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Community and Q&A

Supplemental Hydronic Boiler

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. Expert Member
    NICK KEENAN | | #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.

  4. an123 | | #7

    Any update on what you decided to do? I've been making similar changes (tightening the house, switching to more electricity HVAC) but am stuck on what to replace my oil boiler with. I have single zone system with cast iron rads, used sparingly - only during the coldest days in winter. I have ductless ATAHP for most of my winter heat in Nova Scotia, and use little oil now (DHW now comes from installed HPHW tank). I am thinking of splitting and upgrading my hydronic system to 2 zones, with outdoor resets etc., and installing an electric boiler. My house is not that big - 2 floors, 1200 sq ft., currently upgrading my electric panel from 100 to 200 Amps. I'm hoping to get solar panels, but my roof is not ideal so it won't offset all of my electricity use. They are looking at installing gas on my street this year, but I declined due to a variety of factors (the state of the world, already started electrifying my house and this would be another ongoing expense). Any advice is appreciated.

  5. apsinnott | | #8

    I have not moved forward other than removing my leaky oil tank. The oil burner remains deactivated. The mini splits did a great job heating the house this winter. But as I mentioned I already did all the work to rezone the hydronic baseboards so I really want to have them provide supplemental heat. Also my bathrooms are heated from baseboards only, so relying on ambient air to warm them doesn't quite cut it. Even with all the insulation work I've done. The wife noticed they were chilly so I'm committed to a fix sooner rather than later.
    The electric boiler costs about a third of the gas or oil options (especially since I'd need a new oil tank) and I can easily DIY the installation. The question I can't answer is how much energy will it actually use and should I invest in solar and batteries to offset what would be another increase in my electric bill. I'd size the boiler to act as a redundant rather than supplemental system, so that's a 16kw unit with four 4kw elements. Electric doesn't care if it's oversized and can cycle on/off without any concern which is great for my application. I have to design it using a primary/secondary loop where the boiler is constantly flowing water through a small primary loop and then when a zone calls it cycles through that zone until it's satisfied. The electric boiler I'm looking at cant also happily operate with any of the elements deactivated to lower its output. So until it's installed and running I don't know how hungry it's going to be just to run the primary loop, whether the onboard controller will be smart enough or if I need to manage it differently.
    This led me to post an inquiry on a DIY solar forum to ask about solar arrays and batteries, as I thought with the boiler being an intermittent load, maybe a small solar array could keep a large battery bank charged for the boiler's intermittent use. However without knowing usage at all and such a large potential load, it became very expensive for a solution to dedicate to a boiler.
    So naturally, I'm now considering an adequate solar array tied to server rack batteries and a zero export inverter that can supply the boiler, both mini splits, and electric car outlets.

  6. walta100 | | #9

    Sound to me like you lost you mind at this point.

    Electric boiler power by solar with battery storage and no net metering = crazy talk

    Batteries enough for 3 cloudy winter days, I am guessing the battery room is almost 3000 square feet.

    Seems like a new oil tank will cost less than 5% of what this solar plan will cost.

    Seems unlikely your current electric service is large enough to power a boiler with a similar BTU output oil burner.


  7. yesimon | | #10

    What about a HPWH for domestic hot water and electric resistance floor underlayment heating or electric baseboards for the bathroom?

    You'll incur lots of system losses with hydronic heating for just a few bathrooms. A taco 007 uses 81W to run. SpacePak sells A2W heat pumps in the US but you're probatly already oversized with your existing mini-splits at 64k BTU.

  8. apsinnott | | #11

    Walter, I'm just following my curiosity and I have accomplished a lot in both my hydronic and mini split systems.
    The oil burner and the steel boiler are old, so with a new tank I'd want to do a new furnace.
    The solar/battery was a curiosity if I could use a zero export config to draw from the grid when needed but use solar to keep a server rack battery as the primary power source. That was a pie in the sky theory, but if I expand the concept to feed the mini splits, the boiler, and electric car charging it could be feasible to eliminate those as reoccurring expenses.
    The newest microinverters, server rack batteries, and oversizing your solar input have exciting possibilities for off-grid and zero export configurations. Utilities don't like paying solar customers, so why not store it and use it yourself later while dropping your payment to them.
    I've looked at HPWH, I don't want to match the existing mini splits efficiency profile during very cold weather. Eventually I'd replace by gas hot water heater with a configuration tied to this electric boiler, perhaps.
    All the hydronic work is done and zoned and ran really well for a decade. Just looking towards the future where oil and gas continue to rise and electric from the utility goes up too.

  9. Expert Member
    PETER Engle | | #12

    In post #7, A N asked about reconfiguring his HW system that is currently a single zone using cast iron radiators. This is a significantly different case than the OP on this thread, who has already done quite a bit of reconfiguring and has his own reasons for considering an electric boiler as backup. For A N, I would say that installing an electric boiler as backup makes no sense. The cost of reconfiguring to a 2-zone system with an electric boiler is relatively high for a system that will be used sparingly, if ever. IMHO, it would be far easier and cheaper to simply use electric heaters for extra heating capacity on the coldest days. An electric boiler with outdoor resets, repiping the radiators to separate zones and all the associated work would certainly cost over $10k. A few plug-in electric heaters can be had for less than $100 and the heat is produced at exactly the same efficiency (100%) as the electric boiler. Plus, you could get rid of those ugly space-consuming cast iron radiators for good. I really just don't see any use case where an electric boiler would make sense for this person.

  10. an123 | | #13

    Thanks very much for the update, and your input Peter. I agree, the cost of installing a 2-zone system doesn’t make sense for my situation. Thanks for the confirmation.

  11. system_smart | | #14

    @Apsinott, have you already reviewed John Siegenthaler design concepts for low-temperature hydronics for net-zero homes, or the recent-ish idronics30 details? Image courtesy of Siegenthaler. Given you interest to electrify, maybe could you modify the mod/con+ for a SAHP solar-assisted heat pump, or a conventional HPWH @140-145F winter temps. Mixing valve can modify temps for domestic use, while indirect tank can buffer and hydraulically separate for your zones as backup or supplemental. Seems to reuse most of your infrastructure while only adding a sip of electricity use.
    What am I missing from your existing design or goals?

    1. apsinnott | | #15

      Thank you for that reference. That's a great setup and what I am striving to achieve while also doing what you recognized as important, reusing components of my existing infrastructure.
      I recently reached the completion of a stage of my install, entirely running off my mini splits and the added electric boiler for hydronic heat below a temp threshold. The oil burner and tank are gone. I still have a gas hot water heater for domestic. I leave the mini splits on all the time but they slow down below 45 degrees outside air temp so my warm weather shutoff enables the electric boiler which quickly heats the room when a separate thermostat calls for heat. The electric boiler has an outdoor reset to modulate temp, 4 elements to stage heating and my circulators are delta T models. It's very efficient.
      I actually just got my bill and I reduced my year over year electric usage for February by not relying entirely on the mini splits. As I track my usage going forward, the next stage is considering running off solar (and perhaps battery, but yes I know $$$, maybe DIY is possible) while planning to move domestic hot water into the electric boilers demand loop.

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