My wife and I are looking at a very small house...the house is 850 square feet including a finished basement that is about 400 square feet. It is newly built (2005) and met code so it should be reasonably insulated. The heating system is a munchkin boiler/floor radiant heat system powered by propane.

I checked into the propane usage and saw that it used 100 gallons in march. The house was vacant and kept at about 63 degrees. I live in a cold climate, the average night temperature in march was maybe 15 with the average day 35-40.

I know this isn't a lot to go on, but doesn't this seem like an awful lot of propane to use? I know of a 1200 square foot house that only used 100 gallons in the depths of winter.

Might there be a leak? The propane tank is underground. Would a professional be able to figure out if something is wrong?

 

Although the temps are cold, 100g a month seems exceptionally high. I'd say you have a leak somewhere, or radiant heat is just terribly inefficient causing the boiler to run constantly.

The tank could very well be leaking underground, how old is it? Do you know if it was buried with the zinc pellets to control rust?

Assuming the tank was installed (new) when the house was built, it might be in the piping somewhere. A leak that big though you'd probably smell.

 

When I go over to the house, the boiler is actually never running. I always have to turn up the thermostat to get it to kick on.

Is there any simple way to test for a leak?

 

Personally, I don't think that useage is excessive at all. Do you make domestic hot water with the boiler? If so and you are keeping 40 gallons of water in an indirect tank hot, that certainly accounts for some of the useage.

 

Short of not using it for a month and checking the tank gauge. Call the propane supplier, they will usually come and check for leaks for you.

 

Maybe I did the math right, maybe not. Maybe the assumptions are right, maybe not. But see if this is in the ballpark.

propane has about 92,000 btu/gal
100 gal used in March equals
total input 9.2 million btu or
307,000 btu/day or
12,800 btu/hr

degree days for March using 15F low, 40F high = 1125. That's about 37.5 HDD per day.

For 850 heated square feet, this works out to 9.6 btu/dd/sf heating requirement.

That seems high to me for a recently built structure with decent windows, insulation to code, etc. I would expect something in the 5.5 (plus or minus 1) btu/dd/sf range.

Leaky tank is certainly a possibility, but it would also be worth looking into:

1) heat loss calculation for the building. This would tell you what the heating requirement "ought" to be for your climate. Takes an hour or two; you can do it yourself. Free software at www.slantfin.com. For that size house, you will probably get a number between 16,000 to 23,000 btu/hr on a "design" (coldest) day.

2) real energy audit using a blower door test and thermal imaging. This would tell you how much the house leaks air and whether the builder forgot insulation in the walls, attic, or elsewhere.

3) combustion efficiency and maintenance of the boiler. Propane+Munchkin usually = need for very good periodic inspection and maintenance. Although usually the boiler will go into a fault mode of some sort if it cannot fire normally within its safety parameters.

 

How did you determine that 100 gallons of propane were used over that 30 (31) day period?

Being built "to code" is meaningless. Codes are MINIMUMS and who knows when the state or local energy code was last revised prior to building.

Does this house also have radiant floor heating in the basement floor? If yes, then what kind of vapor barrier and insulation was used under the floor? Was a thermal break used between the floor and the footings? Are the basement walls insulated and if so is the insulation interior or exterior?

What temperature was the interior of the house held to during this time? Is there an attic and what is the insulation up there?

 

How did I miss that this is radiant floor? Oh well.

furd, as usual, nails some of the big issues. It's pretty easy to make a radiant floor system -- especially poured floor in contact with earth -- a very inefficient, heat-sucking monster by failing to properly apply insulation and vapor barrier to the slab, footings, etc.

You need to check out the construction details and field verify as much as possible.

 

Also bear in mind empty houses with no furniture will use more fuel than a lived in house. If the house has furniture I would be concerned with radiant insulation. A leak would be my last concern.

 

thanks xiphias for your detailed calculations and furd for your input. i believe the radiant heat is in the basement floor as well as the middle floor. there is no attic, but there is a loft. the floorplan is basically a 250 sf +- loft over 350 sf +- of living feet over a 300 sf semi-finished basement.

it was a fairly contentious building when it came into town and i definitely think the original owner skimped where possible in construction costs--i wouldn't be surprised if the radiant floor system is installed pretty much totally wrong.

furd, i calculated the 100 gal usage by simply looking at the tank on april 1, read 58%, called the propane company and they last filled on february 29 or march 1 to 78%. it is a 500 gallon tank.

i should be able to get a hold of the original building plans and i'll learn more about the installation of the radiant system.

 

The indicators on propane tanks are rarely exact and you also need to make temperature corrections. The calculation of 100 gallons could be off as much as 20% or more.

Also, since propane tanks are cylindrical you cannot simply take the difference of 78% and 58% (20%) and then calculate that 20% from the ultimate capacity of the tank; which is what it seems you did.

If you had the diameter and length of the tank someone could probably make a much closer guesstimate as to the amount of propane consumed. I could do it but the arithmetic makes my head hurt and I no longer get paid (quite handsomely in my day) for doing these calculations.

I suspect that far less than 100 gallons was used in light of how it appears that you made the calculation.

 

Keep in mind also that propane tanks are only filled to 80%, so a 500g tank only holds 400g.