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Community and Q&A

Adding Heat Strips to a Heat Pump

irene3 | Posted in General Questions on

We have a ducted Mitsubishi heat pump (not hyperheat) that was installed a few years ago. There’s no backup heat source and we had the impression that it would be easy to install heat strips later if it turned out it couldn’t keep up during the coldest weather. (We are in Seattle and generally get only a couple of weeks below freezing, if that.) The last couple of years we have had a few uncomfortably chilly weeks in January or February, so decided it was time to spring for heat strips.

Unfortunately it turns out the strip heater that’s compatible with the air handler goes on top rather than inside, and the original installer is not sure he can make it work with the amount of clearance we have available. It’s six inches tall and we have about 9.5 inches of clearance. So he’s suggesting installing separate electric wall heaters.

It seems to me that for something we really only need a couple of weeks of the year, if we can’t have a centralized solution, it would make just as much sense to use good quality portable heaters. My husband thinks a centralized solution has to be possible and we should look for a different Mitsubishi specialist. What say you all?

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  1. Expert Member
    AKOS TOTH | | #1

    Post a picture of your setup. 9" clearance for a 6" unit sounds like you have space to spare. The heater can be installed further downstream in your main trunk as well, it doesn't have to be right by the air handler.

    1. irene3 | | #12

      I would be interested to hear more about the possibility of installing the heater downstream, if you have time. Is that something Mitsubishi allows for, or would there be a problem with the warranty? Thanks again.

  2. irene3 | | #2

    Hope these are helpful.

    1. Expert Member
      AKOS TOTH | | #4

      Your HVAC person is correct, it won't fit there. There is enough space vertically but that side takeoff is in the way and it doesn't look like it is easy to re-route.

      Your dollars might be better spent improving your building envelope instead of throwing more heat at the comfort problem. Most comfort issues are caused by air leaks, a bit of blower directed air sealing can go a long way in fixing these.

      For example the basement in my home was so cold you could not be down there for any length of time in the winter. Replacing the leaky single pane windows, sealing off old unued chimney and air sealing the rim joist turned it into a decent workable workshop space. Not perfect, it is still uninsulated, but infinitely better. The nice side benefit of this work was the floors of the main level are much warmer, areas that were too cold before are now perfectly fine.

      1. irene3 | | #5

        I have actually done a fair amount of work on air sealing and insulating the upper walls of the basement, which has helped a lot. The roof insulation is in good shape and the newer parts of the house (we did a major addition about fifteen years ago) are not bad. Our house is really quite comfortable except for those few coldest weeks. Obviously there's more we can do down the line (we have about a dozen windows, mostly 1980s vintage and a couple of 1901, that need replacing for cosmetic reasons anyway, and there's one bedroom we really should gut and redo, including putting in insulation).

        I am personally leaning toward oil-filled radiator-style electric heaters, as they're no less efficient than the heat strips are, and as far as I can make out don't cost much to run (given we have pretty cheap electricity).

        It's also possible that our ductwork is less than ideal. The original installer suspects airflow may be the culprit rather than heat. I am not sure I buy that given that it seems just fine the rest of the heating season.

        1. mr_reference_Hugh | | #8

          I suggest reading this GBA article

          It appears to be relevant for all heat pump installations.

        2. Expert Member
          AKOS TOTH | | #9

          That joint on top of the air handler is no the pinnacle of low restriction especially the side takeoff.

          If you don't mind doing a bit of detective work, it is relatively easy to figure out if flow is your issue. You will need a differential manometer available for a reasonable cost from your on-line retailers and the blower curve for the air handler.

          You can get the curve from the engineering data book (the ~600 page one) from bellow. Search for "fan performance"

          Set the air handler to max fan speed. Using the manometer measure the pressure between the intake of the air handler (after the air filter) and outlet (right where the ducting connects). You can then look at the curve from the data book for the corresponding flow rate.

          1. irene3 | | #11

            Thank you. My air handler is PVA-A30AA7. I take it that would be here?

            So the engineering book would be this:\2017_P-Series_Engineering_Manual.pdf and it looks as though my model's blower curve is on page 131.

            I am still not really clear exactly where I would measure (it sounds as though you mean inside the air handler itself, not the ducts? this is confusing, because I thought any air flow problem would be the result of the ducts having too many twists and turns) and what I would do with the results.

          2. Expert Member
            AKOS TOTH | | #13

            The pressure connections should be right at the ducting, no need to get into the air handler. You do want to get as close to the air handler as possible. I normally drill a 1/4" hole into the duct and connect the hose from there to gauge. You never want to drill into the air handler itself as you can easily hit wiring or refrigeration lines. Sometimes you can push one hose between the air filter and the air handler intake and the other through the foil tape into a gap on the top around the plenum.

            You want to make two measurements (both with the air handler at max fan).

            First is the pressure across the air handler (the pressure hose on the gauge connected to the air handler out let and reference hose to the inlet). This tells you how hard the air handler is working and how much airflow there is.

            The other measurement is the pressure drop across the return. Leave the reference connected to the air handler intake and leave the pressure port unconnected so it is reading room pressure. This tells you how restrictive is your return and air filter. Sometimes a couple of small changes such as installing a larger filter can make a big difference in airflow.

            Next you have to figure out what external static pressure your air handler is set to. This is in the installer menu on your remote, the remote manual should have instruction on which parameter to check (setting/mode 08/108)

            The air handler should be set to either for 0.3", 0.5" or 0.8". Once you have this setting you can look at the appropriate curve on P131 and using the measured pressure drop calculate the air flow.

            For example, your air handler is set to 0.5" and you measure 0.45" across the air handler.

            You go to the middle curve (the 0.5" one) on the left hand side since air handler is in up flow orientation.

            Follow along the high speed curve until you hit 0.45" pressure drop and read the flow at the bottom which corresponds to about 800CFM.

            If the pressure drop is off the curve than the air handler setting is incorrect and needs to be adjusted or if already at max (0.8" setting), the ducting is too restrictive.

            A bit too restrictive won't effect performance much, if way off it would be worth to see what can be done to improve it.

  3. Expert Member


    If you can't make it work, maybe consider plug-in wall heaters you can demount and store when not in use. Something like this:

  4. mr_reference_Hugh | | #6

    Hello Irene3, I did a ton a research before installing a Mitsubishi heatpump SUZ-KA18NA2 and air handler SVZ-KP18NA with a heat strip. The duct work is very simple and was designed by an engineer. Unless the ductwork is specifically designed for the system, and installed by a professional - who cares about the quality of their work - there is a very good chance that the issue is related to the ductwork. If you are re-using ductwork from a previous system, the probability of it being a ductwork issue increases exponentially.

    Before identifying the Mitsubishi solution, my preferred option was for radiant heat panels. Check out this GBA article about two types of radiant heating systems: 1) Radiant cover heaters; b) radiant wall/ceiling panels. The article talks about the 2nd option as now having some modulation (adjusting heat being produced), which is even better than the more common on/off type of radiant heaters.

    The article talks about the a prototype home where these were installed. The cove heaters are installed just below the ceiling. The radian panels can be installed on the walls or even the ceiling. They can be hard wired but some are plug-in. If you don't have enough place on the walls because of the furniture or house design, which was our situation, these panels can be placed on the ceiling - entirely out of the way and largerly out of view. The technology is really simple and radiant heat coming down from the ceiling would be pretty sweet. Compared to a portable oil heater, these would likely be more efficient, more effective and "out of the way".

    Hope this helps.

  5. mr_reference_Hugh | | #7
    1. irene3 | | #10

      It's paywalled (though apparently it wasn't back in the day, because I commented on it), but I was able to read the beginning and also to get back to the original Q&A. That person was looking for a much more extensive backup plan for a climate that routinely hits five degrees. I am just looking for a solution to provide some extra oomph for a few weeks out of the year (I literally mean like two, possibly three, weeks). Our ductwork was redone to some degree during the remodel, so it's mostly not that old, but it is certainly not optimally designed. Still, the air gets to the registers fine all the rest of the year. My impression is that redoing the ductwork would be an enormous expense for not that much gained. If we had been able to get a Hyper Heat system in the right size (I think they're available now but weren't then), I doubt we'd have any issue at all.

  6. mr_reference_Hugh | | #14

    I agree that redoing the ductwork would not be worth the expense. A new heat pump with Hyperheat might work and might be the most straightforward solution. Depending on your budget and your preferences, there are many options available - which is good news for you.

    I still like the option of a 120V radiant panel that can plug into an existing outlet. I am not sure if I am allowed to quote a GBA article but in my comments but this below is what i was hoping you could read, which was near the end of the article:

    "Our expert’s opinion
    Here’s what GBA technical director Peter Yost had to say:

    One of my first research projects at the NAHB Research Center (now called the Home Innovation Center) was to evaluate Solid State Heating Corp. (SSHC) radiant ceiling panels in comparison to a conventional forced-air heating system. (You can read a one-page project summary here or the full project report here .)

    The idea is that the appropriate number and size of ceiling heating panels, controlled in each room by a thermostat that sensed operative temperature (operative temperature being the air temperature plus the mean radiant temperature, divided by 2), can be operated like lights. You turn the panels on when you enter a room and turn them off when you leave. The panels achieve their operating temperature of about 165°F in about 90 seconds. Their location — either centered in a room or favored towards areas of greater heat loss (such as large windows or patio sliders) — gives the best viewing angles from the panels to room occupants.

    The panels look a bit like ceiling tiles, are about 1 inch thick, and are mounted to the ceiling, run by either 110 volt or 220 volt electrical circuits. A radiant panel provides most of the heat for my office at home (see Image #2 below).

    Do they work? In a nutshell, yes. Their advantage is room-by-room zoning and very fast on-and-off. "

  7. sk | | #15

    irene3, my neighbor is weighing going with a 18K BTU non-hyperheat Mitsubishi (ducted air handler SVZ/SUZ) with 8k watts in backup resistance heat. The advantage of the non-hyperheat unit is that it has better HSPF than the hyperheat (12.6 vs 10.4). She is thinking the efficiency hit on the hyperheat is not worth the occasional need for subfreezing hyperheat performance in Seattle. 8k watts in backup is effectively 100% redundancy if the heat pumps breaks, so definitely an insurance policy. I had always thought the backup heat was to cover just the shortfall in output capacity in subfreezing weather, not as totally redundant capacity. I assume the 8k watts can be energized in stepped increments? How did your situation turn out? Any insight/experience appreciated.

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