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Domestic hot water from a steam boiler: Finding a contractor

Lyell Slade | Posted in General Questions on

apologies if this has been covered here.  I tried searching but may have been using incorrect terminology.  Currently we have a coil inside our steam boiler for domestic hot water.  It worked well enough in the winter when we were running our steam heat all the time, but now we have added some air-source heat pumps (thanks for a lot of help on here on how to do this!) and are not running the steam heat much.  We would like to add an indirect water heater.  I’ve seen here and elsewhere that one can get a water tank with a coil installed that uses hot water from the boiler and that this would be much more efficient than firing the boiler every time we needed hot water.   I’ve had 3 HVAC people not that interested and even telling me it could not be done or that it was problematic.  Where do I find a contractor in the Metrowest Boston area who knows steam boilers and can tell me if I can do this or not?  Thanks!

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  1. kjmass1 | | #1

    Trying posting on, or using their contractor page. There are some diehard steam guys on there.

  2. Expert Member
    Dana Dorsett | | #2

    This can be done (it's done all the time ), but running an indirect water heater off an otherwise idling steam boiler is EXTREMELY inefficient, barely more efficient than running the tankless coil in an idling steam boiler. If you were still heating most of the time with the steam system the indirect would make economic and efficiency sense, but with the heat pumps carrying a good chunk of the load it's probably not a good option.

    For about the same money it costs to set up the indirect you can install a heat pump water heater in the boiler room, which would harvest the some of the distribution & standby losses of the boiler when it's running, and provide a decent amount of dehumidification during the summer.

    There may or may not be MassSave subsidies for the indirect, and there won't be for the heat pump water heater unless you were replacing an electric water heater, but try not to go down the road of doing the wrong thing just to chase a rebate subsidy.

  3. Lyell Slade | | #3

    Thank you kjmass1 and Dana, I have posted this question on and will search there for a contractor. Thank you Dana for straightening me out on the efficiency of an indirect heater versus the coil now that we are not really using the boiler for heat. I got this advice from a home energy auditor before we added the heat pumps. A question I have about the heat pump hot water heater is that I hear they can make the area they're in a lot colder. Currently with temps 30-40F outside, the boiler room (which is also the laundry room) is about 56 and the finished part is about 54. We don't know yet if we will have to use the boiler for heat during really cold days, but if we don't, does it make sense to keep the boiler just so that a heat pump hot water heater could harvest the standby losses? And if we did get rid of the boiler would a heat pump hot water heater function well? Thank you!

    1. Expert Member
      Dana Dorsett | | #4

      You absolutely DON'T want to idle the boiler just to feed it's standby losses the HPWH- most of that heat will be lost to the great outdoors and the slab, not the water heater.

      Yes, the efficiency of the water heater goes down with temperature, but most models also have a resistance element to operate in "hybrid" mode if the recovery times become too slow at low basement temps. But the notion that a HPWH makes the basement "...a lot colder..." is simply not true. A few degrees colder sure, but not five degrees, and nowhere near ten.

      Based on your reported mid 50s temperatures I'm guessing the basement walls aren't insulated, possibly leaking a lot of air, and perhaps the basement ceiling is insulated(?). Running the basement colder lowers the actual heat loss out of the house, but it's still worth air sealing & insulating the basement walls even if you're never going to finish off or fully heat that space. With an insulated air sealed basement the minimum temperatures won't drop much below ~50F even during cold snaps. At that room temp the slab transitions from being a net heat loss to a net heat source in most of MA, due to the low 50s temperature and high thermal mass of the deep subsoil.

      If the basement drops to the low 40s it's probably an indication that you have a lot of air leakage, and at that temperature the plumbing freeze up risk for pipes near the exterior walls goes way up, so keep track of it. MassSave will subsidize air sealing and insulating the band joist, but not the basement walls. But insulating those walls is still "worth it", and can be done as a DIY fairly cheaply if it's a poured concrete or CMU foundation. If it's rubble/brick/fieldstone the only real option is closed cell foam, which is pretty expensive, but effective. I did my 1920s vintage poured foundation in Worcester with 3" of reclaimed roofing polyiso (~R17) , and it stays above 60F all winter long, even though the uninsulated slab temp drops to about 50F by the end of winter. YMMV

      1. Lyell Slade | | #7

        Dana, belated thanks for your reply. The basement walls are not insulated. The "finished"
        part has dry wall but I don't think there's anything behind the dry wall other than the poured concrete foundation. This part of the basement is actually a little colder than the boiler room (because the boiler is adding some heat). The basement ceiling is not insulated either. During our cold snap around Thanksgiving it got into the low 50s down there. Since reading your warning about pipes freezing, I have been monitoring the temp. down there more carefully.

  4. Tom May | | #5

    Is the boiler oil or gas? I usually recommend adding a conventional gas or electric HW heater in series with the tankless in the boiler, that way there, when the boiler is being used for heat, incoming cold water is preheated going into the tank keeping the tank off or at least minimizing the on time. I usually disconnect the aquastat for the tankless so the boiler doesn't cycle just to heat the water, but leave it connected if you are currently using it for HW. Check and see if there are two differential settings on the control, one for the boiler temp. and one for the tankless. If so you may be able to lower the high limit for the heating portion if you are not using it for heat.
    Then, if you add a tank, you can turn off the boiler the rest of the year and use the HW heater on its own. Using an indirect HW heater would require you to keep the boiler running year round. The money you save not keeping your boiler maintaining temp year round will pay for the heater in a few months not to mention the wear and tear on the boiler itself.

    1. Expert Member
      Dana Dorsett | | #8

      Plumbing the electric water heater in series with a tankless coil results in condensation on the cast iron heat exchanger plates during the summer when the water temp is below the dew point temp. If the condensation is sufficiently copious it can be damaging.

      1. Tom May | | #9

        Tankless coils are usually copper.......bypasses can be installed and the tankless drained if it is a concern.

  5. Lyell Slade | | #6

    Thanks for your reply! The boiler is oil. This is our first winter with heat pumps. So far we have not used the boiler for heat. We are wondering whether to keep the boiler at all, but so far feel it might be good to have as a back up. We don't have gas to the house to if we got a conventional HW heater, it would be electric.

    1. Expert Member
      Dana Dorsett | | #10

      One of our former employees had the same situation- a recent vintage oil fired steam boiler came with the place, and he now heats his ~1700' 1920s house in Arlington MA with three mini-splits. Though he was firing up the steam boiler unnecessarily for the first heating season thinking they weren't keeping up, he has since installed wall-remotes/thermostats, which don't have the drift in temperature offsets that the mini-splits had when sensing the room air temp at the head. I suspect the offset was at least 80% of the "keeping up" issue.

      For hot water he bought a plain old electric tank, and spent the bigger money insulating his basement walls with 3" of HFO blown closed cell foam rather than a heat pump water heater, since that's where the bigger savings (and comfort) were to be had. Now the basement is warmer (warm enough to hold jazz jam sessions down there) and the first floor mini-splits are cycling less, with more temperature margin during cold snaps.

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