Transfer fan to move heat (with just one heat pump)?
anukeen_sprout
| Posted in Mechanicals on
Check out this video from one of our favorite builders:
https://www.youtube.com/watch?v=-nnONuVzNv4
At 1:42, he shows a transfer fan, and gives the example of having a single head of a mini split in a main room, but using this to get heat into a smaller room.
Would this work?
For real-world context, I’m considering this for a new, insulated, 800-sq-ft unit with a great room and two bedrooms that share walls with the great room.
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Replies
I think if you do the math and assume the two rooms have a 5° difference the volume of air the transfer would need to move to change the difference to 1° would be 4 times what is coming from the mini.
Walta
If you install that between the bedrooms and the great room, you've all lost sound isolation between those rooms.
If you've got a super insulated unit, the temp difference is going to be pretty minimal in the bedrooms if you leave the doors open half the time. Just having two people in one bedroom might be all the boost heat you need when the door is closed. I'd install a small radiant cove heater in each bedroom, which will rarely operate and cost much lesser than the transfer fan.
> cove heater in each bedroom, which will rarely operate...
Say you like to keep the bedrooms the same temperature as the main room. The cove heaters will provide 100% of the heat needed in the bedrooms. This will be expensive.
I do not believe that is what he was suggesting. It would be sensible to put the cove heaters below the minisplit setpoint to act as backup heat when the doors are closed and it starts getting too cold. Given that most people like to sleep slightly cool anyway, it is a good solution.
Given that the need for supplemental heat may or may not happen, and if it does will be seasonal, I'd suggest a wall mounted plug-in heater. Something like this that can be put away when not in use.
https://www.convectair.ca/en/products/120v-plugin/apero
Personally, I wouldn't be impressed if my HVAC guy told me I had to bring out an electric heater on cold nights to stay warm in my bedroom. I would want something permanently installed. Electric baseboard is cheap and relatively unobtrusive, why not just use that?
Yupster,
I agree, but whether the heaters actually get used largely depends on occupant behaviour - and if they do get used it will probably be for a very small part of the year.
These aren't portable heaters. They mount to small unobtrusive brackets. The difference between them and cove or baseboards is they are plugged-in so that removing them is very simple.
Anukeen,
Here is a link to an article that addresses your question: "Using a Bath Fan to Equalize Room Temperatures."
The problem isn't heat, that is easy to make up with a couple of well placed electric heaters. The bigger issue is A/C, the one thing most people will not put up with is a hot bedroom in the summer.
I think the key to getting this to work with a transfer fan, is having the intake of the fan located near the outlet of the mini split.
This would give much larger temperature delta than just trying to move air from one room to the next. With a bigger delta T to work with, something like a 150 CFM fan should be able to move enough air to heat/cool a small insulated bedroom.
Again the key is the fan needs to be fed with the air coming straight out of the mini split, not the room air where the mini split head is located.
I agree with the comments about the low temp delta and relatively low air flow rates being a problem when put together. My suggestions:
-go with low static ducted if even temperatures are a high priority. Better to spend money on some ducting, than additional transfer fans that will inevitably add up to a couple hundred dollars each and not be that effective.
-if you go ductless... try and discharge the outlet towards the hard to heat area, (And don't stuff the head against the ceiling, that's asking for control problems. IMHO keep it just high enough to avoid bumping you head on it.) Do everything you can to take advantage of natural convection currents. Make door opening tall, and wide, keep ceiling low to trap the heat at a usable level.
-If you do do transfer fans, consider how the air get in AND out. You might need door grills, or a larger door gap to avoid just build ineffective pressure in the room.
-Ductless CRAZY thought, maybe go with higher end/smaller windows, and better insulation details in the bedrooms, this might help to achieve a natural balance since the heat loss/gain should be lower to match the lower available capacity.
More thoughts... You want to setup it up to take advantage of maximum exergy. The might mean trying to draw the hottest air from up high in the mini split room and force it into the cold room. Or, it might mean drawing the coldest air from near the floor of the cold room and push it into the mini split room. You don't want to defeat convection but enhance it.
Having a wood stove in my house made me appreciate the power of natural convection, you don't want to fight it if possible. It's often better to blow the cold air back towards the stove at floor level, and let convection carry the hot air up high to replace the cold air. I've found running my central fan actually makes the hallway bedrooms colder since it pressurizes my bedrooms with relatively cool/neutral return air and stops the natural convection from the wood stove. The biggest change I made to help my stove work better was to enlarge the opening from barely 3ft wide and 7ft tall to 10ft wide and almost 8ft tall. My wood stove room is usually 23-25C depending on how hard I'm running it and bedrooms are 19-21C, kitchen and dining is 22C. My point is be prepared to have to slightly overheat the mini split served area, to get enough temperature difference to drive meaningful convection currents. And ceiling fans (upflow/downflow) proved ineffective since they reduced the exergy by eliminating stratification, which nerfed the useful convection to the bedrooms. Now I keep the central fan, and ceiling fans off in the winter.
The "transfer fan" described starting at about the 1:42 mark in the video is estimated to be only 50-80cfm, per Risinger. Even at a 5F temperature difference and 100 cfm (possibly 2x the actual cfm you're talking only ~500 BTU/hr.
That's roughly the heat output of one adult human during a work out session, or two sleeping humans.
The transfer fan may be useful for moving ventilation air, but useless for space conditioning.
The "Perfect for room-to-room powered circulation in multi-family projects with ductless heat pumps" marketing fluff notwithstanding, at the 60cfm of a WhisperValue FV-05VS3
it's only about ventilation air distribution:
https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Panasonic-FV-JD-Product-Overview.pdf
Since it's designed to accommodate any of the WhisperValue series, the FV-0510VS1 or FV-0810VSS1 would deliver 100cfm when set at high speed, but that's still all about ventilation, not space conditioning.
The only relevance to ductless heat pumps is the ventilation air distribution. It would be the same issue with any non-ducted heating solution, not just ductless heat pumps. Whether it's a ductless head or radiant floor or a radiator heating the room it still needs ventilation air.