Heat Pump Variable-Speed Controls
Background: registered NYS Mechanical/HVAC PE renovating my families 220 year old colonial farmhouse. Full gut and retrofit. New construction is a “pretty good house” r60ish attic (vented), r30 walls, r20 basement. Main part of house. Manual J gave 18k heating and 16k cooling loads. I pulled trigger on a Fujitsu 18RLFCD heat pump. I will have a woodstove to provide heat for the real cold winter days/nights.
One of the main reasons was its ability to mount vertically. I chose a single zone system with the intent on zoning the system with air dampers but I’m uncertain on the ability to do so. Does anyone have information on how these type of units are controlled, something like a control sequence normally found on a commercial unit? Specifically how does this unit or other variable speed Fujitsu air handlers change air speeds? Is it by a static pressure sensor? My concern is if I install zone dampers how would the unit respond as I know the general idea is to keep the unit running as much as possible. Any insight to the control logic would be appreciated.
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I would look at Fujitsu's Airzone kit as it interfaces with the unit directly.
Usually on most of the ducted units I've worked with the blower speed tends to be proportional to how hard the unit is working. For example, rapming the thermostat up by 4F will cause it to go up max blower until the setpoint is reached and will than slowly ramp down to maintain it.
You can do a bit of DIY zoning without fancy controls as long as you limit how much of the overall flow you are modulating.
What you want to do is over-provision a zone (say 125% of required flow) but set the closed position on the zone damper to deliver say 75% required. Since the zone is never completely shut off, this way there is only a small change in the overall flow rate and most units will easily modulate around this without issues.
Akos,
I have been looking at Airzone specifically because its comparably with Fujitsu. I'm also looking at 3rd party systems that im assuming would be more cost effective. In regards to the Fujitsu system I guess I was expecting there to be a sequence stated in their design and technical manual given how controls are so important with these variable speed and inverter systems.
You can look at Fujitsu's 3rd party thermostat interface module: https://connect.fujitsugeneral.com/uty-ttrx/product/UTY-TTRX
This gives you two stages of modulating heat and cooling as well as direct control over fan speeds. Should be pretty easy to interface any standard zoning kit with.
You'll have to do a bit of tweaking of the dip switches to get the temperature setpoint and staging just right. You want the unit to be running in stage 1 pretty much all the time.
So you could use this and control, for example, a Fujitsu unit with Fujitsu's multi-position air handler using a standard Ecobee thermostat?
And you still get the full benefit of Fujitsu's modulating inverter compressor?
From my quick read of the manual, it seems to be the module puts the Fujitsu unit into heat mode at the temperature selected by the dip switches. This would essentially be the same as turning the unit on with a 70F setpoint using the remote.
As long as this setpoint is selected correctly to match the desired room setpoint, the unit would stay in a modulating mode.
You do have to take some care, say you want a 70F room but select a 81F setpoint, the unit will be only cycling between off and fully on.
Might also have to tweak the settings on the thermostat as well so that it can stay in stage 1 for extended period of time. Some ramp to stage 2 if heat setpoint is not reached fast enough.
I think you might find it much cheaper and simpler to install another smaller unit for a second zone (assuming you want only 2 zones).