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My house has redundant systems for both domestic hot water and hydronic heating (baseboard and radiant floor). I can pick and choose which system I'm running. We also have several mini-split heat pumps with solar making our average $/kwh around $0.129. My goal is to pick the cheapest system(s) to run this winter.

  • System 1: Buderus oil boiler with accompanying hot water tank (87% AFUE).
  • System 2: Baxi tankless propane boiler (95% AFUE)
  • System 3: Mini-split heat pumps (Fujitsu -15)

We live in Zone 6 climate so it gets pretty cold here.

So heres my question to you all. If we are relying on our heat pumps for most of our heat does it make sense for us to use the tankless propane or oil boiler for DHW and/or backup heating?

There are a bunch of articles on how to compare propane to oil such as this one. However, none of these articles factor in the additional potential efficiency gains you get from a tankless. According to energy.gov they claim anywhere between 8% and 34% gains for DHW. I haven't see any such claims for heating.

My gut tells me I should run the heat pumps and use the tankless propane for both DHW and backup heating. However, using todays fuel prices in my area the math suggests it would be nearly half the price to run the oil boiler for DHW and backup heating.

- Oil Propane
$/gal $4.10 $4.87
BTU/gal 137381 91452
$/100k BTU $2.99 $5.33
$/100k AFUE BTU $3.43 $5.61

Does the tankless propane boiler provide any additional efficiency gains over the oil boiler that would make these more comparable options? To be even remotely comparable the tankless propane would need to be nearly 60% more efficient which sounds unlikely but maybe that is the case since it would be just for DHW and backup heating.

Edit

  • Included AFUE and updated prices
  • Specified heat pump type and solar info
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    Factor in the efficiency of the specific fuel-burning devices as well for a proper comparison. Unlikely to make much difference at those prices, but if doing the math, do all the math. And you haven't factored in the heat pumps at all (where the efficiency being well above 100% (yes - Coefficient Of Performance - look it up) tends to make the math interesting. So you might do oil DHW and Mini-Split heating. My electricity isn't even cheap and it still wins at my fuel prices. "Pretty cold here" mostly matters as to whether your MSHPs are designed for that (mine are, I'm colder than you) or not.
    – Ecnerwal
    Sep 30, 2022 at 16:16
  • Good call, let me double check the efficiency of each system. I had just kind of assumed they were comparable. I left the heat pumps out because we'll be using those regardless. We have solar so they are by far the most efficient heating system. This is mostly down to DHW and backup heating.
    – anders
    Sep 30, 2022 at 16:22
  • Heat pumps are Fujitsu -15 extra low temp units
    – anders
    Sep 30, 2022 at 16:25
  • If you have solar and you have adequate power, boring old resistance electric heat for DHW may make sense. It's (IMHO) a bit nutty to use a heat pump water heater (extra complicated and expensive with limited lifetime) in a climate where you're heating the house more than cooling it, since the HPWH is just making more work for the Heating heat pumps,, and options that don't do that are Very Prohibitively Expensive every time I check them (ground source water-to-water, CO2 cycle outside air to water, etc...) - theory good, pricing bad...
    – Ecnerwal
    Sep 30, 2022 at 16:32
  • Some places do have extreme rebates that change the HPWH math. In my area the rebates are not much and have been politically tied to "Bought from & installation by blessed dealers" which means the prices for the unit are jacked up and the prices for installation are jacked way up, so they are useless .vs. just buying one and installing it yourself.
    – Ecnerwal
    Sep 30, 2022 at 16:39

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Typically a crummy furnace is 80% efficient and a good fan-forced condensing tankless is 94% or so, but you really should look up the ratings for the specific fuel burners you have, not hand-wave about it.

At those relative prices, propane is expensive, even if more efficient.

But you have not mentioned your delivered per kWh costs for electricity and what heat pumps you have, nor specific models for the fuel burners, so all is vagueness.

I run heat pumps with no backup (eventual wood boiler planned) in a colder place than you. The efficiency drops a bit at extreme low temperatures (defrosting the outside coils a lot) but is an overall win year round, works at -15°F (because it's designed and built to) and is not worth taking up space for a fuel tank and fuel boiler (other than the planned outside wood I haven't gotten around to yet) for typically less than a week a year at the extreme lows. They cost less than either fuel option.

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  • I added some more information to answer some of your questions. I included costs when factoring in system AFUE efficiency. My question though is are there additional efficiency gains to be had from using the tankless in general? I assume there has to be especially if it is only used for on demand DHW and backup heating. Thats the part I'm having trouble putting concrete numbers to.
    – anders
    Sep 30, 2022 at 16:39

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