Alternative Technology Assn Forums

1. 

   HouseOB
   Member

   Does anyone know a reliable formula for calculating the required kW for radiator panels based on room size, external wall size, window size etc?

   I'm getting varying suggestions from suppliers so don't know what to trust. Some just say 1 kW per 10 sqm, some say 1.4 kW per 10 sqm, one gave a result of 5.4 kW for a 21 sqm room, and then there is a calculator that apparently takes into account windows, external walls, whether you have insulation (see http://www.choice.com.au/reviews-and-tests/household/heating-and-cooling/home-heating/heating-options-for-your-home-buying-guide/page/heating-calculator.aspx)

   E.g, what kW rating would you suggest for this room?
   6m x 3.6m with 2 external walls (approx 25% of the external walls are standard windows).

   - External and internal walls are not insulated
   - Old windows that are not double glazed
   - Ceiling is 2.4m insulated throughout, hardwood floors on piers with air underneath.
   - Maximum difference in temperature from outside in winter to desired temp inside would be 15 degrees C.

   From the formulas I have at least 3 different results, 2.8kW, 3.25 kW and 5.4 kW, so not sure what I really need!

   Posted Saturday 23 Aug 2014 @ 12:51:44 am from IP #
2. 

   cava
   Member

   It may help if you list the location of the dwelling.

   Posted Saturday 23 Aug 2014 @ 10:36:39 am from IP #
3. 

   HouseOB
   Member

   Ahh okay, Victoria Eastern Suburbs. Does that change the calculations?

   Posted Saturday 23 Aug 2014 @ 10:58:18 am from IP #
4. 

   nhand42
   Member

   HouseOB said:
   Does anyone know a reliable formula for calculating the required kW for radiator panels based on room size, external wall size, window size etc?

   I'm getting varying suggestions from suppliers so don't know what to trust. Some just say 1 kW per 10 sqm, some say 1.4 kW per 10 sqm, one gave a result of 5.4 kW for a 21 sqm room, and then there is a calculator that apparently takes into account windows, external walls, whether you have insulation (see http://www.choice.com.au/reviews-and-tests/household/heating-and-cooling/home-heating/heating-options-for-your-home-buying-guide/page/heating-calculator.aspx)

   E.g, what kW rating would you suggest for this room?
   6m x 3.6m with 2 external walls (approx 25% of the external walls are standard windows).

   - External and internal walls are not insulated
   - Old windows that are not double glazed
   - Ceiling is 2.4m insulated throughout, hardwood floors on piers with air underneath.
   - Maximum difference in temperature from outside in winter to desired temp inside would be 15 degrees C.

   From the formulas I have at least 3 different results, 2.8kW, 3.25 kW and 5.4 kW, so not sure what I really need!

   I'd strongly recommend getting an expert - suppliers aren't necessarily experts, and I'm definitely not an expert either - but if you're keen on doing the calculations yourself, you can get a rough idea with the R values. A proper calculation is hideously complicated because it takes into account air-infiltration, solar insolation, thermal gradients, thermal bridging, etc. But the simple calculation will be somewhere in the ballpark.

   You have ~20sqm of uninsulated brick veneer wall (R=0.5), ~5sqm of standard glazing (R=0.1), ~20sqm of ceiling (R=2, generously guessing), and ~20sqm of stumped timber floor (R=0.7 down).

   Watts = square metres * temperature difference / R-value. With 15 degrees you're losing 600W through the external walls, 750W through the windows, 150W through the ceiling, and 400W through the floor. How much heat you lose through the internal walls I couldn't say.

   You'll need at least 2kW to heat the room. More if the room has draughts and air-gaps, which it almost certainly does. The 3kW recommendation sounds about right.

   Posted Saturday 23 Aug 2014 @ 3:58:34 pm from IP #
5. 

   HouseOB
   Member

   Thanks sounds about right, I've found a few more formulas around and there seems to be a pretty common one that gives pretty good results. I've also been able to back this up by checking a similar house in the area that had them installed and comparing their values.

   Essentially the formula (for panel watts) seems to be:
   Floor Area * 80
   + Standard Window Area * 106
   + Double Glazed Window Area * 30
   + Uninsulated External Wall Area (minus windows) * 30

   The multipliers vary slightly between sources and will depend on your insulation etc. The internal walls are ignored for the most part since it assumed adjacent rooms will also be heated, but if they are unheated allowances would need to be made there as well.

   Posted Sunday 24 Aug 2014 @ 2:54:39 am from IP #
6. 

   Rick
   Member

   When our Hydronic system was calculated they may have factored in bedrooms to be heated less than living areas. Also floor coverings may make a difference ie carpet and underlay instead of bare boards.(in addition to the other factors mentioned in previous posts) Ultimately the heat load will determine the boiler size. Radiator dimensions are selected to match the available wall location spaces to the required heat output for each room. Pump speed is adjusted to give ideal release of energy though the radiators.(This was tested with an IR thermometer)

   Here's another rough guide to compare to your current calculations from a suggestion elsewhere on this forum:

   http://www.enter-shop.com.au/catalogue/c27/c69
   There are some simple rules to go by if you wish to install a hydronic system with your own plumber.
   For energy efficient homes you will require approx 600 watts of heat energy per 10m2 of room if the ceiling height is 2.4m, for every 300mm increase in ceiling height add another 100 watts of heat (each radiator has heat output in watts listed in the sizing tables)
   For non-energy efficient homes allow 1000 watts or 1Kw per 10m2 for 2.4 ceilings add another 100 watts every time the ceiling height increases by 300mm.

   Posted Monday 25 Aug 2014 @ 3:45:27 am from IP #
7. 

   HouseOB
   Member

   Yeah that enter-shop has a calculator (that you need to pay for), but from their sample documents, it looks like they use the same formula I mentioned in my previous post.

   Posted Monday 25 Aug 2014 @ 8:07:04 am from IP #

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