Sizing a boiler for UFH and greedy hot water guzzlers

[MrsDeS]

Morning! 

We've built a our new chalet-style bungalow of 190 square metres from SIPs.  Having finally decided on a gas boiler with UFH downstairs (about 110 square metres) and oversized rads upstairs, my next challenge is the boiler.  I want to make sure I get this right.  
 

Our heating demand should be quite low (SAP says 31kWh per square metre) and MVHR will recover some of it for us too.  We have two bathrooms with showers.  
 

Hot water is the issue.  We have two teenagers who can while away hours in the shower and I'm not much better!  We have a combi boiler in our current house and I appreciate that hot water is constant and readily available.  I cannot bear the idea of running out of hot water, so waiting to use my shower on occasion is far preferable to having the water run out on me in the middle of one.  
 

I understand that we should only need a small boiler for the heating to run efficiently and to have it kick into condensing mode as much as possible, but that if we did want to have two showers running and not have a tank, that we should oversize the boiler to accommodate a high flow rate (assuming we have that from the mains once the water is connected!). Is it okay to do that?  
 

I've also just read about using a smaller combi boiler with a small tank that could cope with feeding two showers and allow the tank to be topped up with a half decent flow rate that should prevent us running out of hot water.  Have I got that right?  Presumably that would help the heating run more efficiently, but how much of a concern should that be to me?  Is it going to keep the bills down massively or will the heat loss from the tank end up cancelling out any savings on the heating side? 
 

What size boiler/tank should we fit in each scenario? 
 

Thanks in advance!   

 

Edited May 28, 2020 by MrsDeS

[ToughButterCup]

7 minutes ago, MrsDeS said:

[...]

What size boiler/tank should we fit in each scenario? 
[...]

You'll get a technically correct answer from others on here @MrsDeS.

But the results of heating stuff up depend on how people perceive those results. On how real people feel in situations relevant to them.

I'm suggesting,  based on a trustworthy estimate, you over-specify by a bit.

 

How big's that 'bit' then Ian?   Ask another  plumber - one who hasn't specified or quoted for the system. No conflict of interest that way.

[daiking]

Is anyone able to run 2 showers direct from mains water even with a BFO combi boiler? (no tanks)

[PeterW]

Why the oversize rads if you’ve gone for a gas boiler ..? Standard size will be fine, blending for UFH temps (35c or so) should be done by the manifold. Don’t be tempted to run the rads off the UFH manifold ..! 
 

Oversizing is normally done when you want to use ASHP. 
 

In terms of your hot water, I would not bother with a combi as even a 35Kw will struggle with a pair of showers running - consider a 500 litre UVC, gives you the option to run it hard before the first showers but the tank will stay hot enough to keep you going through the day (with potentially a mid morning boost)

 

Other option is a thermal store as it will allow you to run everything off one tank with just a heat only boiler but that will put more heat into the building 24x7x365. Upside is that if you have Solar PV you can dump to it year round and not care about it being 85-90c as it is just more heat held. 

[Nickfromwales]

1 hour ago, daiking said:

Is anyone able to run 2 showers direct from mains water even with a BFO combi boiler? (no tanks)

Yes. As long as it’s a dedicated ‘high flow’ unit it’ll do the job. Most you can expect in reality is two “ok” showers simultaneously, or one excellent shower, but two is possible.

This is remembering that an instant hot water heater, of whatever fuel origin, is ‘cold mains dependant’, so if during the two ok showers someone flushed the loo, you’re going to know all about it. Same if any white appliances are set to run whilst you’re showering that would be a disaster. 
Discipline is number one in this scenario, so with unsympathetic teenagers in the mix I’d forget anything instantaneous here @MrsDeS.

Boiler + thermal store is what I’d fit here, or a bigger than necessary UVC ( unvented ( mains pressurised )) cylinder, but you’ll then suffer the longer term ownership issues when said water thieves fly the nest and it’s just you guys there. 
As you have UFH ( and why have you oversized the rads if you’re on gas not a heat pump ?!? ) I’d say fit the TS ( thermal store ) as it’ll give you a buffer for running the UFH and give you your condensing range, ( which you won’t get at very low temps btw ). Downside is you need to keep the TS hot all summer for DHW ( same as you would with an UVC, but typically hotter than ) so losses need to be considered and managed. Best way is to put it in the airing cupboard to warm your trollies all year round. 
Cold mains needs to be surveyed before deciding, as you can’t get a pint out of a half pint pot, and you’ll probably need to have all the cold mains in 22mm and pipe accordingly for your needs. 

[Nickfromwales]

Crossed with Peter. 

[Nickfromwales]

Another option could be a floor standing high flow combi , ( Worcester Bosch do a tidy one iirc, used in a lot of B&B’s / small guest houses ), and a 2-300L cold mains accumulator to fortify cold mains delivery, but you’d still need a small / medium buffer to compliment the gas vs UFH situation to get your condensing range reliable. 
The issue is the length of hot water draw off duration stated in the OP. No accumulator will defeat that problem, unless you’ve a room full of them.

[daiking]

2 minutes ago, Nickfromwales said:

Yes. As long as it’s a dedicated ‘high flow’ unit it’ll do the job. Most you can expect in reality is two “ok” showers simultaneously, or one excellent shower, but two is possible.

This is remembering that an instant hot water heater, of whatever fuel origin, is ‘cold mains dependant’, so if during the two ok showers someone flushed the loo, you’re going to know all about it. Same if any white appliances are set to run whilst you’re showering that would be a disaster. 
Discipline is number one in this scenario, so with unsympathetic teenagers in the mix I’d forget anything instantaneous here @MrsDeS.

 

This ^. What 'normal' mains water supply has enough flow for 2 or more outlets at once? The shower is below par (but still usable) when another tap is opened/toilet is flushed but you can't run 2 hot outlets at once, the flow is too low.

 

I will be making the upgrade the system to something with a more robust supply when the time comes to change it. 

[Nickfromwales]

3 minutes ago, daiking said:

 

This ^. What 'normal' mains water supply has enough flow for 2 or more outlets at once? The shower is below par (but still usable) when another tap is opened/toilet is flushed but you can't run 2 hot outlets at once, the flow is too low.

 

I will be making the upgrade the system to something with a more robust supply when the time comes to change it. 

Depends on whether it’s all being squeezed down one 15mm pipe which is T’eed off repeatedly, or through well thought out and dedicated radial supplies. The difference that can make to the same cold mains is significant. 
Most combis have 15mm inlet & outlet whereas an UVC / TS have 22mm so can convey much more water / have lower paths of resistance.

Designing a new system from scratch makes this possible, but solving a retro fit is a bloody nightmare as you’re constrained by the existing pipe work etc.  

[Dreadnaught]

More generally, and not being any form of expert, I found that having a (paid) M&E adviser in the planning stage of my build has proven to be invaluable.

 

A few hours of hourly-rate advice from an expert has impacted on so many areas, not only the choice of a heating system, but everything from penetrations through my concrete raft, to lots of other issues that had knock-on consequences elsewhere.

 

I had picked up a great deal of knowledge by avidly reading this site (thanks everyone!) but that adviser was able to assist in unpicking all of my half-thought-through ideas and string together a coherent concept.

 

I considered it money well spent and my architect complimented me on the approach.

 

 

[Nickfromwales]

11 minutes ago, daiking said:

I will be making the upgrade the system to something with a more robust supply when the time comes to change it. 

Beware fitting a bigger device, if your existing supply and pipe work won’t effect the desired results  

[daiking]

8 minutes ago, Nickfromwales said:

Beware fitting a bigger device, if your existing supply and pipe work won’t effect the desired results  

 

Hence upgrading the system. Its all bollocks as it stands.

[MrsDeS]

Thank you people. 
 

Radiators haven't been purchased.  I thought it would be better if they were running at a lower temperature as well, but I'm more than happy without massive radiators if I should just run them normally.  I'm hopeful we won't need them too much.  
 

Maybe we just go for a 'normal' sized combi and settle with the status quo.  It's been no big deal and if the oversizing will still only result in a fairly rubbish shower, there's little point. As pointed out, the water thieves will be leaving at some point. DS is already saving for a deposit on a house and DD is determined to go to Uni in London and is in love with it, so I suspect we won't see her for dust.   I'll end up following her ☺️

[Alexphd1]

First thing I would do is check your mains water pressure and flow rate then work from there. 

 

Personally I would be more looking at a small system boiler and a 300l unvented cylinder. If the boiler short cycles on ufh then add a small buffer. A UVC should last at least two gas boiler life spans. A system boiler is more reliable (less parts) than a combi boiler and you have always the electric immersion back up for hot water you could also add a immersion to buffer for electric back up. If you have PV it's easily to dump excess in hot water tank. 

 

Piped correctly and decent water pressure and flow rate will give you 2 good showers.

[Mike]

6 hours ago, MrsDeS said:

Maybe we just go for a 'normal' sized combi and settle with the status quo.  It's been no big deal and if the oversizing will still only result in a fairly rubbish shower, there's little point.

You could also consider changing the shower.

 

In particular check out the latest version of the Moen Nebia which uses atomising technology to cut water use to between 3.5 and 5 litres per minute (compared to between 8 and 15 litres / minute for a regular shower head). No direct experience yet, but on balance the reviews seem positive, so I'm likely to install one in my next build.