Just Wonderin' About Propane Usage

Rich Davis_2

Does anyone know how to calculate The difference in fuel usage by difference in thermostat setting.  For example, what is the difference in the amount of gas used to keep my house 66 deg, versus 70 deg.  Not on deg design day, which is 0 deg for me but, on 20+ days. I'm using a TT60 piped direct with the ODR and radiation is BB and one small radiator.  Just something I got thinkin about the other day.  Thanks

Rich Davis

3M's

Jamie Hall

Very very rough

Your heat loss, except for infiltration (which can be significant!) and sun load (which can also be surprisingly high) is directly proportional to the difference in temperature between the two spaces -- in our case, inside and outside.  So, in principle, the fuel usage should also be directly proportional.  Just round numbers in my head, but if it is 20 out and 60 in, you have a delta T of 40; if it's 70 in, your delta T is 50 -- and, in principle, if everything else were constant, you'd use 80% as much gas at 60 inside as you did at 70.



Things ain't quite so simple, though, as the outdoor temperature is anything but constant, and there is a good deal of thermal mass in the walls, and... and... so about the best one can say, without some pretty sophisticated modelling, is that you would use less gas...

pipe4zen

heating degree days

you would need to do a heat loss calc for 70 degree indoor , then a heat loss for the lower indoor temp, figure your fuel use using heating degree days for your area multiplied using the two different outcomes from heat loss, (search the actual formula) .



I once figured a seasonal 55 degree indoor vs. steady 68 degrees for a vacation home , and found only a 20% difference in usage, probably less than that once they bump up the temp when house occupied.

Fortunat

which degree days?

Ah yes, but you also need to account for the fact that a building maintained at a lower temperature experiences fewer degree days (not just a lower load per degree day).



There are a number of degree day data bases which let you figure out degree days based on different indoor temperatures.



Adding that dynamic to the reduced heat load per degree day will increase the apparent effect of lowered setpoint.



~Fortunat

pipe4zen

correction factors

sure there will be a correction factor associated with the fuel use formula,



a degree day is a degree day (average of high and low outdoor temp in a 24 hour period minus 65 degrees) and there will only be that one number regardless of indoor setpoints, slower rate of heat loss , infiltration etc.



In my reasoning , the "effect" in all actuality is diminished due to the indoor set points thermostats are only one aspect of fuel usage and indoor comfort. Plus, off-on control of boiler by lowered thermostat may even short cycle even more, by trying to maintain lower settings. I'm seeing this data logged with online thermostats which is recording runtime of boiler, while maintaining away temperatures. Boiler runs for only 1-5minutes before shuting off, and bounces on/off during the day.



The best way would be indoor feedback controls, with outdoor reset, with an away/vacation mode that would agressively shift the heating curve for optimum fuel savings. Another way to look at it is don't just lower the thermostat , but lower the supply temperatures as well, or solely. Then you'll really have an impact of fuel use.

Rich Davis_2

My Basic Question is?

I wanted to know how much more propane would I use if I bumped my Thermostat up from 66 to 70 deg.  In the milder winter temps.  On average this winter (mild) I've used 2.7 gals/day.  Up until this winter we've never had central heating (wood stoves) so I wasn't used to paying for my heat directly.  When I had a fill up in the middle of Dec I nearly had a heart attack.  Nearly $700, and it looks like I might have to get more before the end of winter.  So Really am I saving much money by keeping the thermostat lowered?

Thanks guys,

Rich Davis

Bart Vaio

PG&E

This is cut and paste from their website.  It is just a rough number as the figures will vary due to outdoor ambient and building specific heat loss.

• Set the furnace thermostat at 68 degrees or lower, and the air-conditioner thermostat at 78 degrees or higher, health permitting. 3 percent to 5 percent more energy is used for each degree the furnace is set above 68 degrees and for each degree the air conditioner is set below 78 degrees.

pipe4zen

Basic answer...

It depends.



What type of boiler due you have, can you lower water supply temperature, do you have

outdoor reset control. With this mild winter, you could have been saving up to 35% in fuel

with outdoor sensor.

Rich Davis_2

My Equipment is

I'm using a TT PS60 with ODR on a one zone system with 4 loops my supply temp is generally 90-130 with a 20 DT. Thanks

Rich Davis

Gordy

So if thats true

 I keep my tstat at 72* then I'm using 16% more fuel. Or my gas bill could have been 23.00 Lower last month. I dont know if I buy those numbers.



Do a heat loss calc at different set points It works out to a little over 1% for every degree increase in temp setting.



Gordy