System boiler size dilemma...any advice?

[Rick734]

Evening good ladies and gents of BH,

 

Question on my new boiler if you would be so kind as to vent your opinions. Doing a full refurb on my 1960s 4/5-bed detached in Devon. Been getting quotes on a new system boiler and cylinder from a few local plumbers and have a bit of a dilemma on the kW of the boiler. So firstly a bit about the house - it does have cavity insulation (albeit probably a bit patchy, is evidently not too new), approx 2200sq ft GIFA, 1 main bathroom (separate bath and shower) plus 2 ensuites each with 1 shower, updated loft insulation (another ~200mm on top of the original), brand new double-glazing all round, radiators will be all-new as well. I need the capacity to, in worst-case scenario, have a house full of people in the middle of winter, with all 3 showers running at once. Cylinder will be 300L.

 

Have had 4 quotes from various firms - 3 guys have quoted for a 30kW, which to me seems reasonable given the size of house and what I've seen in other similar-sized houses (but only gut feel) - however the guy I was thinking of using has quoted on a 24kW. He is the Ideal servicing firm for the area, has multiple engineers and should definitely know what he's talking about, and assures me that he's done all the heat calcs and it's absolutely fine etc....however the fact that everyone else has quoted on a 30kW is leaving me uneasy, and (gut feel again) a 30kW just seems like a bit more appropriate for the size of house (Which obviously you good people can't really judge from afar).

 

What do you guys think is the best approach here, I'm practical on quite a few aspects but plumbing & heating is a bit of a grey area for me. I'm willing to challenge him again, but would like to get some independent opinions from the forum here to give me something to think about. Also not entirely sure what is the best way to do the calcs myself, if anyone can point me in the right direction...tried working it out earlier and it was recommending a 6.3kW or something...any help appreciated! 

 

Thanks,

Edited 13 hours ago by Rick734
Adding detail

[Rick734]

Edited to add a bit of detail.

[JohnMo]

28 minutes ago, Rick734 said:

30kW, which to me seems reasonable given the size of house

First you don't size a boiler based on house foot print or size, you size based on heat loss. Nearly every boiler installed in the UK is massively oversized.

 

So to size well you need to do heat loss calculation. Second at a more average heating day in the UK it's about 7 degs - so heat loss is approx half your design heating day -3(ISH). So you want a boiler that modulates down to pretty close to the output of the 7 Deg day, then it can just tick away adding only the heat lost.

 

29 minutes ago, Rick734 said:

need the capacity to, in worst-case scenario, have a house full of people in the middle of winter, with all 3 showers running at once.

 

So you are looking to size system based on a once a year eventually if ever. This can get difficult, expensive and maybe make system over kill so be careful what you ask for. Two showers at full performance is easy, three may push things at full flow depending on shower heads.

 

I would do your 300L but insist on a heat pump one with 3m² coil - this gives super fast reheats or you can reheat at a much lower flow rate for added efficiency, in a more normal time.

 

Boiler plumbing - Do not accept anything other than PDHW (priority domestic hot water), this allows one flow temp for cylinder heating and a different one for central heating. And you fully boiler power given to the cylinder. So S and Y plan is out. If your plumber doesn't understand priority domestic, strike from the list automatically.

 

 

[Rick734]

Thanks @JohnMo, super helpful. I did wonder about many boilers being oversized. Was going to give Stelrad site a try to do some heat loss calcs and see if I can figure it out myself. Do you know how much should be added to allow for the DHW rquirement?

 

Quote

I would do your 300L but insist on a heat pump one with 3m² coil - this gives super fast reheats or you can reheat at a much lower flow rate for added efficiency, in a more normal time.

Thanks, will definitely consider that one.

 

1 hour ago, JohnMo said:

Do not accept anything other than PDHW

Glad to see that, thanks, as I had come across it earlier and was wondering if it was worth a shout - will definitely request it! I'm redoing all pipework so no excuse not to do it.

 

Feel pretty thick on this subject - no real idea on what things to ask for e.g. if there's any particular specifics to ask for outside of what we've discussed here, to make it more efficient to run, more intelligent controls, Open Therm(?) etc. It will have a Nest or Hive control etc anyway and all rads will obviously be installed with thermostats etc...

[marshian]

41 minutes ago, Rick734 said:

Feel pretty thick on this subject - no real idea on what things to ask for e.g. if there's any particular specifics to ask for outside of what we've discussed here, to make it more efficient to run, more intelligent controls, Open Therm(?) etc. It will have a Nest or Hive control etc anyway and all rads will obviously be installed with thermostats etc...

 

Twice in a day...........

 

3rd party controls don't add efficiency they kill it - manufacturers advertise it as "Smart" but what it really does isn't

 

Size the boiler to one that modulates down to your house heat loss at whatever the worst OAT in winter for your area is.

 

If you can size it to modulate lower than worst case even better but  unless it's a huge leaky gaff you'll have to settle for worst case heat loss because in any half decent house with std improvements of CWI, current regs Loft insulation and double glazed windows and doors you are going to struggle to find a boiler that can go lower than worst case.

 

Size the rads to the meet individual room heat loss at sensible low flow temps 

 

Run Weather compensated with DHWP and balance the circuit and you won't need TRV's as anything other than limiters for solar gain 

Edited 11 hours ago by marshian

[JohnMo]

6 hours ago, Rick734 said:

Feel pretty thick on this subject

Don't, most plumbers are knot well.versed into efficient heating design, and it's their day job. Doing heating well isn't something done in the UK for various reasons.

 

Simple take away if it sounds complex it's often complexity for complexity sake. The most efficient systems are sized well, run low and slow, have limited outside influence.

6 hours ago, Rick734 said:

Do you know how much should be added to allow for the DHW rquirement

With a boiler or heat pump if you allow 2 hrs for DHW heating per day that will be ample in most cases. So if house needs 5kW to heat it at the design day, that is 5 x 24, so 120kWh. To heat house in 22 hrs you divide 120 by 22, so 5.5kW boiler size would be needed.