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Community and Q&A

Mod-con boiler + indirect domestic hot water tank – How to size correctly?

Ali K | Posted in Mechanicals on

Hi There,
I asked an HVAC question a few months ago and received extremely helpful advice about sizing the boiler correctly for our Zone 6, newly renovated Vermont home with r50 ceiling and r25 wall insulation.
Everyone’s advice was helpful to not oversize the boiler, and I decided on the HTP UFT-80. The question I have is around the size of the indirect water heater that I need and how to set it up best with this boiler.

Details: We’ll have 3 zones – radiant floor heat on the first floor (1100 ft2), oversized flat panel radiators on the second floor (750 ft2), and radiant slab heat in the basement with 2″-4″ under slab insulation and 2″ edge insulation (1100 ft2 – but will run at a low temp bc this area will be used less).
The local efficiency VT group ran numbers and estimated our heat load at 27,000 BTU. (I’m not 100% sure this is correct but is probably close enough based on my calculations).

The issue:
My plumber is insisting that I bought too small of a boiler for the house size. He was advocating for a 130 BTU. Additionally, he is now saying that I should get a really small indirect water heater for DHW because he wants to set it to 180 degrees for DHW calls and he then “overheats the hot water and then puts on a mixing valve to bring it back down and thus you don’t need to heat as much water”.  This seems off to me based on all of your advice.

I had chosen a Super Stor 60 or 80 because we’ll have our family plus an accessory unit in that basement, and thus I wanted to be safe in case 2 showers and a laundry/dishwasher were running at the same time. He’s saying that’s WAY too big and that I should do a 40 and do his above solution, and also again, he’s saying that he doesn’t think the HTP 80 boiler is big enough for the house and won’t heat back up again once heat is lost.
Does anyone have any suggestions for me on how to navigate this discussion?
Thanks so much in advance!

PS Here is the link to the previous discussion from my earlier question if that helps:

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  1. Expert Member
    Dana Dorsett | | #1

    You may need to find a different plumber- this guy doesn't seem to do math.

    Under current code ALL storage type water heaters need to have a tempering valve or thermostatic mixing valve and any distribution plumbing that goes to sinks, tubs or showers, but untempered water is allowed (but not required) for laundry and dishwashers, whether storing water at 180F or storing it at 120F. (Code in many locations require the storage temp be set to 140F or higher for legionella mitigation.)

    That said, the 40 gallon indirect is probably about right, even at 140F.

    The UFT-080W serving an indirect as the "priority zone" (recommended) can run a single 2 gpm shower forever on just the burner alone. If a 40 gallon tank can fill the biggest tub in the house at a storage temp of 140F, , with an 80K boiler behind it it will also be able to cover a pair of simultaneous showers with some margin unless both showers are REALLY long. The laundry and dishwasher draws are miniscule by comparison , and don't much affect.

    You really won't need a 60 or 80 gallon water heater unless you're going for the "endless shower" experience, a six side-spray deluxe gusher shower, or have a monster spa tub to fill. And you REALLY don't want to run a storage temperature of 180F, which only increases the standby loss while lowering the combustion efficiency.

    For referenece I helped out a guy in Maine pick out a boiler replacement, currently heating his house with a 50K condensing Bosch that now thinks he should have gone with something even smaller. At the point of boiler replacement the initial design heat load was~32K (later reduced to about 27-28K), which is bigger than your calculated heat load. His original contractor was pushing him to install a 100K boiler using similar arguments to your plumber. The "...won’t heat back up again once heat is lost..." came up- the contractor was suggesting that if storm took out the power grid it would take too long to heat the place up again.

    I pointed out to him that he lived in a first-world country with a fairly reliable grid, and even if after an outage long enough for the house to drop under 40F it hardly matters if it takes 4 hours to bring it fully up to temp rather than an hour or two. Turns out the guy is running one of Maine's combined-cycle gas power generators- he bought the grid reliability argument. :-) Unless the radiation is ridiculously oversized, a bigger boiler isn't going to heat the place any faster.

  2. Jon R | | #2

    Don't know much about it, but prices I see on the cheapest indirect water heaters are higher than many standalone tank water heaters. So why not just install the latter?

    1. Expert Member
      Akos | | #7

      I'm with you Jon. Whenever I looked at the costs, it never made sense.

      If you don't mind an extra combustion appliance, can't beat the cost of a direct vent unit. Efficiency sucks, but natural gas is still cheap. They also last for around 15 years, the additional upfront cost and durability of an indirect is not worth it.

      If you don't want an extra burner in the house, the much cheaper option is either an electric tank or a heat pump water heater with a sidearm heat exchanger connected to the outlet of the boiler. This way in the winter the boiler does most of the heating and in the summer the boiler never has to run.

      With this setup, you can also always toss two or three PV panels on the roof of the garage down the line and heat your hot water for free.

  3. Expert Member
    Akos | | #3

    The installed price difference between a 45 or 60 gallon indirect and the standby energy loss difference is small enough that unless you have a space constraint, there is no reason to go with the smaller one.

    I've had issues with 50 gallon trying to fill two tubs at the same time, unlikely, but could happen in a rental situation.

    Don't listen to your plumber. With a 27kbtu load, the HPT80 is already well oversized even if you add in the load to heat water for two apartments.

  4. Expert Member
    Dana Dorsett | | #4

    A stainless indirect such as a SuperStor will last 2-3x as long as 50 gallon standalone, and a much faster recovery when operated as the priority zone.

    Then there is the condensing efficiency issue. The installed cost of a condensing tank water heater is more than an indirect, and not sufficiently higher efficiency to rationalize it. (There is effectively zero condensing going on with a target storage temp of 180F.)

    The last thing you want to do is upsize the boiler to handle the domestic hot water load- upsize the indirect, if needed, but ONLY if needed. If you go with a 40 gallon and for some reason it's not enough at a storage temp of 130-140F you can gain quite a bit of capacity by bumping the temp up, even if it's running lower efficiency at the the higher temp.

  5. Ali K | | #5

    Great, thanks so much for the responses.
    This might be a dumb question, but if I have to potentially bump the temp up above 140 for a smaller indirect, does that increase affect the efficiency of the whole heating system since I can run the heat below 140F for radiant floors and flat panels?
    I'm thinking of getting the Super Stor Ultra 60 after hearing from you all, just to be sure on capacity. Is the Ultra worth the money in your opinions?

    1. Expert Member
      Akos | | #6


      You generally want to run an indrect above 135F to control bacteria. With a small tank you can get more capacity by bumping that up to 160F/170F but that requires the boiler to put out 180F water. A modcon putting out 180f water into a very hot tank will mean pretty low efficiency (low by modcon standards, around 80%).

      The HTP can run separate temperature set-points for space heat and domestic water, setting the indirect temperature higher than what is required for your space heat won't effect efficiency. Just make sure the plumber sets it up right.

      Overall, to get the best efficiency of your modcon install, the thing you have to watch more than the output temperature is the return water temperature. This is what really effects the efficiency of the setup.

      It is very common for plumbers to over-pump a setup, there is one unit I looked at that had the output set at a fixed 160f and the RWT was 150F, essentially getting zero condensation.

      It is always a good idea to install ECM pumps (they also consume 1/2 the electricity), this way you have an easy way to adjust the flow rates down the line in case the installer doesn't set them up properly.

    2. Expert Member
      Dana Dorsett | | #8

      The temperature of the rest of the system doesn't have to rise to heat an indirect to some other higher temp. Most condensing boilers have hooks in the controls & setup to serve a higher temp to a domestic hot water zone (some condensing boilers can control different independent temperatures to several individual zones.) The UFT series boilers even have a separate port & associated controls for running an indirect water heater, minimizing the amount of external plumbing work.

      With flat panel rads and radiant floor you will be able to take advantage of outdoor reset to run the space heating zones, and it will be worth your while to take the time to fine-tune it to the absolute minimum curve that keeps up.

      The fact that your plumber seems a bit in the dark on these issues means you may have to take over and do most or all of the setup yourself. Make the installation manual your bedtime reading for awhile until you understand it perfectly.

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