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HVAC Planning during a retrofit

kiwiscott | Posted in General Questions on

I’m in a situation where I have to replace my roof, siding, windows and add some interior insulation. My interior insulation retrofit will take a few years as I plan to go room by room. 

As part of the interior work I want to remove my baseboard and install heat-pumped based heating – ducted on one floor and mini splits on the other. 

I have two questions: 
– How should I size my installation? Do I do an installation now based on the current zero insulation or based on the completion of Phase 1 (exterior roof and wall insulation) or based on the final values? 
– Should I both zoning a mini split application if I’m going to air seal the outside of the house and have a whole house ERV? 

FWIW, 
Zone 5a – Northern New Jersey 

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Replies

  1. mr_reference_Hugh | | #1

    user-6760694

    Looks like the HVAC questions don't get many responses or maybe it is just my perception. You may want to cross-reference what I say here with what I shared in another response this morning.
    https://www.greenbuildingadvisor.com/question/sharing-erv-and-hvac-supply-ducts

    For your specific questions, here is my input.
    You asked
    – How should I size my installation? Do I do an installation now based on the current zero insulation or based on the completion of Phase 1 (exterior roof and wall insulation) or based on the final values?

    You also mention: "My interior insulation retrofit will take *a few years* as I plan to go room by room. "

    Response:
    This is what I would do.
    > Select your heat-pump and air handler based on the anticipated final BTU requirements.
    > While most heat-pumps are usually sized for the very coldest of days (or this is how it is done here), I would "undersize" the heat-pump to address a temperature that is below the average and you might expect to experience quite often. In this case, you don't size the heat-pump to the lowest possible temparature.
    > When you "undersize" the heat-pump, you could then have a secondary/back-up heat source for the very coldest days. That could be a heat strip installed in the air handler (Mitsubishi has this).

    Why I would do this?
    Insulation is important, but it is the air sealing that we are learning makes a massive differerence to the occupant's comfort. If you do all the other work first and insulate afterward over a few years, then this strategy could work. The heat strip might work more often while you do the insulation but it would very likely be much less costly than modifying your HVAC system - AFTER - you finish your insulation.

    I am not talking about the A/C here because the heat-pumps normally have much greater capacity on the cooling side than on the heating side.

    You asked –
    Should I both zoning a mini split application if I’m going to air seal the outside of the house and have a whole house ERV?

    Response: It might depend on how you duct your ERV. My response in the link above explains how I feel about ERV ducting. In terms of zoning heat and A/C, I would be cautious for 2 reasons.
    1. Multi-zone heat-pumps hooked up to multiple heads are less efficient because the heat-pumps normally have to run the gas in the line-set through all the lines. Maybe technology will change.
    2. The heads used in rooms often produce too many BTUs for what each room require. This ends up causing short cycling.
    3. Each unit costs extra money and each unit requires maintenance. These heads require regular maintenance.

    If you use a ducted system, you avoid some of the challenges associated with individual heads. If you have one individual head and a ducted unit then you could look at 2 separate heat pumps.

    Cost and and other factor that are difficult to list on the first posting of the questions can make it challenging to provide advice that might otherwise be satisfactory.

    For sure, i would have an engineer design the system and you just be sure to know up front which way you want to go regarding the questions you posted here... or the engineer may have tooooo much influence on your decision.

    1. kiwiscott | | #2

      Thanks - I’m finding it difficult to fond people to work with who want help craft a plan without wanting to charge $5k etc - but I’ll get there.

      Thanks again.

  2. mr_reference_Hugh | | #3

    Are you talking about an HVAC engineering firm?

    The firm we used was pretty good but too busy to spend too much time one on one. The cost was $1000 CAD for a home less than 2000 sq ft. They never set foot on the property and realistically it is pointless because the house was not built in my case.

    I know that these firms can work without a physical presence. You might want to reach out to firms that are outside your immediate area, perhaps even in another state.

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