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Choosing an HVAC System

emma_vt | Posted in Mechanicals on

Hi all,

This forum has been hugely helpful and informative as we build our zone 6A, very good, lower-case p passive house. But my head has been spinning for months trying to figure out what our HVAC system should be, and I would really welcome some input.

House is about 2800sf, including a mostly finished basement. We are right now between interior framing and electrical rough in.

I calculated the peak cooling load to be 10,907 btu/hr, and peak heating load to be 34,174 btu/hr.

When we started building, we had our sights set on an air-to-water heat pump for heating and DHW and a Minotair for ERV and cooling. Neither of which has panned out for various reasons I can elaborate on if anyone is curious. Because that was the plan though we went ahead and put tubes in the basement slab, Wirsbo 1/2″ spaced at 8″. There are six circuits, all at 200′. The plans call for a 3″ concrete slab on the main floor for a solar heat sink; we could also put more tubes in here if we wanted, but GBA articles have really steered me away from doing this, especially in the south facing rooms.

There will be a small wood stove in the living room, but it is intended for joy and extended power outages only – not our everyday heating needs.

Cooling is important to us. We need to have some sort of cooling unit in the master because we sleep with the door closed and loathe being hot at night. The current cooling plan is a 6000btu Mitsubishi Hyper Heat upstairs and a second one in our first-floor master. It’s over-sized for the room, but it’s as small as they go and I figure we can keep our door open during the day to help with the main area load.

So, what do we do for heating? Right now, I’m thinking we put tubes in the north facing main floor rooms and use an ER hot water heater to send appropriately warm water through those and the basement slab. I’m exploring a drain heat recovery unit to feed into that ER hot water heater to add efficiency. Then leave the south facing rooms without tubes so they can absorb solar heat gains into the slab. The north facing rooms and basement will have polished concrete as the finished floor, the south facing main floor will have engineered hardwood over the slab.

However, I’m worried about how little control that gives us and if on cloudy days it will be cold. We could add another mini-split in the main area to address this, but that makes the one in the master feel extra redundant – except that we must have cooling of some sort in there. Do we scrap the master mini-split, put one in the main area, and just use a portable AC in the master in the hottest months? That would work but feels really clunky for a new build.

I’m trying to keep it simple but it seems like no matter what we keep ending up with something that feels clunky and/or over-engineered. Thoughts?

PDF of our Manual J with layout is attached.

Thanks so much!

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  1. frankcrawford | | #1

    In a Passive House the mechanical system is supposed to be the cost saving, so keep it simple.
    My suggestion for the simplest option for heating and cooling is a heat pump. oversize your ERV ducts a bit and use them and a multi-position heat pump air handler with the return air going through the ERV. Add a return air flapper valve for when the heat pump kicks in at a higher air volume. Hot water from a 15 amp rheem hybrid heat pump, 80 gallons as it is a family home.

    You can add zone control to the ducts if you want, but that increases complexity and a Passive House has very little temperature differential.
    Make sure you calculate your overheating potential from your windows using PHPP and reduce that to close to 0%, no more then 5%.

    Hydronic in-floor heating is almost always, too expensive and to slow to respond for a Passive House and is difficult to provide cooling using it.

    1. emma_vt | | #3

      Thanks, Frank. Forgive the following naïve questions - the air handler version is the heat pump variation I'm least familiar with. Would the air handler get installed in our basement mechanical room? And we would do an integrated ERV so that they could share ductwork? We don't really have the space for two ductwork systems. And your feeling then is that we abandon the tubes already installed in the basement slab?

  2. paul_wiedefeld | | #2

    This probably is on the overengineered end of the spectrum, but this wood hydronic setup could work especially if you're going with a wood backup anyway. You'd need some way to generate electricity during outages for the circulators.

    The simplest seeming option would be a ducted minisplit. Mitsubishi/LG/Fujitsu/others all make ducted options that have enough heating capacity for your house and would avoid the distribution and oversizing issues of ductless options. Plus you'd avoid all the complexity of hydronics.

  3. emma_vt | | #4

    I'm in a rapid deep dive on air handler mini splits right now - had been focusing exclusively on wall and ceiling units. The Mitsubishi units say, "This air handler unit features a built-in humidifier, ERV, and auxiliary heat control inputs." Does that mean they have a built in ERV? Or that they have inputs for an ERV? If it's the former they're effectively a magic box like a Minotair...which would be amazing.

    1. brian_wiley | | #5

      That description had me curious, so I looked over the literature for the SVZ. I think that it was just a poorly worded sentence that is supposed to mean that the inputs for controlling auxiliary heat, a humidifier, and an ERV are built in.

      It's listed under sections 9, 10, 11

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