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Sizing a Mitubishi HyperHeat System

Brad B | Posted in General Questions on

Hi Folks:

Hoping you can help me answer some questions.   I’m located Northwest of Boston, MA and have an early 1970s 4 bedroom colonial.  About 2800 sq ft.  New Windows, doors, attic insulation.

The house never had ducting or air conditioning.  Up until now, we heated with our oil-fired boiler with hot water baseboard heat.

My wife and I pulled the trigger on a Mitsubishi HyperHeat system for the house this past fall.   We wanted to be able to have AC at last, but also wanted a more green and hopefully less expensive way to heat during winters.

Our system is large, with 7 zones.   On the second floor, we have three GL06’s in 3 of the bedrooms + one GL09 in the Master.   On the main floor we have an FS12 in our family room and a PEAD-A09AA7 in the living room.   And we have a GL09 in the basement finished room (mostly for heat in winter, with dehumidify in summer.)

We had a few choices in outdoor units.  One was to have a 4 zone hyperheat unit outside + a 3 zone.  So two outdoor units.  But in the end we opted to have a single, larger Hyperheat outdoor unit MXZ-8C48NAHZ driving all 7 zones.   (We didn’t have a lot of options for where to place multiple outdoor units, so felt 1 larger was better. Although I now understand I may have traded off efficiency by doing this.)

Anyway…reading threads on here, and excellent advice from the team (particularly Dana) I am now worried my brand new unit is potentially “oversized.”  And worry this could be (or is) causing short cycling.    It hasn’t been too cold yet in Massachusetts, but the new system does have a tendency to overshoot setpoints.  And I have had to play the “lower the room setpoint” game just to get a room to my target temp of 68deg.    Due to moderate outdoor temps, I’ve really only run the PEAD and FS units on the first floor.  But do notice the temps in these spaces slowly rises over the course of the day.   Setpoint is 67 and room gets up to 70.  (The PEAD uses a wired thermostat where the stat temp feeds back to the PEAD.)

What is the best way to know if I’m oversized?   My contractors who did the install were excellent.  But I don’t know if they did a formal manual J calculation.    I did fall a bit for the “need a head in every room” thing.  At least in my bedrooms.

Thanks!

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Replies

  1. James Howison | | #1

    Run a room by room Manual J. Coolcalc.com is pretty useable for homeowners (although word is that it over estimates a bit). You'll need to know your construction details, though, and accurate sq ft measures per room.

    If you mean "Can I find out from this equipment how much heat it generated" (ie the output prior to distribution losses"? Then no, I don't think you (or your contractor) can. I'd love to be wrong about that, but that's what I've been told. That, of course, seems bonkers: how can the computers on the equipment not know?

    You can see how much power the outside unit is using, which might show "cycling" if you are below minimum output levels. I don't know if you can find that out for each inside unit.

    I actually wonder how they figure the CoP etc, are they measuring the output externally to the equipment (using some standard test setup room sized box?, and comparing the energy in and the temp in the standard room?)

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