Propane In-Floor Heat Operating Costs

jasonwilkerson · October 4

I need some guidance/help in estimating operating costs of in floor radiant heat. I am building my own home which I started 3 years ago. At that time, I decided on a solid fuel outdoor boiler to provide the heat, but I'm getting old and tired and have rethought cutting and splitting 8 cords of wood a year. So I am nearing the point that I need to decide how I will heat.

I am considering a propane heat source, but I am having difficulty with a real-world operating cost. I am also trying to determine if I should go tankless or boiler.

The home is 2300 sq ft. The slab has 3 zones of 1/2" pex installed on top of 3" foam insulation underneath and on perimeter of slab. Walls are R19 and Ceiling is R49. Climate is moderate (western North Carolina, approximately 5500 HDD/year).

The home also will have a wood stove—a very nice and efficient Hearthstone soapstone stove that will supplement heat nicely). It is cooled by mini-splits that also have backup heat strips.

Propane in my area seems to be $3.00-$3.40/gallon right now.

For tankless, I have been TOLD that 2 hours of run time per day is reasonable. I really don't know what to consider for a propane boiler.

I have read the posts discouraging the use of on demand heaters for space heat, and I get that. Right now, I am trying to make an honest comparison of OPERATING COSTS. How many gallons of propane should I expect to use per day/month/year?

I'd love to hear some real-world experience of how many hours your tankless/boiler runs per day.

TIA

Jamie Hall · October 4

In realty, what matters is how many BTU it will take to heat your house for a year. You need, for starters, the heat loss for the house — how many BTUh it takes to keep the place warm with a specific outdoor temperature. That, combined with your known heating degree days, will tell you how many total BTU you need for the year.

So that's one number.

The next consideration, obviously, is how much do those BTUs cost? And that's not at all hard to figure — propane has around 90,000 BTU per gallon. Fuel oil around 140,000. Wood is quite variable, but there are sources which can tell you how many BTUs are in your wood.

Then there is efficiency. Here is where there is a consideration of tankless water heater vs. modern boiler. And not only is a modern boiler built for heating (a tankless water heater isn't) but, with the right controls and sized properly, it's quite reasonable to expect it to use only about 80% of the fuel that a tankless water heater would use. Perhaps less.

Kaos · October 4

Your winter fuel use in Therms is about design heat load / 42.

So if your heat load is say 42000BTU, you need 1000 Therms.

If you have a 95% modcon, that means about 1050 Therms of propane or 1100 gallon. So ~$3500/year.

You can do the above math for your heat loss. Generally with expensive fuel, it is worth while to get your heat load down, so see where you can improve your building envelope.

There is not much efficiency difference between a boiler or a condensing tankless. Efficiency is mostly a function of return water temp, so as long as those are low (~90-100F) you'll 90+ efficiency out of either.

hot_rod · October 4

if you have radiant floors an lp mod con is the way to go. It could get 90% efficiency running low temperatures. A combi would also supply hot water 110,000 btu/ hr us as small as I would go with a combi

That should be plenty of boiler for that sized home.

A heat load calc is the best way to get an approximate fuel usage

Or work the wood consumption backward

12- 32 million btu in a cord of wood. Dry hardwood being on the high side

Green or wet on the low end

40-55 .% efficiency from a typical OWF

The insulation quality, outdoor temperature and your desired indoor temperature will dictate fuel usage

I see a 110- 120,000 combi in your future.

DCContrarian · October 4

https://forum.heatinghelp.com/discussion/comment/1866754#Comment_1866754

This is the right answer. Just to add on, houses are like snowflakes, every one is unique. Two similar-looking houses in the same place can have heating loads that vary by a factor of 10 depending on the choices that have been made during construction and renovation over the years.

Since the house is going to have heat pumps, you have to consider the cost of that as well. This is more complicated because the efficiency depends upon the outdoor calculation. A fairly simple calculation is to figure out what the COP of the heat pump needs to be in order for propane and electricity cost the same per delivered BTU, and then look at the published performance curve for the heat pump you want to use to see what temperature it achieves that COP at. Then you can look at climate data for your area to see how much of the time the heat pump is going to be economically viable.

Your house sounds like it's insulated to modern standards, which is good, you'll find it's more comfortable. Also make sure it's well air-sealed, which will make it quiet and less dusty. In an efficient house it can be difficult to get a wood stove to burn at a low enough level to avoid overheating. You may also find that the heated floors have to be only a few degrees above room temperature to meet your heating load, which may not give the warm-toed feeling you're looking for.

HydronicMike · October 4

You may want to double check that HDD number. Should be in the 3500 range, not 5500.

Propane In-Floor Heat Operating Costs

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