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Replacing a gas furnace with a heat pump, looking for some advice.

Zogy | Posted in Mechanicals on

I am replacing a gas furnace with a heat pump, looking for some advice from the community on sizing and a few other questions.

(I know this is a long post, but I tried to be as concise as possible, and still give all the pertinent details).

A summary of my questions are:

1)      Proper unit sizing (trust local HVAC contractor, or my Manual-J calculations, or neither)

2)      Zoning or not, (and bypass-duct use)

3)      True mini-split vs. Single Unit with zoning

4)      MERV-15 Media Filter or “regular” filter (Media would be in the attic, regular filter more accessible)

5)      Side question about Communicating Thermostats (worthwhile tech, or overly complex)

Pertinent Details:

Current situation: I’m running a 2-stage gas furnace on second stage only (51,000 BTU output); I do not currently have A/C.  The furnace is currently in the garage. The house is a split-level, and is located in Southern CA (Los Angeles); and is about 2100 S.F of living space.   The exterior is stucco, and there is currently no insulation (anywhere); but plan to have dense pack cellulose installed in the walls (which I think get you around R-15) and in the Attic (R-30+).  Windows are low-E.

Planning to install a heat pump to take care of both heating and A/C.

I am very limited on the location of the condenser due to property line offset requirements, so it therefore needs to be just outside of a ~45″ high masonry wall surrounding a deck (so the condenser needs to be shorter than that to remain out of sight).

I’ve gotten a few bids and only one looks like it going to make the cut.  The proposed bid includes installing a 4-ton Daikin Fit heat pump, replacing all the ductwork; and moving the air handler into the attic; with a few potential add-on’s:  Zoning (2-zones), and an “Air Quality” package (MERV-15 filter, and Aerus Airscubber).  I am thinking I would like to go with zoning (2-zones: upstairs and downstairs).

The only other option that I’ve found that fit my requirements, is a Daikin mini-split (MXS series); where I would use (2) ducted units (FDMQ), one serving downstairs (2 ton) and the other serving upstairs (1.5 ton), and a single wall unit (3/4 ton) for a large family room on the lowest level of the house.  The primary draw of this approach would be the potential to self-install (I consider myself fairly handy; but do not underestimate the effort involved, and other issues like warranty coverage etc.), assuming there is sufficient/any reason to go this route.

With that background here are my questions:

1) Unit Sizing with zoning:  The contractor proposed a 4-ton unit but did not run a Manual-J calculation.  I am assuming they sized it based on the size of the house and (hopefully) knowing what works in the area.  I ran a Manual-J calculation using an on-line tool (CoolCalc) and came up with a slightly lower number:  Overall cooling load of 38,000; and a total of ~43,000 BTU by summing up all rooms; I did a room-by-room calculation: (31,000 BTU for 1st floor, 13,000 BTU 2nd floor).  I asked the contractor about using a 3.5 ton system and they said they would sign up to that.  (I’ve read one article that a zoned system should be about a ½ ton bigger than the largest single zone, which would put it around 37,000 BTU).  I hesitate to “suggest” that they install a smaller unit and then have them use that against me should it end up being too small.   Question: What is the correct size heat pump; considering the overall output and desire for zoning?  and should I go with my Manual-J estimate and have them install a smaller unit?  (or maybe insist/pay them to perform their own Manual-J calculations and perhaps have them come up with the same conclusion?)

Zoning:   The basic question is to zone or not to zone?  In addition, they included a by-pass damper in their bid, for the zoned system.  I asked them about the by-pass since I’ve read it’s “bad”; but I can certainly appreciate why it would be needed.  Their response was it is needed to handle any additional flow if only the smaller (upstairs) zone is calling for cooling (or heat).  And that the unit is inverter-based and can reduce output to ~35% capacity (which would be 1.5 tons).  The zones are roughly split 70 (downstairs)/30 (upstairs) using the Manual-J calcs; or 60/40 using floor area.  They have proposed an electronic bypass damper; and a EWC UT3000 controller.  Question: should I do zoning, and if so is a by-pass damper really just “bad”, or are they okay/necessary if implemented correctly?  (not sure I can suggest dropping it if they want to use it; so trying to understand the implications)

Single system, vs. a Split System:  The proposed single-unit, zoned system may provide some versatility (for fine tuning) and additional capacity for worst-case days.  Whereas the “fixed” mini-split installation approach would be a “true” zoned system. Any advantages to a true mini-split system to a zoned ducted single unit installation?

Air Quality Options:  The optional Media filter (MERV-15) would be in the attic, so replacing it would involve more effort.  I assume the other option would be to have the filter installed at the return intake box, which would be accessed from the inside hallway ceiling (and would maybe be up to MERV-8-13, but likely only 1” thick?).  I suspect the added depth of the Media filter would produce less pressure drop, and would require changing less frequently  Question: should I go with a Media filter (with added hassle to replace), or a lesser filter that will likely get replaced more often (cause it’s easily and more accessible)?  And finally should I install an “Air Purifier”? does it provide a really benefit or is it snake oil; or perhaps a hold-out from the COVID-paranoia era?

 (Secondary question) Communicating Thermostat:  The Diakin Fit uses (exclusively) their Communicating Thermostat.  The net is fully on contradicting opinions on Communicating Equipment; the contractor indicates they put in lots, and haven’t had any issues.  Assuming I don’t go with a mini-split option, it kind of comes with the system, so it may be a moot point.  Question: what’s the verdict on communicating equipment?  Wave of the future, or proprietary headache?

I know that’s a long post, and appreciate anyone who reads through it.  Any words of advice would be greatly appreciated on any topic you care to comment about.  Wife wants the A/C in before the Summer heat arrives, so I need to make a decision soon.   Thanks in advance for your comments.

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  1. Expert Member
    Akos | | #1

    I would start by running through the calcuations here based on the gas usage of your fuel burner:

    This will get you much closer real world heat loss numbers than a ManJ which is hard to get right for older buildings.

    As a side note, stucco is not an easy finish to insulate behind. Lot of older installs miss the require WRB and flashing details which can cause issues with water and mold. I would check first before committing to dense packing the walls. Sometimes the simpler option to leave it as is until major reno or exterior work is needed.

  2. jasonknowles | | #2

    1a) I would strongly suspect the contractor sizing is based on your current furnace size and apparent satisfaction with it's performance. I would definitely go with a correct load calculation sized system. Don't forget the importance of following Manual S as well. You can't always just go with a nominal tonnage of equipment because it matches a load calculation.
    If you are going to insulate, the load calc should be based on your post improvement needs. If you are intent on doing it right.

    1b) Without knowing anything about the house it is difficult to give good advice about zoning.
    The purpose of zoning should be to address temperature imbalances in a house. IE. upstairs, downstairs. Most people who desire to set different temperatures in different zones, one zone cooler and one zone warmer, are dissatisfied with zoning. Better separate systems if that is the goal.
    A Fit is appropriate for zoning with the EWC communicating board and does not need a bypass duct. By setting your zone weighting the variable airhandler and heat pump will ramp up or down to satisfy the zone demands.

    The ducting must be right! You definitely do not have the necessary ductwork now. If your ductwork was perfectly designed and installed for the existing furnace only installation then it's undersized for the Fit's needs. I would not expect it was perfect even for the airflow demand of the old furnace and it will certainly not be for the new inverter system. Don't skimp here or you will have comfort AND premature failure complaints.
    Low MERV filters are primarily for protecting the equipment. To truly clean the air you need to go with the high MERV filter. Media filters (Aprilaire etc) are the way to go here. You can go with filtered return grills and standard 1" filters as long as they are sized appropriately. Sizes that normal HVAC contractors will consider to be "oversized" such as two 24x24x1 for that nominal 4 ton system. Then you can run the high Merv 1" filters that are too restrictive for "normal" installations.

    Air purifier is such a broad topic that I have no input on that without knowing what kind of system you're contemplating.

    2) Communicating equipment is the wave of the future. The only way you're going to have a Daikin Fit system is with their complementary controls. You also will be required to use the matching EWC zoning board that is communicating on the same network. As with anything, just as reliable as other options if installed well.

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