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Community and Q&A

HVAC Equipment for Multi-Zone System

SeattleFam | Posted in Energy Efficiency and Durability on

I recently bought a two-level 2500 sf house. It has a single HVAC – no zoning. My mom moved in with us and wants it hot. We have all the vents closed upstairs and are still too hot at night. We also want to add air conditioning, which we will use occasionally over the summer as needed. We live in Seattle, so AC is not needed every day. The most environmental option is important to us. Should we create a bi-level two-zoned HVAC and add cooling, or should we switch to ductless? We had a company come out who only does ductless work, and they recommended 3-4 heads and not to do the HVAC. Also, our furnace is gas.  Thank you for your advice!

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  1. paul_wiedefeld | | #1

    You’re mixing up terminology here, which is okay! It sounds like you have a furnace but no AC? That’s the H in HVAC.

    If so, you can add a heat pump to replace the furnace using the existing ductwork, which solves everything: best for the environment and adds cooling. If your mom wants it hot when you don’t want it hot, I’d get an electric baseboard for her room, which is about $100.

    Ductless heat pumps (also HVAC) have a lot of tradeoffs and they can be justified IF you don’t have ductwork, but with ductwork it’s a lot of unnecessary compromises. In particular, assuming they’re pitching a multi-split (one outdoor unit connected to many indoor units), that’s a proposal with a high chance of failure. I’d talk to different installers.

  2. scottperezfox | | #2

    From an outside reading of it, you're on the right track that replacing your home's existing forced air system with a multizone ductless heat pump system would be a moved toward modernity, and solve the mother-in-law problem to boot. And since you live in Seattle, where it doesn't get Calgary-level cold in winter, you might be able to ditch a secondary heating source altogether and go full-electric.

    But just be mindful that you're talking about replacing an entire one of your home's systems. Does your house have a basement too? It's a lot of work, across all floors, all rooms, and maybe even in the attic too. And then it opens a conversation about air-tightness and window performance.

    It's a big world ... good luck!

  3. Danan_S | | #3

    > We have all the vents closed upstairs

    Doing this causes excessive pressure in your ducts, causing them to leak and potentially fail prematurely. It can also force your furnace to overheat at which point it will shut off it's burner to protect itself and you.

    It sure sounds like you have a poorly designed heating and distribution system, which is often the case when people want zoning.

    The best thing would be to to get whatever heating/cooling system properly balanced no matter what type you use.

    Also, with all the cheap (and clean) hydro power you have in Washington state, a heat pump of some kind is pretty much a no-brainer.

  4. Expert Member
    Dana Dorsett | | #4

    >"Also, with all the cheap (and clean) hydro power you have in Washington state, a heat pump of some kind is pretty much a no-brainer."

    True, but for a heat pump solution to work efficiently & comfortably (in any climate) it has to be sized correctly. The "ductless head per room" solution usually leads to a TERRIBLE, even GROTESQUE oversize factor in locations as temperate as Seattle, leading to higher cost up front, lower comfort overall, and lower efficiency.

    It's highly likely that the existing furnace & ducts are 3x or more oversized, which leads to comfort issues even if there aren't room to room balance problems. To find out the oversize factor run a fuel-use based load calculation as outlined here:

    Run that load math (wintertime billing periods only, nothing after mid-March or prior to Thanksgiving) and compare that to the BTU in/BTU out numbers on the furnace nameplate, then report back.

    A 3x+ oversized system can still run efficiently, but will (almost) never properly heat the rooms at the ends of the duct runs, since the cycles are rarely (or never) long enough for the system to reach equilibrium. Cranking the thermostat up to raise the temp at the far rooms only yields a hot-flash followed by the extended chill, and overheats the rooms nearer the furnace (or those with better air flow.)

    For a more lengthy explanation of the nature of the problem it's worth downloading a few free chapters from Nate Adams' book, and watching his short videos here:

    ...and here:

    ...and here:

    Whatever you do, do NOT replace like-for-like on BTU output, or comfort problems will persist. A right sized single stage furnace will run almost continuously when it's 25F (Seattle's approximate 99% outside design temperature) or cooler, and room-to-room temperature differences will shrink, and become more tweakable by register settings (or better yet, balancing vanes on the ductwork.) But this only works WELL if the ducts are also well-sealed (probably aren't now, but could be.) Duct sealing is a reasonable DIY project for the handy, if you don't mind working with sticky goop like duct mastic.

    It is often possible to modify duct systems to make it two completely separate systems, and that is usually the better solution if the people living in one zone prefer a different baseline temperature than those living in the other zone. If switching to heat pumps (recommended, even though gas is still pretty cheap in Seattle) it would be even more important to right-size the pump to the load, whether it's a modulating heat pump or single or 2-stager.

    The fairly easy to use (and very appropriate) BetterBuiltNW Manual-J-ish load calculator for heat pumps developed by the NEEP consortium of NW utilities, specifically targeted at HVAC contractors for the purpose of avoiding oversizing issues. Run both room-by-room and zone-by-zone load calculations, and compare that to the wintertime fuel use load calc as a sanity check. (<this is a free tool, but you need to share an email with them to open an account.)

    The good thing about starting with a 3x oversized system is that the ducts are likely to be oversized for a right-sized system, yielding lower static pressures, lower duct velocity, which in turn means lower noise, which is even more important if the system is right-sized & modulating and running nearly 100% duty cycles even during the shoulder seasons.

    The notion that a system should run nearly 100% of the time feels odd to those who have grown accustomed to oversized equipment. Late last year I specified a heating/cooling solution for an ancient double-wide mobile home near Bremerton. It had previously been heated with a 1.5 ton ductless (until it died), and an oversized ducted electric resistance furnace. Previously when the ductless was still working the homeowner reported that the master bedroom/bath area became too cold, so she would crank on the furnace, which would take over from the mini-split, but was always in hot-flash/chill mode. The solution we came up with was a right-sized wall-mounted on/off electric panel radiator in the master bedroom suite, (controlled by a plug in thermostat with an RF remote, where the temperature sensor is in the remote), and a 1-ton for the main living/dining/kitchen/work space comprising nearly 2/3 of the house. The house could have been heated with a 3/4 tonner, but large south facing picture windows brought the cooling load up to make a 1-ton rational for managing heat waves like those experienced last summer.

    The panel rad was finally installed about a month ago, the mini-split a couple weeks ago. After the first week of operation the homeowner thought there had to be something wrong with the mini-split because it was running CONSTANTLY, even though it was running so slowly and quietly she couldn't tell for sure. Apparently the 1.5 tonner had never behaved that way, and always cycled, robbing her of both comfort & efficiency. It keeps up just fine (of course it does- it hasn't really been that cold this month), and only rarely steps up the blower speed. She keeps it 72F in the living room, 68F in the bedroom, so the mini-split is at least partially heating the bedroom. Comfort levels so far are reported to be very high- higher than they have been in the ~20 years or so she has lived in the place, and WAY higher than when heating with the grossly oversized ducted furnace.

    I suppose I'll hear from her in January if it doesn't fully keep up during cold snaps (I'm estimating it has enough capacity to cover the load only down to ~+10F or so, which well below the design temp, and pretty rare in Bremerton), but in a "Hail Mary" pinch she can always turn on the electric h0t-blast system, which was not removed.

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