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Using an HRV to create mixing between zones of ducted mini split HP system

asahopkins | Posted in Mechanicals on

We are in greater Boston, climate zone 5. We are insulating and sealing up a 1938 Cape, and expect the heat loads to be about 12 kBTU/hr for each of the first and second floors, and about 6 kBTU/hr in the basement, for a total of about 30 kBTU/hr. (We’ll have an expert run the Manual J soon, and he’ll help with the issues discussed below as well, but I’d love to tap the collective knowledge in this community.) We’re switching to cold climate heat pumps and off the oversized gas furnace. The duct system for the old furnace is pretty (well, really) poor,  so we’re going to junk it and get a system designed to match the heat pumps.

Our challenge is that the upstairs is carved up into lots of bedrooms (which is great because as a result we have 3 bedrooms and 2 offices), but it means the load in any one room doesn’t support a mini-split head. So, we’re looking at a Mitsubishi MVZ series air handler to serve a real duct system in the (encapsulated/conditioned) attic space, and a similar system in the basement to serve the first floor and basement. You can’t run a ducted indoor unit on a one-to-one basis with an outdoor one (for a reason I don’t understand), so these two units would run off a single outdoor unit.

My question relates to finding ways to get relatively efficient (modulating, rather than cycling) behavior at warmer, shoulder season temperatures. At 50 F, for example, load for the whole house would be about 10 kBTU/hr. The MXZ-4C36NAHZ (and all of the other MXZ sizes of relevance) all turn down to 7,200 kBTU/hr, so as a whole-home system it should be in modulating mode. However, the MVZ air handlers don’t turn down far enough to split this load nicely for the loads in each zone (I have gathered — there are no good specs posted anywhere I can find). So, the only way to be in modulating mode for indoor units in low-load times would be to have only one zone (say, the downstairs) run, and the other not. 

A single zone, with shared ducts around the whole house, wouldn’t have this problem, but there is no good way to get well-designed ducts to all of our room without ripping out walls, etc – not going to happen. (We see some benefit in having two zones for heating/cooling, but it’s not really necessary; it’s just impossible.)

But, could we design an HRV system so that it would spur enough exchange between the downstairs and upstairs in these low-load times (by adding to natural exchange via the stairway) to maintain comfort throughout the house? I can think of a couple places in the house where we could sneak a duct from an attic-based HRV downstairs (such as through the “chase” left down the middle of the now-defunct chimney), to try to link the zones in this way. I haven’t been able to find good guidance or examples anywhere about using an HRV in this manner, to create mixing between heated and un-heated portions of the home.

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  1. joshdurston | | #1

    If the temperature Delta isn't very big it usually isn't feasible to move enough air to make an appreciable difference in temperature.
    I'm not sure the units actual work the way you've described as far as modulation. I think the indoor units will basically cycle at their rated capacity no matter what the load is.
    You can get 1:1 lower static ducted units, but the system had to be designed with low static pressure, usually short runs with the unit bring placed in or next to the spaces served.

    But in the shoulder season, warm air wants to rise so I suspect your upstairs will be fine and the downstairs unit will do most of the work. The upstairs unit won't have to start heating until it gets colder

  2. asahopkins | | #2

    Some additional thoughts and research since the original post.
    1) There doesn't seem to be a really definitive answer about how the MXZ/MVZ combos work in modulating or low-load situations. The best thread here appears to be this one: Recent posts there indicate that some modulation is possible, but perhaps not continuous. Perhaps someone could get Mitsubishi to produce something written about how this works?
    2) I found this article about how stairways pull downstairs heat up the stairs, resulting in the first floor carrying a larger fraction of the heat load than you'd imagine:
    [This aligns with my experience in a previous home, which has forced air gas downstairs and electric baseboards upstairs. Bill analysis indicated that 40% or so of the heat load in the upstairs rooms had to be being met by the gas furnace -- the electric bills were too low to be actually carrying those rooms entirely.]
    So, could a downstairs 1.5 ton MVZ air handler possibly carry the whole house down to 40 or so outdoor temp (building heat load ~15 kBTU/hr), and then we can start up the upstairs unit to split the load? Some cycling is fine, of course, so maybe the crossover point to starting up the upstairs unit would really be in the high 40s. This aligns with Josh's conclusion, I think.

    Another option might be to add a bit of heat to the HRV flow via a duct heater, during shoulder seasons (and have it there in case of extreme cold, to help the HPs out) -- the HRV would effectively carry the house (and especially the second floor) during the warmest bits of heating season (October and May), and help out upstairs when only the first floor unit is running.


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