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  1. #14
    Join Date
    Aug 2011
    Location
    Kingston, Ontario
    Posts
    207
    As a Canada only product, I suspect that the details of the Zuba are not widely known by the pros here. You should ask either Mitsubishi Canada or the local area distributer these questions. I dealt with the Ottawa distributor and he was very helpful.

    I don't even have our Zuba installed yet and am no expert (10 days to installation - can't wait!) But as I understand it, the Zuba is designed to run at constant 1200cfm in heating mode and 800cfm in cooling mode. I also read that max static pressure is 0.8". So perhaps at 1200cfm you are exceeding or close to the sp? I think they told me that the air handler will automatically cut back if sp gets too high. But again, ask the experts.

    I do suspect that by limiting air flow, you will be downgrading the capacity. I would have the duct capacity carefully checked. How long are the ducts? Maybe if you could post a diagram one of the pros could advise if the CFM should be limited?

    One other thought - Have you had a Manual J or similar heat loss calculation done on your home? In our case we found we needed 42k btu/hr at -25C design temperature for that wing of the house. (I bought HVAC-Calc to double check it) The balance point turned out to be at 1degF, so Zuba has excess capacity down to that temperature. If the capacity had been limited to 32k btu/hr due to lower CFM, the balance point would have been at 10F. (Balance point is point where plot of heat pump capacity vs outdoor temperature crosses plot of house heat loss vs outdoor temperature. Use say 70F for zero heat loss)

    I asked and was told my Mitsubishi that COP does improve as compressor cuts back, but no numbers were give.

    Where are you located? May be interesting to compare notes if you go ahead.

    Quote Originally Posted by R J Cedar View Post
    I am also looking at the Zuba Central single central ducted system. I am wondering what your air handler cfm will be set to? The Zuba Central is rated at 38000 btus, 40000max, but for that does it need to run at 1200 cfm?

    Because of my limited ductwork - 8 - 5" ducts and 2 - 6" ducts - I am advised that the air handler would be set to run at 1000 cfm. My understanding is that when cfm is reduced, there is a corresponding reduction in max capacity, ie. the 3 ton unit acts as a 2.5 ton unit but is this correct?

    If that is correct, then my question is what happens to COP when the air handler runs at a reduced CFM and btu capacity level?

    Your question was about how COP varies when the inverter driven compressor runs at lower than full rpm in order to only provide the heat required, not the full output. A partial load. Here, the refrigerant flow is varying / modulated. When that happens, is your CFM staying at the same level on the air handler, eg if is set to the optimum for 3 ton capacity setting of eg 1200 cfm level, does CFM also vary up and down or is it constant? So modulation is varying refrigerant flow/ constant fan speed?

    Or does CFM modulate up and down as well as refrigerant flow? The system modulates to meet demand, but I have seen no description of what modulates, when. So any clarification on this would be appreciated.

    In my case, if the air handler CFM is set down to 1000 cfm (or 900 cfm) due to restricted ductwork, and consequent total btu capacity (if I understand this right?) reduced to a 2.5 ton unit btu level, does the same COP table apply at the lower cfm settings?

    It would be interesting to know what the cop effect is when the zuba system is set to operate at partial btu capacity due to restricted ductwork. If the effect is negative, there would be a case to upgrade ductwork to 6". I would think that improving ductwork capacity would also mean less stress on the system, and less maintenance.


    Thanks for any help.

  2. #15
    Join Date
    Sep 2011
    Posts
    36
    Hello Freeagent,
    I phoned a distrubutor and learned a few things, good tip.
    Cfm can be set down with a resulting lower btu capacity, though not a fully proportional btu decline.
    Contractor says for my limiting ducts, he would run system at 1000 cfm. Run lengths range from 1 to 14 feet. Your illustration of balance point shift is what I was thinking would happen. I should do a hvac-calc too.
    I am not understanding how to frame hvcalc results. When you say your heat loss is 42000 btu/hr at your design temp and bal point is 1 degree F, this is running zuba at rated 38000 btu or full 40000 btu capacity up at 1200 cfm. So at 38000 btu/hr supply and 42000 btu/hr demand is it that one just tries to get the supply and demand values as close as possible, in sizing the system, and then you just work with the balance point you get?

    I was concerned that running cfm at 1000cfm would do 2 negative things:
    - this still may be noisy with too high velocities in some ducts
    - my balance point would be raised higher as in your example, so that in turn means in a cold winter the auxillary heat would have to come on more than it would otherwise. But how frequently I have no idea.

    Figuring all this out sounds does sound like a software calculation - something not automatically brought up and offered, so far in my inquiries.

  3. #16
    Join Date
    Aug 2011
    Location
    Kingston, Ontario
    Posts
    207
    Quote Originally Posted by R J Cedar View Post
    Hello Freeagent,
    I phoned a distrubutor and learned a few things, good tip.
    Cfm can be set down with a resulting lower btu capacity, though not a fully proportional btu decline.
    Contractor says for my limiting ducts, he would run system at 1000 cfm. Run lengths range from 1 to 14 feet. Your illustration of balance point shift is what I was thinking would happen. I should do a hvac-calc too.
    I am not understanding how to frame hvcalc results. When you say your heat loss is 42000 btu/hr at your design temp and bal point is 1 degree F, this is running zuba at rated 38000 btu or full 40000 btu capacity up at 1200 cfm. So at 38000 btu/hr supply and 42000 btu/hr demand is it that one just tries to get the supply and demand values as close as possible, in sizing the system, and then you just work with the balance point you get?

    I was concerned that running cfm at 1000cfm would do 2 negative things:
    - this still may be noisy with too high velocities in some ducts
    - my balance point would be raised higher as in your example, so that in turn means in a cold winter the auxillary heat would have to come on more than it would otherwise. But how frequently I have no idea.

    Figuring all this out sounds does sound like a software calculation - something not automatically brought up and offered, so far in my inquiries.
    Firstly, I am no expert and the following was only learned by talking to the Mitsubishi distrbutor!

    What I gathered, was that the capacity data (quoted in my original post) reflects the MAXIMUM output of the Zuba (I obtained those figures from the Zuba submission datasheet) Max capacity stays close to 38k btu/hr through most of the normal temperature range. But the required home heating load drops from a maximum at the design temperature (37k btu/hr - case in attached graph at -23.3C ) to zero at 20C.

    The outdoor unit has an inverter drive and it cuts back the compressor so it only provides just the heat that is needed. This would be the case for all temperatures to the right of the balance point. To the left of the balance point, additional heat would be required (the lower curve is for a Carrier unit I was considering). If you lower the cfm to 1000cfm, the capacity curve (see an example attached) will be lower and therefore the balance point would move to the right (higher temperature).

    Compressor will run continuously until heat requirements drop to the Minimum for the Zuba. After that it will cycle on and off .

    If you determine your design heat load, you can easily make up a similar graph for your home and determine the balance point. You will have to make an assumption as to how much Zuba output will be reduced at 1000cfm. You can then estimate when supplemental heat will be required (temperatures to left of balance point)

    Regarding noise - I changed the registers in our home to ones with more open area and checked the cfm per room using HVAC-Calc. We are going from 2 1/2 ton A/C only to 3 ton Heat pump, so had similar concerns regarding noise.

    It seems to me that this is only theoretical and does not account for heat losses or leakage from ducts. I am working on those and anxious to see just how well Zuba will perform!

    By the way, I have the Zuba submission data sheet and the installation manual, I could email those to you if you wish.

    PS: The attachment is not our final design - just the cleanest graph I could find. Our Design heat load is higher. In that example, I was looking at affect of some other home upgrades.
    Attached Images Attached Images  

  4. #17
    Join Date
    Sep 2011
    Posts
    36
    Thanks for your reply.
    I have a document to email to you, will do now.

  5. #18
    Join Date
    Aug 2011
    Location
    Kingston, Ontario
    Posts
    207
    Quote Originally Posted by R J Cedar View Post
    Thanks for your reply.
    I have a document to email to you, will do now.
    It seems I used an incorrect email address when registering. It has now been corrected. Wanted to send you a pm or email, but don't know how on this forum.

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