Energy consumption: propane fueled hydronic floor heating vs cold climate mini split

| Posted in Energy Efficiency and Durability on

Hello,

I could use some input on a comparison I am doing being two systems.

Assumptions:
-1300 square foot home, slab on grade, with a heat loss of 20,000 BTU
-\$0.1385/Kwh is the average winter cost of electricity in my area
-\$0.698/litre is the cost per litre of propane that I was quoted today

Essentially, I would like to determine which system would consume more energy to heat the home to 20C in a cold climate?
1. Propane fueled hydronic radiant floor heating, with a condensing tankless water heater with high efficiency; or
2. Cold climate mini split, non-ducted, with an HSPF of 10 BTU/Watt-hour

I would like to do an operational cost comparison of both systems. By these rates, and running the math with available conversion factors, I think that the mini-split would cost nearly half as much to run as the hydronic floor heating.

What I am unsure of is, and intuitively speaking, once the slab has heated to temp, wouldn’t it have such a significant thermal mass that it wouldn’t take much additional heat to maintain the room temperature or heating load? I mean, relatively speaking to the mini-split?

I guess what I’m asking is, if the slab is up to temperature, and the home requires 20,000BTU/h to heat, surely that doesn’t mean that the boiler needs to inject 20,000 BTU/h to meet the load? It’s “top up” needs would have to be less than the mini-splits, no? And if that’s correct, then would this mean that an operational cost comparison of the two systems might be too difficult to theoretically calculate?

Any info is appreciated.

Thank you.

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Replies

1. | | #1

"once the slab has heated to temp, wouldn’t it have such a significant thermal mass that it wouldn’t take much additional heat to maintain the room temperature or heating load?"

No. It's pretty simple; you only get out what heat you put in. If the slab is of sufficient mass that the heat won't need to run for a longer time, that just means that it took more energy to heat it up in the first place.

"surely that doesn’t mean that the boiler needs to inject 20,000 BTU/h to meet the load?"
That's exactly what it means. The slab has no ability to generate heat, just hold it. You still have to get it in there. The only exception is if you're talking about passive solar heating, but that really has nothing to do with the primary heat source; it would be the same for the heat pump as for the in floor heating, providing the slab is the same.

1. | | #2

That makes perfect sense. Thanks for your comment!

2. | | #3

The heat pump will be much less expensive to operate. And in most areas, better for the environment.

3. Expert Member
| | #4

You don’t mention your climate zone, so it’s difficult to estimate how well a heat pump will work for you.

Regarding energy use, all your heat source is doing to maintain a temperature inside your home to to put as much thermal energy into your home as is lost through the walls, windows, and any air leaks. If you put in more heat than is lost, the temperature rises. If you put in less heat, the temperature inside drops.

It will take the same amount of thermal energy regardless of the source, with the exception being that heat pumps move heat instead of creating it, like an air conditioner in reverse. Where heat pumps don’t work efficiently is where the outdoor air temperature is so low on average that the heat pump has to use electric resistance auxiliary heat instead of running the heat pump part.

1 liter of propane contains 23,700 BTU. 1kilowatt hour of electricity is equivalent to 3,412 BTU. Assuming you have a 90% efficient propane furnace, you get 21,330 useful BTU per liter of propane, so one liter of propane is equivalent to 6.25kw/h. That means it would cost 86.6 cents to get the same amount of heat from electricity at your rate as 69.8 cents of propane would buy. Propane heat is cheaper than electric resistance heat in your case. Note that a heat pump will be more complex to compare, and may well be cheaper on average to operate over the entire year.

Another thing to think about is that electric rates tend to be more stable over time than propane rates, so propane is less predictable in terms of potential future energy costs than electricity.

Bill

4. Expert Member
| | #5

Using Bill's numbers, a properly sized cold climate air source heat pump will operate above 2 COP even in -15C weather and above 3 most of the shoulder season, the energy cost for a heat pump be about 2/3 to 1/2 of propane price.

As a bonus, heat pump is also a high efficiency AC in the summer.

Propane makes sense mostly if your power is unreliable. Even then, a wood stove is much cheaper to run and better for ambiance.

The way solar prices are headed, going with a fossil burner is making less and less of a sense in terms of long term cost.

5. | | #6

Using Bill's numbers, if you assume a COP of 3 or so, a heat pump is more than twice as efficient as propane.

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