in the garden and then needing to top up the fire regularly. It's not meant as a heating source, purely ornamental as I like a real fire (rather than electric fire) and will be used all of 6-8 times a year. But the heart wants what it wants. 

 

2. Gas hob vs induction - no argument here. I want gas as I like cooking on gas. Induction may be quicker, sleeker but I've never really liked it when we've stayed over in holiday cottages or at my brother's place. He plans to rip the induction hob out when he does the kitchen (inherited a high end Miele which is pretty swish, but no comparison to a gas hob).

 

3. ASHP vs Gas boiler - all this discussion above really underlines the point why I moved away from ASHP. You need to know an awful lot and understand flow rates, and DHW, and weather compensation, and what temp to heat at, and take the risk of legionella into account. On top of that, the rates we've been quoted for ASHP install are stupid and I'm sure I can find someone to do it at a reasonable price. But life is short, I like convenience, and the gas option comes in at a reasonable price that my builder understands and can supply. With the million other decisions I need to make, spending weeks researching and setting up the perfect ASHP system is something I can do without. 

 

 

I did as you ask in a house in Oxford, way back when first dabbling in the new works of building “not shit” houses; this was my first sample of an MBC PH dwelling.

 

Client remit was to be able to have 4 reasonable showers at the same time, without notable dips in performance.

 

Did it, and client was happy.

 

 

Left is a 300L cold mains accumulator, and right is a 500L thermal store with a 28mm cold in / hot out instantaneous DHW coil (high flow / efficiency by design, courtesy of Telford bespoke offerings).

Behind there is the utility room when I hid a 37kw Vaillant 637 system boiler, so when the TS called for heat it got it.

 

This system literally can provide continuous DHW, but the caveat is that the accumulator will eventually deplete. I suggested a 500L acc’r but client said they’d see how the 300L one performed; no issues afaik. 

[marshian]

1 hour ago, -rick- said:

 

Yes, after John's last post I realised that I wasn't thinking/communicating clearly about this. I'm aware how it works but clearly got thrown off track.

 

I think my point stands that if you set your heat source gas or ashp to run during the shower time then it will extend the amount of hot water available from a tank, priority doesn't even come in to this. The caveat is that it needs to be on and near temp before you start and thinking harder I have experienced the cold slug from the heating system coming on mid shower occasionally in the past so I should have been less surprised by John's experience.

 

It's OK - don't worry - mistakes I've made a plenty

 

You know the nice thing about making a mistake is you learn...................

 

Observation skills are paramount - math is everything

 

Oh and to keep @SteamyTea away from the conversation don't mention thermal mass and above all don't get units wrong

-

PS if this seems like a dig at ST it's not - we all need the voice on our shoulder pointing out the error

[marshian]

1 hour ago, Indy said:

A lot of discussion since I last saw but I'll address some of the points.

 

1. Gas fire - was a compromise as I wanted a log burner and the missus wasn't on board with me faffing around finding wood, storing wood outside in the garden and then needing to top up the fire regularly. It's not meant as a heating source, purely ornamental as I like a real fire (rather than electric fire) and will be used all of 6-8 times a year. But the heart wants what it wants. 

 

2. Gas hob vs induction - no argument here. I want gas as I like cooking on gas. Induction may be quicker, sleeker but I've never really liked it when we've stayed over in holiday cottages or at my brother's place. He plans to rip the induction hob out when he does the kitchen (inherited a high end Miele which is pretty swish, but no comparison to a gas hob).

 

3. ASHP vs Gas boiler - all this discussion above really underlines the point why I moved away from ASHP. You need to know an awful lot and understand flow rates, and DHW, and weather compensation, and what temp to heat at, and take the risk of legionella into account. On top of that, the rates we've been quoted for ASHP install are stupid and I'm sure I can find someone to do it at a reasonable price. But life is short, I like convenience, and the gas option comes in at a reasonable price that my builder understands and can supply. With the million other decisions I need to make, spending weeks researching and setting up the perfect ASHP system is something I can do without. 

 

 

 

I'm A OK with all that..............

 

Doesn't change the need to use math.................

[John Carroll]

Just did a few calcs on this elusive unlimited HW.

Assumed, 300L cylinder heated from 7C to 60C by using a PHEX combined with a 20kW boiler with a PHEX secondary flowrate of 5.41LPM discgarging into the top of the HW cylinder at 60C and absorbing the full 20kW boiler output as soon as there is a HW demand, my calcs show that the 300L contents will be exhausted after 22.6 minutes at a HW demand of 30LPM at 40C, say 2 showers which will require 18.68LPM@60C mixed with 11.32LPM@7C to give 30LPM@40C, this gives a total vol of 678L at 40C vs 482L if the cylinder alone was not augmented with the boiler firing at full output, the other advantage with the PHEX (maybe) is that once the cylinder is exhausted then the boiler's 20kW will still provide 8.7LPM@40C continuously but will only feed one shower then at that reduced flowrate,  enough for some if happy with that  flowrate. I don't know how this compares with a conventional coil and again assumes the boiler fires up immediately on HW drawoff, will the total time still be 22.6 minutes before exhaustion and how long is the waiting period to achieve a comparable flowrate at 40C as the PHEX?.

[Oz07]

I watched the urban plumbers youtube video where he made the storage combi. In the comments someone mentioned a couple of high flow combi boilers there was the worcester bosch 50kw which can do something like 22L per min and a Navien 54kw one which also has a high flow rate. Does a boiler like this tick the boxes @Indy? you'd be getting 2 showers at once with minimal complexity

[JohnMo]

3 minutes ago, Oz07 said:

I watched the urban plumbers youtube video where he made the storage combi. In the comments someone mentioned a couple of high flow combi boilers there was the worcester bosch 50kw which can do something like 22L per min and a Navien 54kw one which also has a high flow rate. Does a boiler like this tick the boxes @Indy? you'd be getting 2 showers at once with minimal complexity

Some reading 

I did a similar setup - you don't need huge boilers - could flow enough to run 3 showers on the edge of stability, 2 with ease.

Canetis-SuperFlow-Product-Sheet-WE-050318.pdfCombi-SuperFlow-White-Paper-v1-2-4.pdf

Setup is sold by Alpha Boilers

[Oz07]

6 minutes ago, JohnMo said:

Some reading 

I did a similar setup - you don't need huge boilers - could flow enough to run 3 showers on the edge of stability, 2 with ease.

Canetis-SuperFlow-Product-Sheet-WE-050318.pdf 322.43 kB · 15 downloads Combi-SuperFlow-White-Paper-v1-2-4.pdf 960.61 kB · 11 downloads

Setup is sold by Alpha Boilers

Yeh the urban plumber guy used a 30 ish kw with cylinder, but the person in the comments implied that these bigger combis can just do everything out of the box. I think the Navien one had some kind of hot return loop to fit. They're not even that expensive so is it just better to go with one of these out of the box and job done?

[JohnMo]

1 hour ago, Oz07 said:

bigger combis

Most combi boilers are already huge outputs, great for DHW output but utter rubbish for CH especially in a new build. You have to balance the minimum output and max output. A boiler kicking out huge DHW flows is great, but if it spends the next 22 hrs short cycling that isn't so good. 

[Oz07]

39 minutes ago, JohnMo said:

Most combi boilers are already huge outputs, great for DHW output but utter rubbish for CH especially in a new build. You have to balance the minimum output and max output. A boiler kicking out huge DHW flows is great, but if it spends the next 22 hrs short cycling that isn't so good. 

Yeh I think the ratio these could go down to was advertised as quite good but im not really clued up. 15:1 from memory

[JohnMo]

1 hour ago, Oz07 said:

Navien

Just looked at that boiler and it looks pretty good, goes down to 3.5kW and does full weather compensation. Seems to tick all the boxes. Add a small cylinder upstream of the cold water to keep temperature rise low and consistent all year. Should flow very well indeed.

[marshian]

OP ruled out ASHP on his new build - downside of fitting a bloody great big combi is when he does “have to go” to an ASHP he’s then got to find space for tank which was never allowed for in the build

 

makes no sense to me at all to go down a combi route 

[Nickfromwales]

11 minutes ago, marshian said:

OP ruled out ASHP on his new build - downside of fitting a bloody great big combi is when he does “have to go” to an ASHP he’s then got to find space for tank which was never allowed for in the build

 

makes no sense to me at all to go down a combi route 

How soon (actually) before the OP has to worry about gas disappearing.........altogether?

 

Prob not in my lifetime, and I'm 51. Or at least when it does I will be too old to give a shit.

[JohnMo]

Op has already spoken about huge cylinders, so obviously has space.

On 29/09/2025 at 12:16, Indy said:

Go for a much larger cylinder - 400l or even 500l

 

18 minutes ago, marshian said:

have to go” to an ASHP

That really is never likely to occur. As much as the right thing to do, is to ban gas or oil from any new house. You just don't need fossil fuels.

 

So don't disagree with you at all. But being pragmatic, you do a powerful combi with lots of turn down, or big storage and suitable boiler. The big storage allows a more modest boiler, turn down doesn't matter, as you do the storage as a thermal store as @Nickfromwales shows above, then you can draw the heat for the heating that without having to manage modulation.

[Nickfromwales]

2 minutes ago, JohnMo said:

Op has already spoken about huge cylinders, so obviously has space.

 

That really is never likely to occur. As much as the right thing to do, is to ban gas or oil from any new house. You just don't need fossil fuels.

 

So don't disagree with you at all. But being pragmatic, you do a powerful combi with lots of turn down, or big storage and suitable boiler. The big storage allows a more modest boiler, turn down doesn't matter, as you do the storage as a thermal store as @Nickfromwales shows above, then you can draw the heat for the heating that without having to manage modulation.

That was my idea for that setup. Not pretty on paper so to speak, but did what they guy asked me to.

 

Lower temp (it was a PH MBC job) heating was peeled from the TS on a lower set of tappings; the pipes running low behind the cardboard boxes. Telford made the cylinder entirely bespoke, and sprayed something like 100-120mm of foam (hence it doesn't have the usual posh dinner jacket), as it was a higher temp store in a PH, so wanted to reduce latent losses.

 

High level temp probe pocket for the boiler to kick in ASAP, not the greatest, but with the volume of water in the TS to use for some elasticity it didn't go crazy on short-cycling, so afaic a plenty good enough design / arrangement.

[-rick-]

25 minutes ago, Nickfromwales said:

How soon (actually) before the OP has to worry about gas disappearing.........altogether?

 

Prob not in my lifetime, and I'm 51. Or at least when it does I will be too old to give a shit.

 

I reckon you can be confident that mains gas is available for another 20 years or so. After that I'm less sure. Gas boilers will at some point get banned from sale with incentives for people to switch away. Overtime with less people using gas the economics of maintaining the network (at least outside industrial areas) become iffy and I don't think residential cooking is enough to keep it going.

 

I hope you will still be giving a shit at in your 70's, it's not that old any more

[Nickfromwales]

2 minutes ago, -rick- said:

I hope you will still be giving a shit at in your 70's, it's not that old any more

With my knees I'll be signing up at Dignitas at 65 😆

[-rick-]

1 minute ago, Nickfromwales said:

With my knees I'll be signing up at Dignitas at 65 😆

 

Get new ones!

[Nickfromwales]

Just now, -rick- said:

 

Get new ones!

Indeed, lol. That's if the NHS outlives the gas supply.......

[marshian]

18 minutes ago, JohnMo said:

Op has already spoken about huge cylinders, so obviously has space.

 

I feel you need to read the first post again

 

On 29/09/2025 at 12:16, Indy said:

Ok so title is a bit vague but the summary of our new build is that we're going to have:

 

My point which I will make again is that if the space is not made for a tank due to a hoofing great combi and no tank - it's going to be much harder at a later date to fit in a tank into a house that wasn't designed to have one.

 

18 minutes ago, JohnMo said:

That really is never likely to occur. As much as the right thing to do, is to ban gas or oil from any new house. You just don't need fossil fuels.

 

So don't disagree with you at all. But being pragmatic, you do a powerful combi with lots of turn down, or big storage and suitable boiler. The big storage allows a more modest boiler, turn down doesn't matter, as you do the storage as a thermal store as @Nickfromwales shows above, then you can draw the heat for the heating that without having to manage modulation.

Not disagreeing with you - more than one way to skin the proverbial - but it's a forum which shares different opinions - the discussion of pros and cons - will hopefully lead the OP to a decision that he has to live with preferably happily!!

 

As far as fossil fuels go (and I have recently replaced a gas boiler with a gas boiler) it might not be a ban on fossil fuel boilers it could as easily be the increased cost of fossil fuels as a result of ASHP becoming the norm - the costs of the gas grid would then have to be covered by the remaining gas customers............... This could result in the "economics" of gas driving further conversions to ASHP.......

 

JM2pW

[marshian]

42 minutes ago, Nickfromwales said:

How soon (actually) before the OP has to worry about gas disappearing.........altogether?

 

Prob not in my lifetime, and I'm 51. Or at least when it does I will be too old to give a shit.

 

I refer the honourable gentleman to my reply to @JohnMo I specifically did not say anything about a ban on gas or it running out

 

The economics of ASHP conversion are currently being bolstered by a £7500 BUS grant (Ignoring the grant harvesting impact) it wouldn't take a lot of movement in gas price or more importantly the standing charge  to further drive the move to ASHP's - especially when the BUS grant scheme ends and equipment cost falls

[Nickfromwales]

3 minutes ago, marshian said:

I specifically did not say anything about a ban on gas or it running out

You said when he 'has to go to ASHP', so I'm charging you with entrapment.