Propane costs for an LP fired tankless boiler

[ConcreteJordan]

I live in a country home in Southwestern Ontario. I currently have an outdoor wood boiler which works very well, but I am tired of the work involved to keep it fueled with wood. Also, I don't have a wood lot so I have to pay for the wood. It can cost $2000-3000 per season. And we make due with space heaters in the home during the early fall and late spring, so the household heat isn't always where we would like it to be.

Our home has a new addition on it. The old section and the new each have their own forced air handlers with separate thermostat. Each cares for a space of 1000ft. I also have a third space of 750ft that has indoor radiant heat.

I understand that switching to an lp fired tankless boiler will have some major up front costs. But my question is...How much propane can I expect to burn through? Is there a way to figure out a rough estimate? If it costs the same as wood, I am happy to make the switch and save the labour and space needed to supply wood. If I can expect it to be a lot more than my wood costs than I would have to reconsider.

Are there any other suggestions as to what I could consider switching my heat source over to? I am just tired of wood.

Thank you for any input provided.

 

[Dana]

Heat load and fuel use is not a function of the floor area square footage of the conditioned space. It's all about the indoor to outdoor temperature, and the heat transfer characteristics of the exterior surface types & areas of the building. An R30 roof doesn't lose nearly as much heat as a U-0.35 window, or 2x4/R11 wall per unit area. The loss when it's -30C outdoors is very different from the loss at -10C outdoors.

To even so much as get the order of magnitude correct either a Manual-J or I=B=R type load calculation (Hot2000 works) at the 99th percentile temperature bin (aka "99% outside design temperature") for the location, as well as the 25 year average of annual heating degree-day (HDD) data (base 18C/68F.)

Once you have the heat load at a particular outdoor temperature, divide the load by the temperature difference. Say the load works out to 40,000 BTU/hr @ -20C outdoors, +20C indoors, that's 40,000 BTU/hr/40C = 1000BTU /degree-C-hr. A heating degree-day is the amount you would need in a whole day at 1 degree temperature difference (I'm simplifying here), so at 1C temperature difference it would take 24 hours x 1000BTU /degree-C-hr = 24,000 BTU/HDD.

So if your local climate is similar to Toronto which averages ~3800 HDD/year, your heating system needs to deliver 24,000 BTU/HDD x 3,800 = 91,200,000 BTU/year. If your condensing boiler is operating at 95% efficiency that takes 91,200,000/0.95= 96,000,000 source-fuel BTU/year. A liter of liquid propane has about 24,200 BTU, so it would then take 96,000,00/24,200= 3967 liters of LP. Conventions on units at which LP is sold vary widely in Canada, so you'll have to figure out the conversions to come up with the right units for pricing to make the energy cost comparisons.

Per source fuel BTU propane is usually quite a bit more expensive than cord wood (often 2-3x as much), but the as-used efficiency of an outdoor wood boiler is usually pretty low compared to right-sized condensing propane boiler or furnace that is completely indoors.

In many parts of Ontario it can be cheaper to heat with cold climate air source heat pumps than with a propane boiler or furnace. There is a growing number of heat pumps with a specified capacity at -25C, and even some fully rated at -30C. If it gets colder than that for an extended period of time in your area (say 3+ days where it barely breaks -25C outdoors even during the "warm" part of the day) it's important to plan for having sufficient backup, but there are several options. Even at -30C most cold-climate heat pumps are still more efficient than electric furnaces & baseboards. There are both ducted and ductless versions of this type of heat pump, but sizing it optimally for your climate is a bit of an art, but it all starts with the heat load calculations- without them it's just a WAG, a shot in the dark, and getting it wrong (on either the oversized or undersized side) cuts into both comfort and efficiency.