Having looked at various replies to the question' How do I work out what size boiler I require?' it's noticeable that many take the form of 'I haven't trained as a heating engineer for 30 years to tell you how to do it for free! Get yourself a book or go to college.' What's wrong with doing a bit of research when you're going to spend £2000+. If they don't want to help, why don't they just mind their own business? Oh and is there anyone who has some pointers? I've tried a 'Whole house calculator' for my semi and it comes out at 7.7KWs, which seems low but then I'm not an expert !!!
So if I wanted to know about say Smart TVs and ask on a public forum about them, it's OK if TV engineers come one and say, get stuffed? Sorry, I don't get it. If they aren't interest in sharing their expertise, just don't reply. Is that so difficult?
That does seem very low. Might be worth a double check on your input figures and units (feet/metres etc?)
I can possibly understand the reaction from pros because there is no quick answer so some work is needed and there are an abundance of calculators online for DIY use.
I've done mine recently and got about 18 for a 4bed detached - mixed new and older construction. I had an old 18kW boiler in there and I knew it was struggling in the depths of winter so I've just put in a 24kW - time will tell. The advice I was given was not to go too big on a just in case basis otherwise the boiler does not run efficiently.
7.7Kw would be low. Around 20Kw would be more averaqge.
It is not a topic that can be explained in a couple of paragraphs. So you won't get an anwer here.. Assuming you have basic arithmetic/ science, at least four hours explanation to cover most aspects. So the correct answer IS to buy a book or go to college.
that sounds about right. I did one of those (had it done for me) and it came to 10KW, so in went a 12KW boiler.
NOW it is certainly true it can hold the temperature at 19C when its -5C outside, that's ALL it can do,. and heaven help me if the temp goes lower or the house needs further warming.
7.7KW is the steady state heat loss. You should aim at least 50% over that to ensure the boiler isn't running flat out in winter all the time, and possibly not at its most efficient if its a modulating boiler. And you have fast warm up times.
Fortunately I have wood burners in three rooms for when its extra parky.,
Yes, bit pointless to try to explain it on the internet when there's lots of books in the library and lots of websites explaining it.
Do an Excel spreadsheet.
Heat loss through each structural element is Q, where Q = UAdT
Heat loss due to ventilation is (IIRC) 0.33 x N x Vol x dT.
That's it.
Most heating engineers don't calculate it, they rely on rules of thumb, experienec and oversize everything (you can turn a too-big rad down, but you can't turn a too small rad up).
Yes, I've read that too. Looks like I need a book ! I did rate all the radiators at one point, when I shifted a few around, but I just wanted a rough idea, which can't be that difficult in a pretty standard, brick-built semi with cavity insulation and adequate thickness in the loft. Good job the electricians here aren't so sensitive ! ;-)
Thanks for that, it all makes sense. I've also got a gas fire for a bit of a boost but I'd prefer not to use it and have the extra extra boiler capacity. Looks like some sums are necessary. All I want is a ball-park figure in mind, for when I'm getting quotes from the experts.
Just to note that the radiators may not have the biggest demand on the boiler if it is a combi.
Hot water for two good showers running at once takes a lot more poke than that required to heat the radiators.
This does of course mean that the boiler is over sized for the radiators so you lose some efficiency.
I sized the boiler for two showers, and the plumber doing the install said "I assume you sized the boiler for the showers not the radiators?".
I would assume (although I haven't been down that route) that if you sized individual rooms for radiators then there should be a straight forward calculation for how much boiler you need to heat all the radiators.
On Tuesday 09 July 2013 16:58 Andy Cap wrote in uk.d-i-y:
Well, where did you post? Trades forums are notorious for having a good few folk doing an impression of a Klein bottle. The IET forums can be a little like that and it is noticeable that the more helpful people are either industrial sparkies or electrical engineers.
Whereas the "WTF do you think you are doing touching electricity - you'll kill everyone and go to prison!" are more often than not domestic sparkies who've never done anything else (with no implied disrespect to Adam who is a fine gentleman and not at all like that).
look at where the boiler you intend to buy is at its most efficient, and size for that output into the winter.
Assuming its a stored hot water thing.
The most fuel is burnt in the months of November December January and February. That will be over half the total heating bill - maybe 2/3rds. Forget about summer costs - you want to be as efficient as possible heating against a 0-5C ambient outside. If the calcs show 7.7KW look at a boiler than can maintain that efficiently, but can peak well over 12KW. Thats still not a huge boiler especially for a combi. But unless it's a one person flat don't install a combi.
(a Combi +heat bank is just as expensive, if not more so, than a system boiler + pressurised tank system, and pumps are more expensive than a water softener to KEEP that pressurised hot water system scale free. Unless you have intermittent or very low pressure water, soften it, heat it and store it)
Buying bigger wont be a disaster., Buying to small may well be.
And the more zones you have, the better control over room temps and fuel burn you have too. If its a total reinstall, think in terms of running 4 core wires everywhere to have motorised valves and stats on almost a per room basis.
To be able to turn spare rooms down completely is a good thing. And a sight easier than TRVS on multiple rads.
When I did my C&G 6084 Energy Efficiency for Domestic Heating back in
2005 (when the Building Regs were changing to mandate mainly High Efficiency boilers) our instructor said that the Whole House Boiler Sizing Method[1] had been designed and checked to give an accurate estimate compared to the elemental method. Still, if one distrusts it's easy enough to do a simplified elemental for a house. I think I did check them out side by side a couple of years back and they corresponded reasonably well.
[1] which I've written up on the wiki
Thanks. I've now done a room by room assessment and the requirement works out even slightly less. The 15KW Vaillant 415 open-vent boiler I'd been considering would be more than adequate. I like the simplicity of system I have and only really want to change the boiler as the Glowworm's getting a bit long in the tooth now.
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