Boiler sizing calculations

[Z]

What measurements would one need to calculate radiator output HW output and henceforth boiler output for a domestic premises.

Room volumes, u values?

[John Rumm]

Create yourself a spreadsheet with a set of entries for each room. You need three bits of information to workout the heatloss:

The surface area The temperature differential The u value of the surface.

Multiply the lot together to get a figure of watts/hour transferred through the surface.

(if a wall has a window then subtract the area of the window from that of the wall, and count the window as a separate surface).

One you know the loss (or gain) through each of the surfaces add them up to get a nett loss/gain for each room

Lots more info here-

[J.Milton.Hayes]

Try 'heatloss manager' from

[Andy Hall]

It is free softeare, though, and I have a copy that I can email to anybody wanting it.

.andy

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[Peter Taylor]

John Rumm wrote

Don't forget the air change factor!

Peter

[Andy Hall]

Both of the other other replies get you to knowing the heat losses for the rooms - essentially the Myson program has a set of U values and does the sums for you.

Once you know the heat loss for a room you can size the radiator.

What you go with depends on the boiler type and operating temperature, because the actual radiator heat output depends on the mean temperature difference between the radiator and room.

If you look at radiator manufacturer data sheets, you will find that there is a main table of ratings. However these come from a test which is done at a higher temperature than is normal for domestic installations and has to be scaled with another table.

The two typical designs for UK installations are based around a non-condensing boiler and a condensing boiler.

For a non-condensing boiler, the flow temperature is taken to be 82 degrees and return is 70, making a mean water temperature of 76 degrees. With an air temperature of 21 degrees, it means a Mean Water to Air Temperature of 55 degrees. So you can refer to the table and you will find that the derating factor is 0.89. This means that the radiator gives only 89% of the test value as an output. So, if you are starting from your heat loss number, you have to divide it by 0.89 to determine which radiator to choose because it will need to be a higher number than the main table suggests. Then you go to the main table on the data sheet with your number and pick from there. In other words, if your heat loss is 890W then you would need a nominal 1000W radiator.

For a condensing boiler, the ideal design temperatures are 70 and 50 degrees. For that, the derating factor is generally about 0.6 which implies a rather larger radiator. However, you have a choice here. Condensing boilers will run up to the traditional 80 degrees if needed and still be somewhat more efficient than a non-condensing one. So if you already have the radiators then it doesn't mean you have to change them. Also, the heat loss calculations assume -3 degrees outside which is not for much of the time - in effect you are designing for a worst case. For large parts of the year, less heat is needed and the boiler will modulate its temperature down into a more efficient lower temperature range. At this point, the radiators are plenty adequate anyway. So the sensible thing, is that if you are putting in new radiators, size them for 70/50 operation, but it isn't mandatory if not.

For the hot water, if you are using a cylinder and are able to change it, use a fast recovery one. This is able to accept a lot of heat from the boiler - perhaps 20kW or more.

Therefore it is not unusual to have a boiler sized at 25-30kW to do that, when the CH requirement might only be 10-15kW. If the boiler is a modulating type, this will work properly because it may be able to drop its output to 4-7kW.

.andy

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[J.Milton.Hayes]

: >Try 'heatloss manager' from

This is useful, but unfortunately not up on their site at the moment. : It is free softeare, though, and I have a copy that I can email to : anybody wanting it. : :

That's odd, I've just been to their homepage and entered via the 'homeowners' link & it is at the top of the page ;-)

[Andy Hall]

Are you sure you don't have it cached?

I tried via installer and homeowner options, the page appears but the HeatLoss Manager link (button in the middle on the right) appears as Under Construction.....

.andy

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[J.Milton.Hayes]

: >: >Try 'heatloss manager' from

>: : >: This is useful, but unfortunately not up on their site at the : >moment. : >: It is free softeare, though, and I have a copy that I can email to : >: anybody wanting it. : >: : >: : >

: >That's odd, I've just been to their homepage and entered via the : >'homeowners' link & it is at the top of the page ;-) : >

: Are you sure you don't have it cached? : : I tried via installer and homeowner options, the page appears but the : HeatLoss Manager link (button in the middle on the right) appears as : Under Construction..... :

My apologies, I didn't realise that the page was offline temporarily.

[Andy Hall]

No problem at all. I thought that maybe they had put it back up again. Before the site was reconstructed, they did have a beta of a new version written IIRC in Java, but it was pretty slow and ropey.

I grabbed a copy of the earlier one some time ago which is reasonably good. .andy

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[Peter Taylor]

The Barlo calculator is still available Andy. Although not an inspiring program and written for Win 3.1, it does work.

[Andy Hall]

the U value tables, although you can correct them. It's a while since I used the Barlo one, though

.andy

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[Christian McArdle]

TBH, there is little point calculating the boiler requirement nowadays. Almost all modern domestic system boilers are modulating (variable) types of around 24kW. These will do a fairly large and poorly insulated house. Even if the requirement exceeded 24kW, you would still fit the 24kW boiler, but would just improve the insulation to bring down the requirement. Obviously, hotels or other large or commerical buildings may genuinely require more. Also, if installing a combi boiler, you'd probably want more than 24kW.

Of course, if you need to calculate individual radiator sizes, then you would need to do the calculations. There are programs available to do the calculations. You need to know the various dimensions of the rooms and openings. It isn't done on volumes, but on wall/floor/ceiling areas, the construction of those areas and the temperatures on the other side. You need to know the materials used (or the appropriate u-Values) and details of insulation. You also need to decide on the outside temperature you expect it to work in. People generally design to -3C now, although -1C used to be common.

When choosing radiator sizes, don't forget to derate to the average of flow/return temperatures. Preferably design to 60C (or under) to allow even greater efficiency for a condensing boiler installation. Even if you don't install one this time (you should) then you'll be forced to next time, so you might as well optimise radiator sizes for them. However, even if you don't optimise radiators for condensing boilers, you'll still get much better efficiency than with a non-condensing type.

Christian.