Heat Loss... again...ish

Hilly

So I recall on previous posts and conversation that when do a heat loss for a job you will use different values based on whether the HTU is a radiant, baseboard, fan-coils, etc. Does it make a big difference? Should you be overly concerned with it when you have radiant in basement slab, staple for titled areas and then cast iron for the looks and heating the mittens in your entrances? A good friend is building and it looks like it'll be a hybrid of HTU's... but I'll get more into that when the time comes. I just got the plans so I wanted to focus on the heatloss and potentially doing a low-temp design.

icesailor

First things first:

IMO, you need to do a very accurate and comprehensive heat loss on the building before you decide on how to heat the spaces. The heat loss doesn't change, just the way to heat them does.

Minnie Pearl or some old wag at The Grand Old Oprey used to tell stories about two smart brothers. One day, one brother saw the other dragging a big long logging chain down the road. The other brother asked what he was doing dragging the big long chain down the road. To which the other brother responded, "Have you ever tried pushing one"?

Sort of like designing heating systems.  

furnaceNewby

Is it possible to measure the heat loss of a home? At a given outdoor temperature, and a particular thermostat set point, can we measure the heat loss by recording how much natural gas is needed /used to keep the house at the set point temperature?

Rich_49

Unfortunately that does not work in a situation where work will be done and changes to the structure made . Also not possible on a set of plans .

furnaceNewby

It'd be great if you could help me understand any error that I might be introducing with my method. I'll try to explain it here.

When I make the measurements, I record the number of minutes that the furnace gas valve is open (gas is flowing), and the number of minutes the furnace gas valve is closed. My accuracy is to about 30-60 seconds. I also record the exterior temperature, and the setpoint of the furnace/T-Stat (inside temp).

For example:

Ext Int ON OFF
7F 64F 03:03 03:10
7F 64F 03:27 03:35
7F 64F 03:52 03:59
7F 64F 04:14 04:22
8F 64F 04:38 04:45
8F 64F 04:59 05:07
10F 64F 05:22 05:29
12F 64F 05:43 05:51

Over the course of nearly three hours, this shows the gas valve was ON for a total time of 7+8+7+8+7+8+7+8=60 minutes.
Total time for the recording was 2 hours and 48 minutes (05:51-03:03=02:48) or 168 minutes. We can round this to 3 hours since the gas won't have turned ON again for another 15 mins after the gas last turned OFF (05:51) in that chart. i.e. the next gas ON time would be after 06:03 in that chart, making the total reading time 3 hours (or more).

Over the course of this three hour window the external temperature went from 7F to 12F and the gas ON time periods didn't really change. I still recorded 7 or 8 minutes of gas ON time.

furnaceNewby

Continuing my calcs, the gas ON time was 60 minutes out of the 180 minutes of recording time in the chart above. That turns out to be a gas ON duration of 33% of the time.

My 143,000 btu/hr input gas furnace running 80% efficient puts 115,000 btu/hr into the home. Multiply that by .33 and I get a consumption of 115,000 x .33 = 37,950 btu/hr.

Do you see where I have mistakes or introduced errors?

thanks.

furnaceNewby

Thanks for the reply!

If it matters, this is a forced air natural gas furnace.

The cycles occur due to thermostat operation. The thermostat closes and the system comes on. The system heats the house, the thermostat opens once the room temperature reaches the setpoint, the system turns off. Nothing abnormal as far as limits.

The cycles may be short but, from everything I have read, the system is not "short-cycling".

furnaceNewby

Ahh, sorry, I've just now realized that I'm posting in the "radiant heating" forum. My bad. I saw the topic of "heat loss" and thought it'd be a good place to discuss my readings. Apologies for taking up your time!