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Community and Q&A

Supplemental Hydronic Boiler: Electric or…

apsinnott | Posted in Mechanicals on

I’m looking for advice on a supplement boiler for my 4 zone hydronic heating system. This is my secondary source of heat as for the last few years I’ve been using my LG mini split to as the primary way to heat and cool my home on Long Island, NY.
The hydronic system was originally a very old 2 zone system connected to an old oil burner. I modernized the zoning myself by splitting it into 4 zones for 3 floors (separate zone for bathrooms) with new baseboards and their own circulators controlled by a Taco switching relay with a Taco boiler reset to the old burner. This was my primary source of heat for several years before I installed the LG mini split. As a secondary system, it worked well for the coldest days and if I wanted to make the bathrooms extra toasty on a cold morning. The outdoor reset and zoning also vastly decreased my use of oil to the point a tank would last 2 years. I knew the old oil burner’s days were numbered. However it turned out the oil tank has failed first so I want to rip out the oil burner and replace it with something modern.
As I went along I modernized my home’s insulation with Low-E windows, R-15 exterior walls (most of the house is brick siding too), R-30 ceilings, at least R-60 in the attic, an insulated basement using R-10 foam and R-15 walls. I’ve done my own Manual J calculation with an estimate of 50k BTUs for my 1850 sq ft. The mini split is actually two Multi F units (30 & 34k BTU) with 4 indoor units each which I have distributed throughout the house. I am also upgrading the mini splits with drain pan heaters to allow them to heat from 14F down to -4F which should cover my climate zone all year.
I connected my hydronic system manifolds to the oil burner with couplings so it’s practically a plug and play installation to change the burner. As you can understand it’s really a standby system for the coldest days and will be called on even less with the drain pan heater upgrade. However it’s nice to reuse the existing baseboards to boost the heat in an area, like the bathrooms and to even out the heating from the mini split wall units which don’t always distribute heat as well as the baseboard does. It’s also my understanding that mini splits lose their efficiency as the temp heads below 40F so perhaps I could setup some sort of crossover between the two systems.
I’ve looked at electric boilers and the rarer air to water mini splits. I could probably undersize the hydronic boiler since it will be infrequently used, but I like the idea of it being able to handle the whole house in the event of a mini split breakdown. I would really like to just hook it up to my Taco zone unit with its existing outdoor reset.
I have an older gas hot water heater which is why I could stomach an air to water system’s expense if it could also provide hot water. I’m not really interested in a gas burner as I’ve been told I’d need upgrade my gas line and move the meter outside. Moving to electric only for utilities would also motivate me to research and install my own solar plus battery system. However, my house is a split so roof coverage is not ideal for a lot of panels. Perhaps that’s a pipe dream.
I do all my own work after meticulous research and I’ve just found myself getting stuck trying to decide how to proceed. I can share more but I’ve written quite a bit already. Any perspective is welcomed. Thanks in advance.


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  1. DCContrarian | | #1

    When it gets cold, two things happen to heat pumps. First, their capacity decreases, they can't produce as many BTU's per hour. Second, their efficiency suffers and they use more electricity to produce the same number of BTU's as at a warmer temperature.

    An air-to-water heat pump isn't going to be a good back-up system to an air-to-air system because it will have a similar performance curve. It does help with the capacity drop simply because it's another heat pump with more capacity, but that's a pretty expensive way of doing it. Similarly, if you enjoy the comfort of the hydronic heat, there's no point in using it as backup to the minisplits if your heat source is a heat pump, because they have similar performance curves.

    At a certain temperature the COP -- coefficient of performance -- goes below 1.0, and at that point it's more efficient to use electric resistance heat than a heat pump. What's common with heat pumps is to have electric resistance coils that kick on when the COP drops below 1.0. They can also be used to generate additional capacity at low temperatures, albeit at lower efficiency. If you want to keep your hydronic system as backup capacity a resistance electric heat source would seem to make the most sense.

    1. apsinnott | | #4

      Thank you. I started with looking at electric boilers but was distracted by the high temp air to water systems (especially since I built experience on mini splits and acquired tools). But as you have outlined it clearly won’t be a good fit.
      I think I’ll plan on solar sooner rather than later to offset any fears of a larger than expected electrical bill. At least sized initially to offset the electric boiler.
      My other concern was it appeared to me electric boilers come with a duplication of what I have with my Taco control unit to power the circulators and have their own outdoor reset connections. At 50k BTU the ones I reviewed are 2-4 elements. I was concerned connecting the Taco to the TT connections might not now allow for the boiler to properly step up between the number of elements used. Am I on the right path?

  2. Expert Member
    Akos | | #2

    I would have a hard time justifying much more than a resistance boiler if it is only for backup.

    If you didn't already have the mini splits, something like a SpacePak LAHP-048 would probably be in the ballpark. Not cheap but not that expensive either if you can DIY the install.

    Keep in mind that most air to water heat pumps don't like to make water much above 120F when cold outside, so you need some extra radiator size to compensate. Since the radiators are sized for older leaky structure and minimal insulation, your upgrades might be enough.

    The simplest for you hot water is probably a heat pump water heater. Hard to justify the complexity and cost of an indirect with an extra in-line heater to bring the water up to 140F.

    1. apsinnott | | #5

      Those are nice units. I read about the low vs high temp air to water units. I tend to complete projects in stages and I had to learn everything on how to install mini splits, acquire the right tools and see if I liked it as the project proceeds. I bought one LG multi unit for one side of the house and the project grew into an 8 zone monster. But I have a heated and cooled garage and basement now, which is great. I am also looking at the heat pump water heaters but the priority now is to restore the hydronic boiler.

  3. Kyle R | | #3

    I would investigate your gas line size. If it’s big enough to run your water heater it very well might be big enough to run an HTP combo unit or 80 kbtu/hr boiler.

    1. apsinnott | | #6

      You are likely correct. I have a gas line in the boiler room which feeds my hot water heater but my service also does my gas drier, oven and stovetop. If I can replace the hot water heater and boiler with what you suggest with a direct connection to the existing pipe it may be my best fallback if I’m unhappy with my greener electric options. If I have to make pipe changes it’ll wind up including hiring someone to move my meter outside so I’m avoiding that.

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