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Help with ccASHP model for CZ7A. Mitsubishi Zuba PUZ/SUZ or Tosot Apex?

Andy2022 | Posted in Mechanicals on

We’re doing a deep energy retrofit/rebuild of an older home in CZ7a (design temperature of -26c/-15f). Home is a bungalow – 1400ft2 on the main and 1300ft2 in the basement, with upgraded insulation to ~R40 and triple glazed windows.

The HVAC system will be a centrally ducted system reusing much of the existing duct work. The calculated heat load is around 18,000-20,000BTU/h at the design temperature – 18,000 with ACH50 of 1 and 20,000 with ACH50 of 2.5, so we are aiming for the 18,000BTH/h figure. The current furnace is a 60,000 BTU mid efficiency gas unit which will be removed and replaced with the heat pump and backup resistive heat.

We have spoken to our builders HVAC installer and unfortunately he doesn’t have a huge amount of experience installing ccASHP so it’s fallen to us to help with the equipment specifications.

He has installed Tosot Apex units before (rebranded Gree Flexx units according to a poster on here) and we are looking at them as an option – the HVAC installer says they are a lot cheaper than Mitsubishi Zuba units and may perform slightly better at colder temps (rated to 78% capacity at -30c).

The Tosot Apex unit would be a TUD24-36AADU with a rated capacity of 24,000BTU/h and output of 22,000BTU/h at -26c (see attached), so a little over capacity for what we need, but 24k is the smallest option.

We’re getting stuck on the Mitsubishi Zuba Central units. The primary question is whether the PUZ or the SUZ range are a better choice for our usage scenario. Once that decision is made, from what I can tell we should be looking at either an 18k BTU/h model or a 24k BTU/h model. Unfortunately most of the information available only goes down to -15c/5f so it’s difficult to see which would best fit at our design temperature. I’m assuming the 18k unit will be a bit undersized, and the 24k unit may be about right…?

We’re not too worried about cooling as we don’t currently have an A/C unit (summers rarely get above 25c) and the unit will primarily be used for heating.

Any help or recommendations on this would be much appreciated.

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  1. maine_tyler | | #1

    I don't have any answers for you, but have you seen this series of articles by Jon?

    Have you done anything to verify that your existing ductwork is adequate for a mini split?

    1. Andy2022 | | #2

      Hi, thanks, yes I have read through that series. It's definitely useful.

  2. Andy2022 | | #3

    We've decided to go with a Tosot unit. Now it's a case of debating with the HVAC installer as to what should be installed.

    He's recently sent me the submittal for the APEX 36k unit TU36-24WADU (outdoor) and TUD36-24AH2ADU (indoor). He's arguing that's the one we need and as it's a modular system it'll be fine. From the submittal it claims a min rated capacity of 18,000BTU and max capacity of 38,000BTU, which to me seems WAY too high for a house with a design load off 18,000BTU at -16f (-25c). It seems to be one of the few models NEEP actually have data for below 5f (!/product/34231/-16/18000/MT/727770/1) but it shows it having a greater turn down ability (down to 9,800BTU) than the submittal. According to NEEP it'll have 90% annual low load cycling, which seems bad? From my understanding that may significantly reduce efficiency and increase running costs, as well as reduce comfort in the home?

    Tosot do a 24k BTU unit (mentioned in the first post) but their public information is a bit patchy. It looks like it may be the same exterior unit, but a slightly different interior unit. Both appear to have the same capacity at design temp (~18kBTU), but the 24k looks capped at 24k BTU.

    He also sent some information about backup resistive heating. The smallest one they do is an 8kW system, but he's recommending a 10 or 15kW (50k BTU?) system instead. Am I right in thinking both those are WAY overkill for an 18k BTU home.

    1. Expert Member
      Akos | | #4

      The problem with the dual range units is that it is still a 36k output compressor even when set to 24k. At the lower setpoint efficiency is better but minimum output is the same, something with decent turndown at 36k starts looking not so great capped at 24k.

      If you are looking for detailed info on the Mitsubishi units take a look at:

      The engineering manual will have output data including modulation range down to -13F. The P series do go a bit lower but tend to be more expensive. I think you are better off with the M series with a strip heater sized to carry your place in case it cuts out during a cold snap.

      You do want to get the sizing correct, cycling on a heat pump does kill efficiency. I have run into this with a heat pump with low turndown. When it is running within its modulation range, energy use tracks outdoor temperature. Once weather warms up and the unit starts cycling, the energy use flattens out even as the outdoor temperature rises. 90% cycling is not a good start.

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