HVAC Zone Controller Confusion
SLCraftsman
| Posted in Mechanicals on
I have an air to water heat pump that I am preparing to install. The house has two floors as well as an attic. The 1st and 2nd floors will have radiant in-floor heating for the primary method of heating. Each floor will be on a separate zone.
I also have a hydronic air handler that I plan to use for supplemental heat, via a hydronic coil, and auxiliary heat via heat strips. The forced air portion of the system will be split into three zones, one zone per floor of the house via dampers, including the attic. The air handler also has built in humidification/dehumidification. The air handler would also be used for air conditioning in all three zones as well.
Ideally I would like to only have one thermostat per zone (floor) that could control the floors as well as the forced air system. I have looked at a few Honeywell zone controllers that support three heating stages. I am trying to figure out if this is what I would need to make this work where the 1st stage is the radiant heating and the hydronic heated forced air is a 2nd stage and finally the heat strips are the third stage. Would this work? Would the thermostat also need to support 3 stages of heating?
One thing that I don’t see on the Honeywell controllers is a pin for humidification/dehumidification. Is there a way to work this into the zone controller or what is the best way to take advantage of the built in humidistats on the thermostats? I see where many are saying to just wire in one thermostat to the central dehumidification. I do want to keep the basement dehumidified but it seems like I would also want to monitor the conditioned attic in the winter?
Perhaps the simple answer is to have two thermostats per floor but I’m hoping to avoid it if at all possible. Thanks!
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Replies
You would just set slightly different setpoints for each heat source. Set the source you want to run the most to have the highest temperature setpoint, so that would mean the radiant system. Set the hydronic forced air system slightly below the radiant system, set the heat strips below that. In this way, the radiant system will do all the heating until it starts to "lose the battle", and additional heat energy is needed, at which point the temperature will drop a bit and trigger the next heat source in sequence to come on.
For humidity control, you need a thermostat that can also act as a a humidistat. Such units do exist, and would allow you to do everything. The issue you're likely to have is finding a unit that can do three stages of heat AND humidity. Most residential thermostats will do two stages of heat and humidity, I have not seen one that can do three stages and humidity. You could step up to a commercial system, since those systems tend to be modular, and build up what you need. You could do all kinds of fancy multizone stuff with one of the networked commerical systems using one of the open protocols like BACnet. The downside is that these systems are significantly more involved to setup than a typical residential thermostat, and they cost a lot more too.
I see no issue using an additional "thermostat" to act as a humidistat on each floor, although you could probably do just fine with one for the entire house since humidity is difficult to control with the same precision as you can control temperature. If you use something like the Ecobee, it would be easy to automatically copy your program setpoints between the different thermostats in your home. I have two Ecobee thermostats in my house, one for each of two zones, and they have a nice mix of "same and different", allowing for pretty seamless control between the zones while still allowing the different zones to do some things differently as desired.
Bill
How is the air handler dehumidifying? Unless some fancy heat exchange in there, this is not happing with a standard hydro coil. A standard hydro coil will be able to dehumidify while cooling which can be controlled by a regular thermostat.
Most two stage heat + aux thermostats are meant for air to air heat pump, this means they drive the y and y1 signals for both heat and cool, this won't work for your application.
Some will have an emergency heat mode, this might work better. The emergency mode disables the two stages of heat, so something to keep in mind.
Do you have a diagram of how you are planning to plumb all this up. AWHP is not the easiest system to get right.
That is a good question. The air handler is advertised as having built in humidification/dehumidification, but.... you kinda have me wondering now. I can see that there is definitely a built in humidifier. There is also labeling inside on the terminals for dehumidification. How it will actually work, I do not know.
The wiring diagram for the thermostats is very different than a standard thermostat. I haven't found out yet exactly how many stages that it supports. Below are links to both products. Don't expect the manuals to help much. Assuming that the air handler does in fact dehumidify as they advertise, how would you connect the dehumidification wire from each thermostat to the air handler? I am looking at a Honeywell Zone controller like the hz432 or the hz311 but I don't see any inputs for dehumidification. I'm assuming you wouldn't just wire all of them into the dehumidifier terminal on the air handler?
As far as plumbing the system goes. I'm just going to follow the diagram from the manufacturer. Is there any area in particular that you feel that I should pay extra attention to with the design? I bought their pre-plumbed pump station so there is a supply and return that goes to the heat pump on one side, domestic hot water supply and pump, and a radiant supply and pump. The air handler is just plumbed into the radiant supply according to their diagrams. There is a buffer tank on the return side of the radiant as well.
Thanks for both of the responses. Definitely good food for thought.
https://www.mbtek.com/collections/fan-coil-hp/products/apollo-central-air-handling-ahu
https://www.mbtek.com/collections/fan-coil-hp/products/wifi-fan-coil-thermostat
If the pumping is pre-engineered, you are good. Things you have to watch for with heat pumps is mixing water and limiting flow. You never want to tamper the water supplied by the heat pump with return water from any of the zones as this effectively reduces the COP of the unit. AWHP are also very sensitive to water flow rate, so you must always have the minimum flow rate through the unit.
Their fan coil does dehumidification by running the cooling and aux heat at the same time. This technically works (aux does the reheat after the air is cooled and dehumidified) but it takes a lot of energy to do so. I would see how much dehumidification you can get by running the FCU in normal cooling made first. You can improve the dehumidification of the FCU by setting up the cooling staging to control fan speeds with fixed coil water suppled by your AWHP. This way on low fan speed you'll have much higher humidity removal.
For humidifier control, you can run a stand alone humidistat to drive the fan and humidifier input directly.
The Honeywell zoning controller does support 3 stage heat, so you can have them run as you propose.
"Their fan coil does dehumidification by running the cooling and aux heat at the same time. "
Well that's just wacked. A dedicated dehumidifier would work much better.
This is a common way to do dehumidification in many systems, since it allows the unit to perform the function without adding an additional coil. The equipment is cheaper and less complex, but the tradeoff is increased energy consumption while the unit is running in dehumidify mode. Datacenter air handlers commonly do dehumidification this way, but it's acceptable in that application because it is rare to need to dehumidify in a datacenter -- since datacenters run cooling 24x7 all year round, humidification is usually much more important.
For the OP: If you expect to be running the dehumidification cycle very often, I would consider installing a dedicated dehumidifer which will be much more energy efficient. A dedicated dehumidifier works by acting like an air conditioner that puts it's heat right back into the same space instead of pumping it outdoors. With electric reheat, what your proposed system does, you're running the cooling system AND the electric resistance heat at the same time, so you are effectively wasting the heat that the cooling system pumps outdoors. If you expect to run long or frequent dehumidification cycles, you'll likely save a good amount of energy (so lower operating costs) using the dedicated dehumidifier that essentially only runs the cooling system and doesn't waste any heat.
Bill
Looking at the manual for the air handler, it looks like the thermostat for it has a thermostat output but that's the extent of the controls.
For what you're trying to do I think a single large air handler is going to be a poor fit for hydronics. Hydronics would work better with a right-sized air handler in each zone. If you're going with a single large air handler there's no reason not to use a conventional air-to-air heat pump which would give the same functionality for a third to half of the cost.