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Understanding HVAC and heating/cooling zones

airfix | Posted in Mechanicals on

I posted a question a few weeks ago about radiant floor combined with forced air:

Taking the advice on that thread I’m moving away from the radiant floor concept and going either forced air or a mini-split setup.

One comment from Dana on that thread requires more clarification for me and I’m hoping the readers can help me understand.  Dana said:

Designing the mechanical systems and deciding the zoning BEFORE the Manual-J is done is a serious waste of effort!

I’ve now got my Manual J back and I don’t understand why planning a zone based on intended use is a waste of effort.  When I wrote that thread I had planned zoning based on the following use:

Zone 1 – South side of the house is guest bedroom and bathroom and laundry area.  This will typically be used less than the main part of the house and so I anticipate keeping the thermostat in that zone around 60F in heating season and 80 in cooling season.  When guest are staying they can choose a temperature that suits them.

Zone 2 – Great room, kitchen.  This is a large volume and typically we would want it a little warmer than our master bedroom in both cooling and heating season. We typically set our living area thermostat to around 65F in heating season and I’ve never had AC before but I’d imagine about 75F in cooling season.

Zone 3 – Master bedroom, bath, closet and office.  Typically we’d have our master much cooler than the rest of the house.  We’d normally do a setback (in our old leaky house) to 55F but I anticipate having our thermostat set to about 60F during heating season and 70 during cooling season.

When looking at my manual J that I’ve just received and totaling up the heating/cooling loads for each of the zones I was planning I get the following:

Zone 1 – Heating – 11,600 Btuh ,  Cooling – 6,200 Btuh
Zone 2 – Heating – 32,300 Btuh, Cooling – 22,300 Btuh
Zone 3 – Heating – 25,300 Btuh ,  Cooling – 11,100 Btuh

Can you help me understand why designing zones based on usage is not a good idea and how to design the zones based on the manual J?


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  1. Yupster | | #1

    People often mis-understand the point of zoning. The general idea seems to be that it's for maintaining different temperatures in rooms. While it can be used for that, the main purpose of zoning is to ensure that rooms with wildly different loads (such as a great room with 100 windows as opposed to a bathroom with a small window) maintain a comfortable temperature. You don't want a/c blasting into your bathroom, freezing your exposed tootsie, because the great room is trying to stay at 75F. So you group rooms based on loads, not use. Sometimes you can make the two coincide, but the load is more important. Because in reality, zones that are connected have a hard time maintaining very different temperatures anyway, since the heat/cool flows from the conditioned area to the less conditioned area. If you try to group them by use, you might combine rooms that have very different load requirements, negating one of the best qualities of a zoned system. Also, grouping by use tends to micro-zone a forced air or even ductless mini-split system. For instance, if you have a two-stage 2 ton a/c unit, it might drop back to about 60% on the first stage. That's still 14,400 btu/h, 230% of your Zone 1 load, on the hottest day of the year! This can cause severe short cycling. Also, if you have a tiny zone, you still have to move the required airflow. So you either have to oversize that zone's ducts to handle 480 cfm, which can create balancing, space, and cost issues, or you have to bleed that excess airflow and with it capacity into another zone, making temperature differentials even harder to obtain. For various reasons, don't do a bypass duct. And obviously you'll need an even bigger a/c than 2 tons, so the problems are just further compounded.

    Hope that helps...

  2. Expert Member
    Dana Dorsett | | #2

    Most homes discussed on this forum don't have a heating load of 69,200 BTU/hr for a 3900' house. That's almost 18 BTU/hr per square foot, which implies a code-min (or worse) building envelope. (That's roughly the load/square foot ratio of my 2x4 framed sub-code antique at -10F outdoors. You probably have WAY more window area pushing the load numbers.)

    But now at least you know the magnitude of the whole house load and the zone load, which allows you to make more informed and intelligent choices. You'll be able to figure out how much of the heat could be delivered with the radiant floor (maybe all of it, maybe not) and not oversize the auxilliary heat to cover the shortfall, etc. Just going at it willy-nilly with multiple systems without knowing the load numbers is prone to gross oversizing with lower efficiency, higher up front expense, and lower comfort.

    The contractor who backs up radiant floor heating with hot air doesn't have much confidence in hydronic designers, eh? :-) Done well hydronic systems are very reliable and super-comfortable.

    With a design load less than 70K an 85-100K modulating condensing boiler is about right. If the peak water temps never need to go higher than 130F (TBD) you could pull it off with an 80K modcon. With the design load of the smallest zone being under 12KBTU/hr ideally you'd find a boiler that modulates down to 10K or less to minimize cycling during the shoulder seasons. A 100K boiler with a 10:1 turn down could get you there. Most 120K boilers would not. But with the load numbers you know know you don't need anything bigger than 100K- even at non-condensing temps a 100K boiler delivers more than 80,000 BTU/hr

    Any number of HVAC contractors would throw a 150KBTU/hr condensing furnace at that even WITH the radiant floor. It'll heat the place, sure, but it won't be as comfortable as right-sizing. Comfort is far more than just a number on the thermometer.

    Zone 1 could be heated & cooled solely with a 3/4 ton cold-climate mini-split, but if it's cheaper to heat it during the coldest weeks when the COP of the mini-split is less than 2 with the radiant floor that works too. Cooling that comparatively tiny load with a zone-dampered duct from central system that's also cooling zones 1 & 2 could be done, but it's problematic due to the very LARGE differences in zone loads. (Zone 2's cooling load is more than 3.5x that of zone 1.)

    Zone 2 might not be able to cover the full heating load with a radiant floor without frying your feet, a common problem with tall rooms with lots of window area in cold climates. Run the numbers, figure out what the FLOOR temperature needs to be on design day to cover the whole load. If it's 80F or higher you might want to add just enough radiator as a second stage to cover the load with a more comfortable floor temperature. Some flat-panel rads under tall windows in a tall room counteracts the cascading draft when it's sub-zero outside too.

    Zone 3 might be able to heat solely with radiant floor and cooled with a 3/4 ton or 1-ton mini-split.

    With mini-split cooling for zones 1 & 3 a much simpler 1.5-2 ton single or 2 stage AC for zone 2 would work. If it's a heat pump or (AC with 10kw of resistance heat strip, Florida style) rather than strictly AC it could be your freeze-protection for when the boiler failed, even if it doesn't keep the place at 70F without engaging some heat strip.

    Without a lot more information it's not possible to divine the "ideal" solutions here, but with the numbers in place you have something a heluva lot better the a WAG and a "Hail Mary" 6 ton air conditioner and 150K hot air furnace "solution" to the multiple design constraints.

  3. airfix | | #3


    Thank you, that helps me understand the big picture and overall logic. We were not planning a bypass duct as I've read up on the downsides of that approach.

    If I were to do three zones based on load I'd have:
    Zone 1: laundry, WC's x2, closet, bathroom, powder room all in one zone each close to 1000 Btuh load.
    Zone 2: , kitchen, stairs, pantry, master bathroom, bedroom, office in zone 2 - between 5000 and 8000 Btuh per room
    Zone 3: great room, dining room, master bedroom, - between 9000 and 12,000 per room.

    The problem this creates is that the rooms within each zone are spread from one side of the house to the other. I'd have a spaghetti of a ducting system with likely multiple crossovers and returns. It seems really impracticable. Is this really how people run duct work in their homes or are most people smarter than me and group all their rooms together based on heat load rather than function?


    Like I mentioned I'm moving away from the radiant floor heat with back up forced air. If I do anything with radiant floor it would be to do the basement in radiant only, and do forced air upstairs only but that's yet to be determined.

    We're in climate zone 6a. Design points used were:
    Heating - Indoor 70F, 30RH, Outdoor -7F, 15mph wind
    Cooling - Indoor 75F, 50RH, Outdoor 93F, 7.5mph wind

    For heating glazing accounts for 29% of the load, the walls 21% and ceilings 6%
    For cooling glazing accounts for 54% of the load, the walls 3.5% and ceilings 3.2%

    These numbers indicate I should spend more on my glazing and less on my walls but going up from a .31-.33 U rating costs a lot of money.

    Windows I'm thinking I'd go with Windsor with a U between .31 and .33. Their quote was about $30,000 cheaper (half the price) than Alpen's double glazing and inner film (U factors of about .19). It would be nice to find something in-between Alpen and Windsor.

    Wall's above grade I'm doing 2.5" of polyiso with a 2x6 stud cavity filled with BIBS fiberglass.
    Walls below grade I'm doing 1.5" of polyiso and a 2x4 stud cavity filled with BIBS fiberglass.
    In our un-vented cathedral ceiling I'm planning 5.5" of closed cell spray foam with 8.5" of BIBS filling out the remainder of the rafter cavity.

    According to our ResCheck we are 17% better than code.

    The recommended equipment that my HVAC consultant came up with is a 105,000 Btuh output lennox model furnace model EL296UH110XV48C and for cooling a 47,000 Btuh Lennox XC16-048-230-04 condenser and CX34-62C coil.

    I've not had input from my HVAC consultant about using minisplits yet but I plan to spend more time understand them.


  4. Expert Member
    Peter Engle | | #4

    You mention polyiso on your below-grade walls. Is that inside or outside? You don't want to use polyiso below grade outside. It's not even a great idea inside, because of moisture uptake, though foil-faced is probably OK inside.

  5. airfix | | #5


    Yeah that's on the inside of the basement walls and indeed it was planned to be foil faced. I was planning to leave a 1/2 to 1" gap at the bottom of the polyiso to the concrete slab to prevent moisture uptake and sealing to the concrete floor with tape.


  6. Expert Member
    Dana Dorsett | | #6

    The dominance of glazing in both the heating and cooling loads indicates a high glazing fraction, which accounts for the high load.

    A square foot of U0.30 glass moves ~7x as much heat as a square foot of code-min wall and probably more than 8x that of your better than code min wall. Shrinking the total window area is one way to lower the loads without blowing the budget on windows. (BeOpt is useful for figuring out the right balance.)

    A heat load of 70K would call for equipment with 98K of output at the ASHRAE recommended 1.4x multiplier, so a condensing 105K-in gas furnace would be about right. Zoning a gas furnace with zones of widely disparate loads is still a non-trivial design problem.

  7. Yupster | | #7

    For an example, here is a house with a 3-zone system that I did recently. The master bedroom load was a bit small, but we handled that by having the damper for the large main zone always open 30%, allowing excess airflow to bleed into that zone. Since it had a heat gain much larger than the master bedroom, it was unlikely to over cool that zone. Obviously some sense has to be used when grouping rooms because of the constraints you just discovered.

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