Posted By: Tracy Oldfieldheating 22,047kwh/yr
water heating 5,450kwh/yr
My stab at calculating heating has come out at 2.5kw which seems very low for heating a whole house.

That's because you can't just take the annual total and divide by the number of hours in a year. All you did was find the average over the year. Obviously in the height of summer your heating requirement is going to be less than it is in the depths of winter. You need to find out what the heatload is on the worst case winter day and use that to determine the capacity of your heating system.

Water heating is less seasonal so it's probably OK to find the daily average - though if you're using solar collectors you have to factor in the lower sunlight available in winter.

Hope this helps,

Paul in Montreal

The standard method is to work out the heat loss for each room under what is chosen as the worst case conditions, add them all up and add a fiddle factor to get the boiler size. It's easy to do, if rather tedious, but it can give useful insight into were the heat losses are.

You decide on the temperature you want each room to be, say 21C, decide on the worst case outside temperature that you want the system to cope with. I used -1 but I think lower temperatures are usually used. You need to know the thermal resistance and area of all the elements of the house, walls, floor, ceilings, doors and windows. For each element you multiply the area by the temperature difference and by the thermal resistance. Add all the elemental losses for each room together and add the heatloss for air changes and that gives you the heat required for each room. Add all the room heat requirements together and that gives you the output of the heat source.

In a well insulated house there should be no difference in the efficiency of radiators v underfloor heating. In an old house with poorly performing windows properly positioned radiators can be more efficient, as the high surface temperature radiator can compensate for the cold surface temperature of the window and a high temperature surface can provide a good degree of comfort with a lower air temperature.

Bill

Posted By: Ed DaviesThermal conductance (U-value), not resistance (R-value).

Doesn't really matter as one is the reciprocal of the other. But doing a simple room-by-room calculation by hand is tedious. Better to use a modeling program like hot2000 - which incorporates air leakage losses, insolation, ground conduction, wind exposure as well as daily climate data to give you a worst case heat load (you can edit the weather data files too if you want to change the outdoor temperature to suit your particular location). I've used hot2000 to model my old house and the estimate is within about 5% of actual measured consumption over three years.

I must say the 22,000kWh per year for a UK location that's supposed to have "very high levels of insulation" sounds rather high. I'm at around 2.5x that for an old house with not much insulation in a much harsher climate (worst case temperature for heatload calculation is set at -21C). How many square metres is the entire house?

Paul in Montreal

Posted By: Tracy OldfieldPaul, I am worried by your comment on the insulation of our build as we have aimed for high insulation through the whole house (with windows we have bettered the figure in the SAP). The SAP gives a volume for the house of 512cubic m with heat loss through the structure at 199W/K but ventilation rate of 132W/K. The ventilation figure seems high to me do you think we should try and reduce this?

What's the floor area of your house? What air leakage figures did you assume in the calculations? Building airtight with mechanical ventilation is a big win and the energy used to run the system will be way less than the energy saved. Where is the house located? If you send me some details of the construction (floor plan, window size and location, wall sections etc.) I could try and put a ho2000 model together to give at least some idea where to focus on. My email is in my profile (it might take me a couple of weeks to get it done though :) ).

Paul in Montreal.

If you want to do a SAP calc - and don't mind using the unofficial SAP software you can use the workbook for the Eire governments energy assement method called DEAP. It borrows heavliy from SAP (in fact I think it is SAP) but you may find some minor discrepancies. Eg. I have a feeling the degree days assumed, may be different for Eire.

http://www.sei.ie/index.asp?locID=1011&docID=-1

The main benefit is that's it a free piece of software and comes with the manual. If you're a whiz with excel you could recreate your own version using the SAP specs.

However I wish the gov would pay BRE to do the same in the UK like they did with SBEM. Be nice if lots of us could do our own "rough" calcs in excel and only get an accredited assesor when you need to submit officially for building regs compliance or for an EPC.

Kind of gives me an idea that a few of us should get together knock up and test an xls workbook and then pay BRE to get it accredited and make it "open source". Or at least free to use. I worry that giving the source away means people could modify their SAP score :-(. I might have to suggest that one next time I'm in a meeting with CLG!

sorry - forgot to mention that using teh workbook means you can see some of the interim numbers as they get calculated - rather than just the final score.

Simon.