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 Wednesday at 19:57 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 Wednesday at 22:16 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.

[-rick-]

Something to bear in mind. A lot of discussion on this forum related to heating is based around trying to build the most efficient systems an for most situations that is to run heating systems "low and slow", ie, at a low water temperature continuously, rather than the traditional UK approach of only putting the heating on for relatively short periods (morning and evening) and letting the house cool down at other times. If you have a very uninsulated house then the low and slow approach will possibly consume more energy but for any house with reasonable insulation/airtightness (nothing cutting edge) then the low and slow approach will be much more efficient and comfortable* but it feels 'wrong' to many people who think it will cost a lot more to have the heating on continuously. A benefit of designing any system for the low and slow approach is that this is the best approach for heatpumps and a swap to a heatpump at a future point should be fairly simple.

 

For a low and slow approach, even a big house will likely be fine with a small boiler, but if you are really attached to the traditional burst heating approach where you put out a lot of heat in a short period of time more might be needed.

 

As you are doing a refurb, if you haven't put much effort into the insulation/air-tightness yet, now is the time to do it.

 

* It results in the structure of the building warming up which can reduce dampness/humidity issues (though ventilation required as always), avoids big temperature gradients (feet colder than head), etc.

[Rick734]

9 hours ago, marshian said:

 

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 

Thanks Marshian - will get my head around weather compensated but PDHW definitely sounds like the way to go here.  Hopefully the gaff won't be too leaky and definitely isn't huge so as you say will have to get one that modulates down to house heat loss. 

 

Have run heat loss calcs based on Stelrad's online tool which suggests 75deg flow temps as standard...

[Rick734]

9 hours ago, JohnMo said:

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.

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.

Thanks JohnMo super helpful, so just so I've understood your point about the DHW requirement correctly...my rad kW requirement x 24 then /22 should give me the DHW element of the boiler size? So as below...would you say that's correct?

 

[Rick734]

3 hours ago, -rick- said:

Something to bear in mind. A lot of discussion on this forum related to heating is based around trying to build the most efficient systems an for most situations that is to run heating systems "low and slow", ie, at a low water temperature continuously, rather than the traditional UK approach of only putting the heating on for relatively short periods (morning and evening) and letting the house cool down at other times. If you have a very uninsulated house then the low and slow approach will possibly consume more energy but for any house with reasonable insulation/airtightness (nothing cutting edge) then the low and slow approach will be much more efficient and comfortable* but it feels 'wrong' to many people who think it will cost a lot more to have the heating on continuously. A benefit of designing any system for the low and slow approach is that this is the best approach for heatpumps and a swap to a heatpump at a future point should be fairly simple.

 

For a low and slow approach, even a big house will likely be fine with a small boiler, but if you are really attached to the traditional burst heating approach where you put out a lot of heat in a short period of time more might be needed.

 

As you are doing a refurb, if you haven't put much effort into the insulation/air-tightness yet, now is the time to do it.

 

* It results in the structure of the building warming up which can reduce dampness/humidity issues (though ventilation required as always), avoids big temperature gradients (feet colder than head), etc.

Thanks my fellow Rick

 

Makes sense. I guess I am somewhere in between accepting the logic of the low and slow approach, while also liking the ability to warm up the house quickly if desired!

 

Regarding insulation/air-tightness, I have been: renewing some of the 200-300mm loft insulation, replacing all windows & doors with new double glazing, adding expanding foam every gap I can find 😅, dry-lining walls with DPM and 50mm PIR where I am converting single skin areas into living space. Any other suggestions you would add? Thanks!

[marshian]

5 minutes ago, Rick734 said:

Thanks Marshian - will get my head around weather compensated but PDHW definitely sounds like the way to go here.  Hopefully the gaff won't be too leaky and definitely isn't huge so as you say will have to get one that modulates down to house heat loss. 

 

Have run heat loss calcs based on Stelrad's online tool which suggests 75deg flow temps as standard...

 

Heat loss calcs

 

A great video by Urban Plumbers covering how to do a basic heat loss calc - worth watching

 

There are other tools on the net like Heat Punk who offer online modelling of heat losses

 

@Jeremy Harris Tool on here is often highly praised but I've never used it

 

1 minute ago, Rick734 said:

Thanks JohnMo super helpful, so just so I've understood your point about the DHW requirement correctly...my rad kW requirement x 24 then /22 should give me the DHW element of the boiler size? So as below...would you say that's correct?

 

 

All fine until the end - based on the above you need a 16kW boiler max

[marshian]

Oh and I do think those calcs are a little on the strong size

 

Based on Heat Geek Guidelines 

 

 

Quick and Dirty from your earlier calcs Says for Renovated property with CWI and over 75mm of LI - best case 40 W/m2 worst case 65 W/m2

 

So Your worst case would be 11.5 kW best case 7.5 kW (not including HW Demand)

 

I have an early 80's 4 Bed detached house which is a stupid T shape so lots of rooms with 2 or more external walls and my overall W/m2 is 35

 

Improvements post construction are CWI, 300 mm of LI, SUDG Windows and Doors, 52 m2 of 75mm PIR under the suspended ground floor - not much more I can do to it now.

[-rick-]

52 minutes ago, Rick734 said:

Have run heat loss calcs based on Stelrad's online tool which suggests 75deg flow temps as standard...

 

75oC is wrong for a new system. A lot of existing systems run like that which I guess is why they use it (though when I just tried it it defaulted to 70). Either way if you are designing for efficiency, the lower the better. IIRC @marshian's system runs at something like 35. This does mean you need to specify bigger radiators.

 

47 minutes ago, Rick734 said:

Thanks JohnMo super helpful, so just so I've understood your point about the DHW requirement correctly...my rad kW requirement x 24 then /22 should give me the DHW element of the boiler size? So as below...would you say that's correct?

 

 

I think this sounds a little high, checking out their tool it doesn't ask how much insulation you have or how good the double glazing is for example. So they are going to assume for an average property, which as you are in the process of upgrading yours probably won't be. New double glazing is a lot better than 20 year old. A single skin wall with 50mm PIR on top better than an uninstulated cavity wall and likely better than a lot of older insulated cavity walls. I think it's worth trying out some other calculators to see how things line up.

 

Jeremy's one mentioned above is here but it's less user friendly than others:

 

42 minutes ago, Rick734 said:

Regarding insulation/air-tightness, I have been: renewing some of the 200-300mm loft insulation, replacing all windows & doors with new double glazing, adding expanding foam every gap I can find 😅, dry-lining walls with DPM and 50mm PIR where I am converting single skin areas into living space. Any other suggestions you would add? Thanks!

 

Others here are better to ask as I've been more focussed on new build than renovation but you didn't mention anything about the floor. If you have a wooden ground floor which is ventilated then insulating that and especially putting in an air barrier to stop the air from the ventilated space exchanging with your warm air from the room is something to look at. To be clear, you don't want to block the air bricks, you want to stop the air moving from below the floorboards to above. You mention adding insulation to single skin walls, but not to other external walls, there might be scope to beef up insulation in other areas (though sounds like you might be a bit past that point now).

 

Edit to add: if you have fireplaces/chimneys thats an area to think about. If you are maintaining the use of a fire then unless you are using a completely sealed unit you will need to maintain airflow into the room to provide air for the fire (which is a problem if you want to air seal), and if you are air sealing you also need to think about maintaining enough ventilation to stop problems with humidity/stale air.

Edited 11 hours ago by -rick-

[JohnMo]

50 minutes ago, Rick734 said:

DHW requirement correctly...my rad kW requirement x 24 then /22 should give me the DHW element of the boiler size?

Not quite correct. The calculation gives you your boiler size. If you need 13.7 for central heating - 13.7 x 24 is 329kWh for your daily heating.  To complete the central heating and allow 2 hrs for DHW, you divide 329 by 22, that is 15kW.  This is your boiler size.required for central heating and DHW. 

 

The above assumes you have done your heat loss calculation correctly.  Using the Jeremy spreadsheet is good if close to passive, but can be quite a way off if you are not. I would use the info posted by @marshian to get a best guess.

 

Your calculation doesn't seem to include much in the calculation, cannot see floor or wall or roof U values or ventilation losses, hence saying using the approx information above by @marshian.

[marshian]

Link to Urban Plumbers Heat Loss Calculation Video on YT

 

 

Watch it - you have most of the dimensions you need and for building fabric there are plenty of resources on the net for W/m2 

 

I started to build my own spreadsheet years ago which was actually fairly accurate on the rooms I did but then I found I could pay £12 for a single use of Heat Engineer software and plug all the info in there and get a very accurate result (Provided you understood the inputs) however they now charge £100 for single use so I can't recommend them anymore.

 

In a nutshell it's just Math but be very careful with ACH (Air Changes per hour) many on line tools massively over egg this element and as a result heating systems are over sized.

[Rick734]

Good evening gents, many thanks for your advice - afraid I haven't had a chance to go through it properly yet today but just wanted to acknowledge your assistance and links/details provided - this is what BH is about! 😎  Will go through and follow-up hopefully tomorrow if I have time.

[marshian]

7 hours ago, -rick- said:

New double glazing is a lot better than 20 year old.

 

Interesting point - maybe there is something I can improve as our current SUDG windows and doors were fitted in 1997 so are coming up for 28 years old................Question is how much better because there are no drafts from around the closures (the window seals are good) they haven't gone yellow like some cheap windows do.

 

I'm thinking its going to be mainly thermal improvements in the glass and gas between the panes and they aren't big windows (circa 1.7m2 in total apperture area for each one)

 

I'm guessing the payback for that level of capital investment would be a bloody long time

[-rick-]

12 minutes ago, marshian said:

Question is how much better

 

Indeed. Depends a lot on what you have. Just to note, I'm not particularly knowledgeable about this so just repeating things I've seen written here by others.

 

Common improvements over last 30 years:

1. Low E glass coatings etc

2. Argon filling vs plain air

3. Larger gap between panes

4. Better insulated spacer bars

5. Better frames (especially if you you look at the older metal frames but UPVC too)

 

and probably more

 

12 minutes ago, marshian said:

I'm thinking its going to be mainly thermal improvements in the glass and gas between the panes and they aren't big windows (circa 1.7m2 in total apperture area for each one)

 

I'm guessing the payback for that level of capital investment would be a bloody long time

 

Suspect so, especially if they are small windows and seem well sealed.