So, when performing heat loss calculations, we make quite a few assumptions, especially for an older house where insulation/air infiltration details might not precise. But in the end, we re trying to figure out the amount of energy each room/whole house looses. So I was wondering, wouldn't it be easier and more precise to shut the heat off and just measure room temperature change over a period of time, keeping track of the outdoor temperature as well. Then, based on the room size and the outdoor temps, it would be fairly trivial to calculate exact heat loss for a room? I'm thinking this could even be automated with a simple arduino based device with temperature probes.

This has been bugging me for a while. Does this sound reasonable or am I missing some crucial details?

 

Hi vitals,
Finding an easier method would be nice, but your proposal would only be a rough approximation. The sun, the wind, and the rain are just a few of the variables. But one of the crucial details is that calculating the heat loss is more than just getting a number, it is getting the same number others would get and that is important. Current methods have been tested against the real world and fine tuned so they provide reasonable results.

I can't speak for the manual J calculations, I come from the energy auditing side, but the step by step process to determine the heat loss also provides a look at where solutions might be hiding.

Bud

 

A problem would be measuring a few degrees temp drop precisely. And with two adjacent rooms, one may gain heat from the other.

Are you on natural gas? If so, better idea is to clock the meter during a long heat call. Adjust for boiler efficiency and burner run time at design ambient to estimate total heat loss.

Doing a detailed heat-loss calc, using the slant/fin program is pretty simple. See link at one of the stickys on this site. It guides you on the assumptions appropriate for your older house.

 

Another method which can give excellent results is to measure the fuel burnt versus degree days for the immediate area over a long term. Ideally this would be over several heating seasons and at least one that includes a really cold winter. The longer the time frame the more accurate the results. The problems with this method are the difficulty in obtaining the degree day figures for the immediate area and the fuel consumption. If you burn oil ONLY for the heating system (no domestic water heating) this is much simplified but if you burn gas with other gas-burning appliances then you need to install a secondary gas meter for just the heating system. Another method would be to collect the actual time the gas valve was energized and then multiply that by the clocked firing rate to come up with the fuel consumption. If your system also heats the domestic water then you can add a fudge factor derived from the summer time fuel consumption to the calculation to approximate what amount of fuel is used for the domestic water.

Since the above described method takes a significant amount of time to accumulate the necessary data the Manual J heat loss calculation is much easier and/or quicker. It isn't quite as accurate but who has time to amass data for five years when they need the answer tomorrow?

 

Great thoughts everyone! Just to be clear - I do not have a problem with doing a manual heat loss calc. This is more of a 'what if' question in a quest for more precise measurement. The technology to measure, store and analyze large amounts of data is becoming very affordable, so I'm thinking of ways to apply that.
I really like the idea to measure fuel burnt over time. Seems like a way to arrive at pretty precise data.