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Can I use tankless WH for DHW and low temp radiators?

aMogaxXBRg | Posted in Mechanicals on

Any thoughts on using a tankless HW heater for combination DHW and low temp (120º) radiator space heating?

Reason question arises: I have a multi-family project with small unit sizes (500-1100sf) with tight envelope and high insulation levels and would like to have individually metered gas/electric to units. Heating loads are minimal, demand for HW is dominant. I would like to use more efficient, demand-driven system for DHW (tankless) and space heating as well, but every pairing I have seen with an hydronic AHU (Rinnai/Rheem) far exceeds the Btu/hs required for my space heating loads (as low 4500btu/h peak), even at the lowest flow rate and temp (100ºF and 1.5gpm) across the coils.

So, I thought of low temp radiators (perhaps with DHW priority valve or even indirect storage tank or heat exchanger for the space heating loop, if required) coupled with the tankless heater. Made sense to me (especially since I could forgo adding ducts for space conditioning) except I cannot find any information on it. Seems common in the UK (or at least not uncommon, based on internet postings) but not in US.

Any thoughts?

I am about to “give in” and use mini-split ASHPs (it is retrofit project) for heating and cooling but do not think they will work optimally in winter conditions (mixed humid, mid-Atlantic climate) and just have the tankless heaters for domestic water heating.

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Replies

  1. Riversong | | #1

    Tankless, or instantaneous, water heaters rely on a high delta-T between inflowing water and the heat exchanger surface for quick energy exchange. So, while they can be somewhat efficient in certain applications for heating domestic water they are not well suited for space heating where the return water is not much cooler than the outflow.

    And they require dramatically oversized burners in order to provide sufficient quantities of hot water with very little dwell time, so they are not sensible for low heat load space heating.

    Either a small modulating boiler or an efficient tank-type water heater like the Polaris makes more sense.

  2. Robert Orr | | #2

    Thanks, Robert. That makes sense. I have seen configurations with a brazed plate heat exchanger used to isolate the closed loop radiant system from the tankless WH. However, as you noted, the returning water temperature for the tankless system is likely still too high for efficient heat exchange.

    Question about the Polaris: are there other manufacturers that make a similarily efficient gas storage WH? Any schematics showing a recommended interface to use the system for both DHW and space heating?

    Thanks for the help. I have read several discussion threads on this site before but never posted myself. It is a TREMENDOUS resource.

  3. Jesse Thompson | | #3

    If you're in the mid-atlantic with expectations of a cooling load, the air-source heat pumps deserve another look. If you or your tenants will be expecting AC it typically makes economic sense to provide it from the beginning and provide your heat from the same source. This allows easy individual metering, low first cost, and the ASHP units are only going to improve with time in efficiency and low temp capacity.

    The modern air-source units will provide plenty of heat in your climate in a low-load building. If they work fine in Canada, you'll have no problems in the mid-atlantic.

  4. Robert Orr | | #4

    I think your reasoning makes sense, Jesse. And yes, I would not want to do without AC in the building or tenants would simply install window units. As you said, the ASHPs have much to recommend them from many different vantage points: first cost, operational simplicity, ability to meet low heating loads. Anyway, the only (code compliant) way I have found to add a space heating loop to DHW tank is by using a heat exchanger (plate, eg) with a separate pump and closed loop for the low temp radiators. Does not seem worth the extra cost and complexity, given the low loads. Money would be better spent reducing heating loads further (better windows, more insulation, superior air-sealing). Thanks for the advice.

  5. NRT.Rob | | #5

    If the load is very low, you might do this without the upcharge for a massively efficient tank heater. Basic tank heaters typically operate 75% to 80% efficient in a heating application as the standby loss component becomes a smaller and smaller percentage of the overall load (relative to its percentage on tank heater EF ratings, which is based on a very small amount of usage).

    I would look at the ASHPs but if a hydronic solution may make better sense, you can do DHW and space heating for a very small load easily with a mid range tank water heater, which sure is cheap.

    by the way you note high insulation and good construction, then note a 45kBTU peak load for 1100 square feet. those statements do not match very well. I've never seen an average heating load of 40BTUs/sq ft in modern construction. I normally guess blind at 20 BTUs/sq ft for typical construction and if you're not counterbalancing your tight construction with tons of glass then you might even be closer to 10 BTUs/sq ft. Did I misunderstand what you were saying?

    IF DHW demands require more "oomph", then the bigger efficient water heaters like the Phoenix Evolution can do this as well (which has the heat exchanger and tank side pump built in).

  6. Robert Orr | | #6

    Thanks for that response. I had not seen the Phoenix line of products. Very high performance. I will look into the combo space/water heater.

    As for the peak BTU heating requirements, the 4500BTU/h was for the 500sf unit, which means a heating load of 9 BTUs/sf - so slightly below the 10 BTUs/sf figure you indicated above. And that is during peak heating demand, not average usage.

    The units are in a row house (so parti walls reduce envelope load). However, the exterior walls are solid brick (ie, no insulation), but the plan is to frame out interior face of exterior walls and use sprayed insulation. Looking at Air-Krete, but do not know anyone with much experience with it as an insulating product. That is a whole other line of research...

    Still, I am just getting a feel for the Manual J calculation process and, especially since I built my own spread sheet, there is a good chance something is off!

  7. J Chesnut | | #7

    NRT.ROB note that Robert Orr said 4500btu/hr not 45kBTU.

    Tankless, or instantaneous, water heaters rely on a high delta-T between inflowing water and the heat exchanger surface for quick energy exchange.

    This is the first I heard this. Would this mean that for a solar hot water setup a storage tank with a backup heating element is more efficient than using a tankless to boost water temp?

    Mr. Orr great to hear about a high performance multi-unit project.
    I don't know about their efficiencies but tankless units are commonly used in Korea for both DHW and space heating in high rise apartments. Mini splits are common as well.

  8. J Chesnut | | #8

    (I guess I don't know how to use the HTML tags.)
    "Tankless, or instantaneous, water heaters rely on a high delta-T between inflowing water and the heat exchanger surface for quick energy exchange."

    This is the first I heard this. Would this mean that for a solar hot water setup a storage tank with a backup heating element is more efficient than using a tankless to boost water temp?

    Mr. Orr great to hear about a high performance multi-unit project.
    I don't know about their efficiencies but tankless units are commonly used in Korea for both DHW and space heating in high rise apartments. Mini splits are common as well.

  9. user-659915 | | #9

    "Seems common in the UK (or at least not uncommon, based on internet postings) but not in US."

    Tankless units providing DHW and supplying hydronic radiators for space heaters have been more than common, in fact pretty much universal in the UK for the last thirty years. Don't know why it should be problematic here. Nevertheless it would seem to make more sense in this case to use a heat pump system, whether conventional or ductless, in order to integrate the A/C.

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