I need to size a new boiler, hence I need to size the radiators for each room.
There are a lot of heat loss calculators out there, but, depending upon the one I use, I get values of between 733W and 3.3kW for the same room. Unfortunately, they don't say what assumptions are being made to get those results.
Has anyone used one that they can recommend as giving results that actually work in real life?
Colin Bignell
The only way I found was to DIY in a spreadsheet. At least that way you can model the actual construction as accurately as you like.
See example sheet here:
I was hoping to avoid the work :-)
Colin Bignell
There's not that much in it really - by the time you have measured everything and worked out what its made of, all the glossy program does for you is look up the u values.
On Wed, 31 Mar 2010 17:50:07 +0100, Nightjar I need to size a new boiler, hence I need to size the radiators for each
The standard method for sizing boilers is not to do a detailed heatloss for each room but to use a differnet calculator. I think ther are links to some online ones on the wiki but I can email you my spreadsheet version.
... if you drop me an email (my address is valid but the obfuscation of yours defeats my poor knackered brain :-/)
Correct. The boiler has simply to be large enough to manage the house total heatloss at say -5C ambient, and have a bit spare for reasonably fast heating.
Radiators are sized to the room, boilers to the whole house..
In message , "Nightjar "@?.?.invalid> writes
I've got a copy of the old Myson Heatloss Program, passed onto me by the late Andy Hall. Old, but still seems to do the job. You can set the variables (U values, room temps etc.) so you know what's happening.
Drop me an email (reply to is valid) if you'd like a copy.
Thank you. I have a copy I downloaded from their site ages ago. The problem I have is knowing whether to believe that, or the one provided by Plumb Center, or the one given by Ultraheat, or the City Plumbing one, or the one at Heatlosscalculator.co.uk etc. They all give different results, some wildly different. I suspect I shall have to do as John suggests and do the calculations myself.
Colin Bignell
Thank you. I did look at boiler sizing calculators, but they seem to assume a fairly consistent form of construction. My house is about 80 years old and has been extended at various times, under various sets of building regs, so I have insulated and uninsulated cavity walls, to differing levels of insulation, an insulated loft, an uninsulated flat roof, an insulated flat roof, single glazed wooden windows, double glazed uPVC windows, and a heated double glazed conservatory. I think the only way is to work out the loss room by room and add those to get the loss of the whole house.
I am coming to the conclusion that I will have to follow John's suggestion and put it all in a spreadsheet. That would also have the advantage that I could quickly see the effect and savings, if any, of different types of additional insulation. The main problem is that I tend to get carried away writing spreadsheets and end up with huge ones.
Colin Bignell
On Thu, 01 Apr 2010 02:12:13 +0100 "Nightjar wrote: ...I suspect I shall have to do as John
I strongly suggest you do, and then maintain the spreadsheet. I am forever returning to mine, and doing 'what if' type things with it. E.g., this bedroom seems cold, what if I added 25mm Celotex to the outside wall when I decorate? 40mm, 60mm, etc..
I also started making it ever more detailed and checking each room for conformance with the model. By doing this I have discovered that the source of the coldness in the kitchen, which the model says should be
18, is the meter cupboard (1970's bungalow, wiring runs to the loft via the cavity, so no cavity fill behind the meter cupboard) - the cold air is actually coming from the cavity through the CU! Solution cheap and obvious!R.
The one on the wiki page has a link to a google docs version. Failing that I have one here I did for the last house. You are welcome to a copy if you want.
John - could you forward a copy to me please? Just remove the word REMOVE IYSWIM!
Thanks, Rob
I use the Myson program and it delivers results which I believe. I'm not familiar with the others which you mention, but I know that some on-line calculators seem to use "rules of thumb" rather than doing the calcs rigorously - and so might get widely varying results.
The Myson program (and also the one from Barlo, which is very similar) take account of all the necessary parameters - external and internal temperatures, areas of walls, windows, ceilings, floors - all with appropriate U values, air changes, etc.
It has standard U values for all standard types of wall, window, floor, roof, etc. construction - but you can subsistute your own if you prefer. [You'd have to do that with a spreadsheet, anyway].
Having worked out the steady-state heat loss for each room under the specified conditions, it adds on a bit more for transients, and a bit more still for pipe losses - but you have control over those, and can change or omit the grossing up factors if you prefer. It's probably in this area where the different programs give different results - because they make different assumptions about the amount of grossing up required (and possibly about air changes - and allowances for DHW heating when sizing the boiler). One good thing about the Myson program is that you can *see* how it is doing the calcs, and change the assumptions if you don't like the default ones.
I'd not thought of allowing for adding extra insulation inside the wall. So, that is a check box to remove and another drop list to add. So far, I have drop lists for four different insulation levels for the suspended floors, along with different U-values depending upon the perimeter to area ratios, - I can't do much about the solid ones - six different ways to improve the main roof insulation, four different options for the uninsulated flat roof, including two that involve replacing it with a lean-to pitched roof, three different options for improving the original windows, four different outside temperatures and several different options for room temperatures, as I have never found the standard set of room temperatures comfortable.
I've got something similar, except that the hole is above the CU. The hole is flagged for covering if I get the cavity wall insulated, if only to stop the stuff pouring out into the cupboard.
Colin Bignell
I have just been through the same experience. I used about a half dozen of them and the results varied by a factor of three. I found that the most basic programs pretty much ask you for the room size and that's all, they then just assume worst case ie. no loft insulation, wall insulation etc and will give you the highest BTU readings. The better one's ask more questions. ie. number of ouside walls, floor type etc. I think I used radcalc.com which gave the lowest values and then added
50% for good measure. The boiler and rads are now running and it is plenty warm.
Hadn't seen that article: nice one John. But the google docs spreadsheet at
I tried a few of these heatloss calculators and, like you, I got values all over the place.
Mine's a chapel conversion with a double-height main area, very high ceilinged downstairs rooms, whilst part of the upstairs room follows the roofline - i.e all stuff that an online calculator won't follow well.
In the end, I had a think about "how many 2kW fan heaters to pull this room up to temperature from cold, on a very cold day" - i.e I pretty much went for my gut feeling for what sort of heat output I wanted for each radiator. Then added a bit.
Then had a think about what physical size of rad would fit each location. Then went round again and worked out what standard size of rad I could use in as many locations as possible. Then worked out how to get them in heat-pack deal from an online supplier.
Finally result had virtually nothing to do with the calculated size, other than its somewhat bigger.
Boiler I ended up buying a long way in advance of all of this - sizing based on similar "big enough and add a bit".
The insulation levels are now also considerably higher than originally planned (but depending on the weather, there may be drafts from the under-building void through the original timber boards downstairs).
The final outcome? Cosy, and very modest bills indeed.
My conclusions - the calcs are more using for giving plumbing students something to do in their college exams - and estimates based on personal impressions are as good, when considering a problem without a comprehensive set of accurate values to base calculations on.
Its a live "view" on a sheet you can't edit ;-)
I will stick a link to downloadable sheet up as well!
ok, done - my full spreadsheet is now linked from the wiki for download. See :
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.