Unlimited hot water with 4 bathrooms - is it possible?

[Indy]

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

 

1. Gas Boiler - for various reasons, I don't want an ASHP (gas fire, gas hob and generally not liking the over inflated quotes we've had for ASHPs).

2. UFH on both floors - which probably means 2 manifolds

3. 3 full bathrooms + 1 guest cloak room.

 

The general recommendation is to go for a system boiler plus something like a Megaflo cylinder so we have pressure when more than one person is in the shower.

 

In our current house, we have a combi boiler which basically means that we have unlimited hot water when we turn the shower/taps on. Works quite well but does mean that no more than 1 person can really use the bathroom at one time - which happens very frequently. We have 2 young kids who love nothing more than standing under the shower for 15-20m or even more pretty much daily and this may prove to be quite limiting when it comes to having a cylinder of say 300l which is going to get emptied in 2 showers, and then we'd have to wait for the water to heat up again.

 

Patience is not my strong point so I'm looking for suggestions on what others have implemented that allow them to overcome this problem. From what I can see:

1. Go for a much larger cylinder - 400l or even 500l (prices do jump up quite significantly).

2. A separate combi boiler for the kids bathroom and the standard setup for everything else - seems like overkill and I'd like to keep it to one boiler if possible.

 

Any suggestions?

 

 

[JohnMo]

Simple way

With UFH you need low flow temperature, so weather compensation is ideal. To get weather compensation you need to priority domestic hot water (so X or W plan). Definitely do not go the normal S or Y plan.

 

To keep boiler happy and not short cycle you need to run zone fully open system. So switch your head off to multiple thermostats one will do.

 

Cylinder choose a heat pump cylinder - yes a heat pump cylinder. It will have a 3m² coil so hot water recovery is quick.

 

Boiler choose, Viessmann, Atag, Intergas. If you plumber looks blank at you when you ask for priority domestic hot water, get a different one, they won't do you any favours.

@marshian will be able to tell you facts about figures on the reduction in gas consumption going this route.

 

A other way to do hot water with a combi boiler is to have a pre heat cylinder. It moves away from the single shower situation and does DHW. Alfa boilers sell a stand alone cylinder, but Vaillant, Viessmann and others do storage combi boilers. This will give you never ending hot water and with an external sensor weather compensation - a simple solution.

 

The important bit is look at min turn down rate, you don't want a boiler kicking out 12kW for a house that needs 4 or 5kW or less.

 

2 boilers - no

[-rick-]

What @JohnMo said but also if you have a household with many people having long showers, may be well worth your time looking at whether a Waste Water Heat Recovery System might be worth it. They claim they can save up to about 50% of the energy used to heat shower water. Pricey but with your water usage might pay for itself quite quickly and much easier to install if planned in advance.

 

The solution I liked the best when I spent some time looking at this is: https://recoup.co.uk/products/pipe-hex-range/pipe-hex/

 

Don't have personal experience though.

Edited September 29 by -rick-

[SimonD]

You won't get unlimited hot water as such but you can have a decent DHW strategy that will satisfy you as a family.

 

Similar to what @JohnMo suggests, make sure you have a priority hot water installation. As I know Viessmann's and I assume your new build is pretty well insulated and not too massive, I reckon you could get away with an 11kW 100-W system boiler. Modulates from 11kW down to just over 1kW on central heating but will give you 19kW output on a 4 pipe priority hot water setup. The system with then start heating your hot water to your cylinder while showers/baths are being used so will give you much longer run times from your cylinder. This way you don't have to massively oversize your cylinder either.

 

Personally and professionally, I'd suggest staying well clear of any combi setup for a large family house with multiple bathrooms and high hot water demand.

 

 

[Temp]

Perhaps check that the cold supply to the house can deliver enough for 2+ showers at decent pressure.

 

Then what @SimonD said. Think you will need a decent sized tank. 

[marshian]

5 hours ago, Indy said:

In our current house, we have a combi boiler which basically means that we have unlimited hot water when we turn the shower/taps on. Works quite well but does mean that no more than 1 person can really use the bathroom at one time - which happens very frequently. We have 2 young kids who love nothing more than standing under the shower for 15-20m or even more pretty much daily and this may prove to be quite limiting when it comes to having a cylinder of say 300l which is going to get emptied in 2 showers, and then we'd have to wait for the water to heat up again.

 

I guess because the kids aren’t paying the bills they aren’t bothered about wasting energy and water

 

I’d be going one boiler and two tanks - a smaller one for the kids bathroom - when the shower goes cold they’ll soon learn to be less wasteful with energy and water 🙂 

 

On a more serious note one boiler - with a good turndown rate for the CH demands of the house but a decent output for water reheat times to be quick 

 

urban plumbers did a set up a while ago with a fairly small tank that could be re-charged almost as it was used - worked like a giant storage combi but not a combi was how he described it at the end.

[JohnMo]

14 hours ago, marshian said:

urban plumbers did a set up a while ago with a fairly small tank

Here you go 120L tank to support 2 showers 

 

 

[Indy]

18 hours ago, marshian said:

I guess because the kids aren’t paying the bills they aren’t bothered about wasting energy and water

 

I’d be going one boiler and two tanks - a smaller one for the kids bathroom - when the shower goes cold they’ll soon learn to be less wasteful with energy and water 🙂 

 

On a more serious note one boiler - with a good turndown rate for the CH demands of the house but a decent output for water reheat times to be quick 

 

urban plumbers did a set up a while ago with a fairly small tank that could be re-charged almost as it was used - worked like a giant storage combi but not a combi was how he described it at the end.

 

That may be but it's unlikely to change their behaviour any time soon  And to be fair, even though I threw them under the bus a bit, I'm the same and can take extended showers pretty regularly so the answer is clearly that we need a solution to meet our demand/behaviour. I realise it's not the most efficient way of doing things but happy to pay a (small) premium to make the system work for me rather than the other way around. 

[marshian]

5 minutes ago, Indy said:

 

That may be but it's unlikely to change their behaviour any time soon  And to be fair, even though I threw them under the bus a bit, I'm the same and can take extended showers pretty regularly so the answer is clearly that we need a solution to meet our demand/behaviour. I realise it's not the most efficient way of doing things but happy to pay a (small) premium to make the system work for me rather than the other way around. 

 

I was being a little tongue in check with my response  

 

I was however being serious about the two tank scenario - would mean the re-heat for each tank is much shorter and finding space for two smaller tanks is a heck of a lot easier than finding a space for one 400 or 500 Litre tank (which would then require an extended period to recharge) 

 

Heating one big tank quickly is going to need a much larger boiler than is needed for CH which could potentially compromise CH efficiency if the boiler is not chosen wisely with both HW and CH requirements in mind.

 

Caveat to that is if you go for something like a Viessmann or similar high modulation boiler then large doesn't matter when the whole range has a turndown to anything from 3.2 kWh to as little as 1.4 kWh depending on type/model of boiler.

 

You'd need a good gas engineer to work out the logic of managing two independent tank heating processes but I'm sure it's do-able

 

Watch the video linked by @JohnMo 120L tank heated by a 30 kW boiler in 10 mins that 120L tank at 60 deg C would be blended down to max 38 Deg C

 

so effectively incoming cold water 20 deg, HW 60 deg for every 10 litres of shower water used 4.5 litres would come from the HW tank and 5.5 litrs from the cold

 

As a result 120 Litre tank supplying the shower would be blended to 266 Litres of shower water - at 10 LPM at the shower head that's 26 mins of shower time without any re-heat.

 

Caveat - I'm not saying 120 Litres is the tank size you need - just that you need to do some maths around your needs and how best to meet the demands of the house - if it's a new build then consideration to where the tank/tank could be situated and the pipework feed and return as well as proximity to the boiler to minimise losses

 

I'd also want to know what the mains incoming flow is for the property because ultimately that's going to have the biggest impact on being able to run multiple showers at the same time (or any activity involving HW usage)

[Oz07]

Good answers here interesting thread. Without wanting to derail too much how do ashp users cope with this scenario? Can they heat water just as quick as boiler? Is it all just about having enough kw? Or do you need massive tanks with elec only system. 

[JohnMo]

31 minutes ago, Oz07 said:

Good answers here interesting thread. Without wanting to derail too much how do ashp users cope with this scenario? Can they referring heat water just as quick as boiler? Is it all just about having enough kw? Or do you need massive tanks with elec only system. 

Does anyone really need unlimited hot water, maybe not, most likely not. They may think they do. 15 to 20 mins in a high flow shower can easily eat through 100L of hot water if stored at 45 to 50 degs. The hotter you store the less hot water you get through. So 400L would allow 4 long showers (just about). I'm normally done all I need to in under 5 mins. Wife nearer 20 mins.

 

You can get a big cylinder, have waste water heat recovery, low flow shower heads. A combination of or all three. Heat it hotter is the other option. You can heat more than once per day.

 

Big cylinders take a while to heat up, smaller one take less time. It's just a matter of balance to suit your circumstances.

 

Not mentioned it yet, but as you brought up ASHP.

 

Assume OP is a new build 

Gas fire - thought those stopped being installed in the 80s, who needs one in a new build. No-one.

Cook by gas, just do LPG no gas standing charges. Or better still move to induction.

Gas boiler - in this day and age, you need an ASHP, especially with all UFH. Size it right, SCoP of 4.

 

Shop about, you can install a heat pump for the same, or close to the same cost as a decent gas boiler.

 

[-rick-]

One other thing to add is that if you have say a 7kw ashp in a priority hot water setup, even though it won't be powerful enough to provide continuous hot water, 7kw of heat input into a cylinder will at least slow down the rate of heat loss as you use water. So a 300L tank will go a lot further than the same tank without priority hot water.

 

In the event you have guests it's unlikely everyone wants a shower at once. They will naturally space out a bit as people talk, people fuss over the mirror in the bathroom to shave/apply makeup/use the toilet so that 7kw heat input goes a lot further than you might initially think.

[JohnMo]

16 minutes ago, -rick- said:

7kw of heat input into a cylinder will at least slow down the rate of heat loss as you use water

I thought that until I tried it. Scheduled DHW heating at same time as a shower. As soon as the heat pump kicked in to heat cylinder,  the hot water cylinder got churned up and the stratification went, so did the hot shower. That's heating via a coil not a PHE.

[-rick-]

11 minutes ago, JohnMo said:

I thought that until I tried it. Scheduled DHW heating at same time as a shower. As soon as the heat pump kicked in to heat cylinder,  the hot water cylinder got churned up and the stratification went, so did the hot shower. That's heating via a coil not a PHE.

 

Well that breaks my mental model of how this works and would seem to invalidate the whole concept of priority hot water (which loads on hear swear by). Have you come up with any explanation as to whats happening?

 

Edit to add: I have my boiler providing DHW heat when I shower as a matter of routine and have never noticed any problem with that.

Edited September 30 by -rick-

[marshian]

9 minutes ago, JohnMo said:

I thought that until I tried it. Scheduled DHW heating at same time as a shower. As soon as the heat pump kicked in to heat cylinder,  the hot water cylinder got churned up and the stratification went, so did the hot shower. That's heating via a coil not a PHE.

If I did that to my other half I’d be dead - HW re-charge is done well outside of any shower reqts

[-rick-]

Oh, maybe it's because @JohnMo has really long pipework to the ASHP so there is likely a lot of cooler water in the lines that initially warms from taking heat from the cylinder?

 

Edit: If so, could probably eliminate this effect with a bypass valve near the tank that bypassed the tank until the loop was up to temp.

Edited September 30 by -rick-

[Oz07]

@JohnMo you've not really answered my question, are you a politician by any chance?!

In all seriousness im beginning to come round to your ashp way of thinking but It seems to me you have to be quite clued up, almost an engineer to make sure its designed and set up right. 

Can they not recharge water as quick as a gas boiler then even if rated to same kW?

Does op want the gas to get away from all this boffin stuff @Indy?

[JohnMo]

3 minutes ago, -rick- said:

Oh, maybe it's because @JohnMo has really long pipework to the ASHP so there is likely a lot of cooler water in the lines that initially warms from taking heat from the cylinder?

 

Edit: If so, could probably eliminate this effect with a bypass valve near the tank that bypassed the tank until the loop was up to temp.

Maybe? But when you look at how the ASHP heats the cylinder it starts at the lowest temp it can, based on return temp. So if bottom of cylinder is 20 degs and top 45. Heat pump will flow at 25 (dT5) plus the temperature difference across the coil walls, so more like 30 degs, at the start of the heating cycle, then ramp up as return temp rises. So it's more dependant on the coil configuration, mine is a slimline cylinder and the coil runs from very bottom to nearly the top.

 

Have thought about the bypass valve arrangement, but think it would kill CoP. Return water would also drop the flow temp as soon as the bypass opened.

[-rick-]

3 minutes ago, JohnMo said:

But when you look at how the ASHP heats the cylinder it starts at the lowest temp it can, based on return temp. So if bottom of cylinder is 20 degs and top 45. 

 

In a priority hot water setup the bottom of the tank shouldn't get anywhere near that low should it? The temperature sensor is in the bottom third of the tank and should start the heating process as soon as the temp drops below the setpoint + hysterisis. At least that's how I thought it worked. My current system is s-plan but plan to switch to PDHW when the warranty runs out on the install and I get around to buying a new controller.

[JohnMo]

2 minutes ago, Oz07 said:

@JohnMo you've not really answered my question, are you a politician by any chance?!

In all seriousness im beginning to come round to your ashp way of thinking but It seems to me you have to be quite clued up, almost an engineer to make sure its designed and set up right. 

Can they not recharge water as quick as a gas boiler then even if rated to same kW?

Does op want the gas to get away from all this boffin stuff @Indy?

Difficult to answer really. But have attempted to above. No it's unlikely to happen your heat pump isn't going to be big enough to do unlimited hot water. Heat geek have their mini cylinders which will produce a steady flow of hot water enough to full a bath or a single shower on demand. They use a bypass valve to force ASHP to flow at a high temp before being added to cylinder, the cylinder is not an UVC it's more thermal store, small water content high coil size. But not sure I would risk it, I'm in the same boat as @marshian I would be carrying my head around the first time it didn't perform.

[JohnMo]

2 minutes ago, -rick- said:

a priority hot water setup the bottom of the tank shouldn't get anywhere near that low should it?

Mine went from 50 to 13 at the sensor, after the wife's shower this morning (15 to 20 mins) - high flow shower head doesn't help. I just set the times when no-one is likely to use hot water. Morning heat 5am and afternoon heat 2pm.

 

5 minutes ago, -rick- said:

At least that's how I thought it worked

The main advantage of pdhw is the two distinct running temperatures. I also tried to heat on demand, think it tried to heat cylinder 4 or 5 times in a day and used loads of energy. Reverted to timed slots the next day. A gas boiler depending settings can just ramp up to 70 to heat cylinder so you don't get the cooling effect of an ASHP. When I did hybrid DHW I set the boiler to ramp up slowly so it stayed in condensing mode as long as possible. Cylinder heating took 20 mins and boiler never went above 60 degs.

[-rick-]

Gotcha, well back to the main topic. If you wanted to make the tank water last as long as possible wouldn't turning on the heat pump before the shower to make sure it's running at temp get back to the situation as I originally suggested?

[JohnMo]

18 minutes ago, -rick- said:

Gotcha, well back to the main topic. If you wanted to make the tank water last as long as possible wouldn't turning on the heat pump before the shower to make sure it's running at temp get back to the situation as I originally suggested?

The most economical way to do hot water is to heat just before you need it, then reheat again only when you are likely to need it again. We are pretty consistent, wife showers in morning I do so at night. Hence my heating times, also both are in cheap rate periods.

 

You can heat once per day, but you have a trickle of heat being lost all day, you also need a bigger cylinder.  Heat just before you need it with a suitable sized cylinder, your daily standing losses drops as your cylinder spends most the time cooler.

[marshian]

1 minute ago, JohnMo said:

The most economical way to do hot water is to heat just before you need it, then reheat again only when you are likely to need it again. We are pretty consistent, wife showers in morning I do so at night. Hence my heating times, also both are in cheap rate periods.

 

You can heat once per day, but you have a trickle of heat being lost all day, you also need a bigger cylinder.  Heat just before you need it with a suitable sized cylinder, your daily standing losses drops as your cylinder spends most the time cooler.

 

Absolutely this ^ before I had DHWP and WC for CH - I was heating the water the previous evening because I was managing the flow temps for CH manually - it worked from an energy usage in CH but having a tank sitting there overnight meant standing losses were much larger and a 117 litres of water didn't manage 4 showers a day

 

Right now with DHWP - water is heated at 7 to 7.30 am based on a schedule for showers at 8am - typically recharged from mid 20's to low 50's - by 11pm same day tank temp no longer will support another two showers at a comfortable temp (up to 11pm it's fine) energy usage is typically 4 kWh to 5 kWh for a recharge

 

Like @JohnMo when I was starting out with DHWP I set the schedule to allow HW cyl re-charge whenever the tank stat triggered a re-heat. Absolute bloody waste of energy - it massively increased the standing losses and increased energy used for HW by 50% for no real benefit.

 

From memory it recharged 4 times in a day - each time taking the tank back up to 50 deg C - some of those losses were due to heating up the primary circuit and firing up the boiler but it didn't work for me. 

 

Most economical way for me is heat the water I need to the temp that covers the for the whole day in one hit but I'm sure if my HW usage was different and not fixed other ways may work better. 

 

 

[marshian]

2 hours ago, -rick- said:

 

Well that breaks my mental model of how this works and would seem to invalidate the whole concept of priority hot water (which loads on hear swear by). Have you come up with any explanation as to whats happening?

 

Edit to add: I have my boiler providing DHW heat when I shower as a matter of routine and have never noticed any problem with that.

 

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

 

DHWP is about maximising CH run times at low flow temps, accepting a loss of efficiency when doing HW - the priority is getting it done promptly - ie don't muck about trying to go low and slow - hit the HW with higher flow temps - stir the tank up with higher temp differentials and get it done

 

I'm definitely in the camp of not trying to re-heat a stratified HW tank when HW is being demanded (ie showering) that's a recipe for getting lynched by my partner.......

 

Lets say the evening shower time - tank was heated to 50 deg at 7.30am - we've had two showers and taken ~50 Litres out - tank is stratified into 48 deg C for top 50 litres and 20 (ish) deg for the remaining 60 Litres - I try to recharge the cyl and the first thing that happens is the first bit of heat that the coil gets starts a mixing process - top of the tank that was 48 deg drops rapidly to mid 30's 

 

OK Gas boiler not ASHP - maybe with ASHP the process would be much more gentle and less stiring the pot...............