Zoning + Humidity Qs for New Build
Hi All,
Talking w/ 22 HVAC shops has gotten me three quotes for HVAC in a new build, and there are two outstanding questions I’m trying to get my head around.
1) Some vendors are proposing multiple zones with various heat pump systems, while others are saying to do a single system per floor, no zones. People at the HVAC-talk forums say that zoning is almost always done wrong and that it’s harder to do w/ heat pumps vs. a furnace + central air, and that I should go with the 1 floor-1 system approach – does that hold true in a tightly-built and airsealed home? I’d like to avoid three systems if possible (basement + 1FL + 2FL)
2) Some vendors are saying to get a humidifier, some to get a dehumidifier. How can you know what you’ll need before the house is built and you can measure humidity?
Thanks!
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Replies
A Manual J or BEopt model should help you with latent load calculations, to see if supplemental dehumidification (or humidification) is necessary. You can specify the target airtightness for the home in these calculations. What IECC climate zone are you in?
A proper ACCA Manual J, S, and D (roughly in that order) will take care of the load calculations, sizing, and duct distribution, respectively. You will be able to see the amount of cooling/heating being supplied to each room and know if zoning is necessary. I highly doubt the local HVAC shops will perform these calculations for you though. You should consider a 3rd party HVAC designer. Alison Bailes and Corbett Lunsford are two options based in Georgia that are mentioned on this site a lot. Positive Energy in Texas is another. They all do work for clients nationwide There are more out there.
I've already paid an independent engineer for a Manual J/S/D, and the HVAC contractor who seems to have their head on straightest did their own calcs and their numbers are pretty close.
I reached out to Alison's company - they do great work, and his blog is a valuable resource - but it's really pricey, to the point where paying them (or Corbett, etc) was going to be around $5k.
The Manual D doesn't touch on zoning, and I'm not sure how closely my engineer followed Manual Zr. That's what I'm trying to get a handle on that software can't tell me - do variable-speed heat pumps fare well with zoning, or not?
This is my thought process on questions like this: What is the purpose of zoning?
1. To reduce unused capacity of your system and prevent short-cycling? A VRF system automatically adjusts refrigerant flow to the load required in real-time. There should not be (much) excess capacity in a VRF system.
2. To intentionally over- or under-condition a portion of your home? You can either take this into account on the front end with your load assumptions if you know you want to do this, or you could upsize the ductwork slightly and install manual butterfly dampers at the take-offs of your trunk line to adjust airflow to each room. Partially closing the supply registers might also work but it's important to upsize the ducts so that when they get proportionally MORE airflow than they were originally designed to you don't add too much friction to the system.
3. To load-match areas of the home that might receive more sunlight, or more exterior door opening/closing, or face the West/South, etc. more than other parts of the house? All of that should have been accounted for the Manual J load calcs.
4. To change supply rates at certain times of the day for certain zones of the house (i.e. keep bedrooms cooler at night, keep office or living room cooler during the day)? This is probably the best use case for zoning. Depending on how the rooms in your home are divided would inform the best way to split it into zones. If all of the bedrooms are upstairs, splitting by floor would make sense and you can relax the thermostat setting in the downstairs area during nighttime. Stuff like that.
We're looking at #4, but the question is still whether or not variable speed communicating units are difficult to zone properly.
It's been a nightmare even getting quotes in the first place so I want to keep things as simple as possible without breaking the bank.
To #3, there's no way to account for solar gain in the Manual J, because it depends on whether the sun is shining. It also depends upon the angle of the sun, which varies every day of the year.
All that Manual J can do is make sure on the hottest day of the year you have enough cooling. If you need to dynamically allocate that cooling depending on conditions you need zoning.
The way I see it zoning makes sense if different parts of a building have very different loads or on different levels If part of the building is a greenhouse and is all glass on 3 sides it needs its own control system. Because heat rises different levels will have different loads.
With that said things tend to even out on their own. If you need every room the be exactly the same then you want multiple system or zoning controls.
I tend to think separate systems work better than zoning but are no doubt more expensive to install.
How much will you really use the basement? Would the basement be unusable if it were 2-3° cooler in the spring and warmer in the fall than upstairs?
Even the best new home could benefit from the use a dehumidifier thru the end of the second heating season. If you live someplace like Houston a dehumidifier seems likely to be a necessity.
A humidifier becomes a necessity in a leaky house in very cold climates I see humidifiers as an act of desperation. If a house is tightly sealed in cold climates ventilation is required to lower the indoor humidity under 55% to prevent condensation in the walls.
I think heat pumps and AC systems work best when the thermostat set point is unchanged for weeks or months at a time.
Walta
"With that said things tend to even out on their own. If you need every room the be exactly the same then you want multiple system or zoning controls."
In an open plan that's true. With bedrooms, most people close their door at night and the air exchange is limited. And people vary in terms of what they consider a comfortable sleeping temperature. The ultimate in comfort is to have each bedroom have its own thermostat.
Thanks for your input, Walta. The basement will be the kid's playroom and TV area, so it'll be used pretty much year-round - are you suggesting it's fine being in a single zone with the 1FL, or that it should have its own system? We could absolutely live with a small delta.
My house is a single floor with a full basement single zone. The basement was intended to be semi conditioned. I find my basement is 2-3° cooler in the spring and warmer in the fall than upstairs. It is about 2/3 open shop space 1/3 finished. I only have 3 supply registers that are closed and no return it is a walk out on one side. My home is very open and the doors are rarely closed.
+ or - 3° seems good enough to me for the basement usages you described.
If I had a second floor it would have a separate system.
Walta
Zoning with heat pumps is possible but you do have to design it properly.
You have a couple of options.
Simplest is to over provision the zones you want to control with airflow and set up the damper so when closed it sill allows about 50% of the needed airflow. The idea here is to modulate the airflow instead of completely shutting it off. The duct pressure and overall flow does change a bit but most heat pumps have problem modulating around this bit of airflow change.
The nice part about this setup is the zone dampers and their thermostats don't need to talk to the heat pump but it limits the temperature control range you can have (ie it works well in the winter to make a bedroom colder but can't make a bedroom significantly colder than the house in the summer).
The other option is to size each zone to handle the full airflow of the air handler so even if only one zone is calling for heat, it won't restrict the airflow. This setup does require the zone thermostat to communicate with the heat pump, usually this is done by adding a thermostat interface module which limits the amount of modulation you can get out of the heat pump. In this type of setup you can also allow some airflow to all zones all the time which does reduce your duct size.
A unit per floor does work but you have to be careful with sizing. It is very easy for an installer to put a pair of 3 ton heat pumps into a 3000sqft newbuild, which would be grossly oversized in most cases.
The reason most older basements are unpleasant is the large air leaks around the rim joist area, stack pressure in the winter pull outside air in through these which settle near the basement floor.
This should not be the case for a code min basement with an insulated slab and well sealed rim joist and bottom plate. It will be comfortable even with minimum heat, the heat load is low enough that a couple of takeoffs from the main supply trunk can easily heat it. A walkout would need more heat but that should fall out of your man J.
Thanks so much for all the detail, Akos. I think you and Walta have convinced me that a system for the second floor, and a system for the first floor and basement either as a single zone or two zones will be fine for me.
As always, appreciate the great community here! If anyone ever has questions about commercial solar financing (and to a lesser degree, resi finance), let me know, as it's all I can offer on the forum :P
If you do decide to go with zoning look for zoning system made by the equipment’s manufacture and integrated into the system not a third party add on system. I see that Rheem has done this in the last few years and likely most other brands.
Walta