# Using interior “bath” fans to send warm air to other rooms?

| Posted in General Questions on

The house we are designing is a single story rectangle – 28′ x 48′, that will be pretty tight and a bit better insulated than code. Estimated Manual J is around 14,000 BTU. We are in zone 4a in Virginia.

Half of the house is one large open room. The other half has 2 bedrooms, 2 baths, and a utility room. We plan to have the main source of heat in the large open room.

We are interested in using a couple of Panasonic bathroom fans to send warm air from the main room to each bedroom, using 6″ metal ducts going straight about 15 feet (within the building envelope).

The fans have adjustable speeds, and at the highest speed of 150 cfm they could change over the air in each bedroom in just 7 minutes (which might be too much). Seems like that could do a fairly decent job of evening out the heat between the main room and the bedrooms. I don’t care if the bedrooms are a few degrees cooler, though.

The bathrooms and utility room can have small baseboard heaters, if needed, though open doors should provide much of their heat.

Does this sound feasible? Anyone else use something similar to good effect?

Should I get a professional to run the numbers and confirm the duct design, to see if I can get this approved by the building inspector? Otherwise, I’ll have to install baseboard heaters with programmable thermostats in both bedrooms, too. Thanks!

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### Replies

1. GBA Editor
| | #1

Debra,
This issue has been discussed at length on GBA many times. The short answer is that the plan doesn't work.

There are two dominant factors in this discussion.

1. The first relevant factor is the specific heat of air, which is relatively low (in other words, a cubic foot of air can't hold much heat). The specific heat of air is:
0.0182 Btu/cf/°F.
The amount of heat that a fan can move is calculated using this formula:
Heating BTUs = (cfm of the fan) x (delta-T) x 1.08
It isn't much.

2. The second relevant factor is the low delta-T. Another way to express this: It's really hard to raise the temperature of a 65°F room using 72°F air. It's a lot easier to do that with a furnace, because the furnace has access to 150°F or 160°F air.

We could mention other factors, like the fact that a furnace uses a big honking 800 cfm fan, not a little 150 cfm fan. But you get the idea.

The bottom line: Install a ductless minisplit in the main living area. Your bedrooms will probably be comfortable if your house has a good thermal envelope, and if your bedroom windows aren't very big. If you are worried, install a small electric heater in each bedroom. You probably won't need to turn on the electric heater very often, if at all.

2. Expert Member
| | #2

REASON #1: THE CODE DOES NOT ALLOW IT! (Simple Google would do)
M1501.1 Outdoor Discharge
The air removed by every mechanical exhaust system shall be discharged to the outdoors in accordance with Section M1506.3. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.
M1507.2 Recirculation of Air
Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors. Exhaust air from bathrooms and toilet rooms shall not discharge into an attic, crawl space or other areas inside the building.

3. GBA Editor
| | #3

Armando,

Debra is not using the fan as part of a "mechanical exhaust system." Nothing is being exhausted. Air is simply being wafted from one part of the house to another part of the house, just as when the furnace fan is used to mix air in the house.

Nor is Debra suggesting that the fan be used to move air from a bathroom or a toilet room to an indoor location.

4. Expert Member
| | #4

Sorry, my bad. I misunderstood the question.

5. | | #5

What if Debra wanted to do this for the purpose of keeping the bedrooms smelling "fresher"? If the doors were shut at night or a teenager wanted the door shut all the time, then it seems to me that an extra 30 to 50 cfm per bedroom might be nice.

6. GBA Editor
| | #6

Bill,
To reduce odors, the best approach is to install a mechanical ventilation system. Of all the ventilation system choices, a balanced ventilation system (an HRV or an ERV with dedicated ductwork) would be the most effective.

If you have odors, the idea is to remove stale air from the smelly rooms, and introduce some fresh outdoor air.

7. | | #7

I'm sure Dana Dorsett is going to weigh in on this at any time, but I had a question like yours and Dana recommended a mini-ducted unit:
"If the real heat load numbers come out at 20KBTU/hr or less (probably will) a single 3/4 ton mini-duct unit mounted in the "possible furnace location" could probably serve all 3 bedrooms as a single zone with very short duct runs (either soffited or in the unconditioned crawl space) with a 3/4 ton- 1 ton wall coil (or floor mount) serving the common area.
https://www.master.ca/documents/regroupements/12Fujitsu_Submittal_AOU18R...

8. | | #8

Mark,
I have been looking at something similar for our new house, worried about cold bedrooms. Thanks to Dana's input we are planning to go with the Fujitsu horizontal ducted units. We are in zone 4c and our heat load calculation came out to 21,000 btu/hr for a 2,200 sq. ft. house. The Fujitsu ducted units look good with good static pressure, better turn down, and better HSPF numbers than Mitsubishi. We are going to use a 2 ton unit with a 9,000 and 2, 7,000 btu/hr ducted units. I believe the units modulate all the way down to 3,100 btu/hr so if we split a unit between two bedrooms it can get all the way down to 1,550 per room. In our situation I expect the ducted units to also help us with circulating the air throughout the house allowing us to duct the fresh air from the HRV to a central location. We are going with conditioned attic spaces to give us room for the ducting.

9. | | #9

I would decide how much too cold/hot you are willing to tolerate in bedrooms, decide if you can leave bedrooms doors open and then calculate if it will work.

A 150 cfm fan will move roughly 1/2 the heat of an open door.

10. | | #10

Good suggestions and thoughts, folks. Thanks. Martin's formula is very helpful. Basically tells me that those fans could produce at least half the heat the bedrooms need when the doors are closed at night (one of us snores loudly). So, the fans would reduce how cold the rooms will get. And the baseboard heaters could supplement that, if needed. I might not mind bedroom temps down to 60 F, but my sister won't.

We'll leave the bedroom doors open during the day. We can't afford a second mini split for the bedrooms, but are seriously considering using one for the main living area.

11. | | #11

Be careful with the electric heat. Say you want a bedroom 1 degree warmer than the rest of the house. The electric heat will be providing more than 100% of the room load all the time.

Fans into closed door rooms work best in matched pairs - one for intake, one for exhaust.

12. | | #12

Hmm... Never heard the suggestion for matched pair fans before. I'll look into that. As for the electric heat, I plan on setting that well below the temp of the main rooms.

13. Expert Member
| | #13

With a an estimated 14K heat load on the Manual-J you're well within the capacity of a 1.5 ton Fujitsu at VA type outside design temps.

If you're SURE that it's only 14K and your 99% outside design temp is above +15F you may even be able to drop back to the 1-tonner:

The layout matters and don't put the ducts up in an unconditioned attic. The mini-duct cassette need not be placed in the utility room- they're pretty small, and can be installed in the ceiling of a closet or in a small drop down soffit, but it has to be planned for. Unlike their competitors, the Fujitsu units can be mounted vertically in a small side-compartment to a closet or wall, making access for maintenance & service pretty easy.

These things aren't nearly as powerful as bigger-deal air handlers and it takes some amount of careful design and installation competence, but it seems like the "right" solution to your house. Mounting it somewhat central to the house with VERY short duct runs helps. Doing the traditional duct-designer's approach of running the ducts all the way to the exterior walls placing the register under/over a window probably won't work. See John Semmelhack's picures of a vertically mounted mini-ducted unit in response #14 in this thread:

Note the ceiling-mounted register is at an interior wall, but designed for a high amount of "throw" directed toward the exterior wall & window.

With VERY low heat loads in the doored off room you can sometimes still get there without baseboard heaters and a wall-coil mini-split in the main area, but that usually involves limiting the window sizing in the bedrooms and using triple-pane glazing in those rooms.

14. | | #14

Thanks for the shout-out, Dana. I agree, the slim-duct Fujitsu systems would probably be a good fit, as long as the air handler and ducts are entirely in conditioned space...several different ways to do this for a 1-story house in Virginia, including putting the air handler and ductwork in a conditioned crawlspace.

I'm always a little baffled by the transfer fan concept, since it adds back ~half of the cost-difference between a ductless and ducted mini-split, while solving much less then half of the problem.

Being in Virginia, I'm surprised Debra didn't mention cooling/dehumidification distribution (even more of an issue than heating distribution with point-source equipment)...unless she's up in the mountains in southwest VA.

15. | | #15

Dana and John, thanks for sharing more suggestions! It looks like I should hire a professional that really knows how to properly design this kind of system. I've been badly burned with extremely poor duct design by local HVAC companies in the past, even with regular heat pumps.

My 99% outdoor design temp is 16 F, and my best shot determining my Manual J using CoolCalc comes to about 14,300 BTU's. CoolCalc doesn't let me enter my R-23 mineral wool insulation inside a 6-inch wall, only R-21. But I don't think that will make much difference in total BTU.

And yes, I do live in the cooler mountains of SW Virginia. So I don't need much actual cooling - but our outdoor dew point is often in the mid-60's throughout the summer, so dehumidifying is important to me. I have lived here without AC for 15 years, and have experienced indoor mold due to constant high humidity levels.

I'll have a vented attic, and a closed conditioned crawl space. But I'm a bit concerned about installing the air handler even in a conditioned crawl space, as I'm rather hyper reactive to mold and still paranoid about exposure to crawl spaces. That's one reason I'm considering installing the interior fans, to help circulate the bedroom air back to the main room for conditioning and dehumidifying.

I have a 15 foot long interior hallway where I could create a dropped ceiling, if that helps. But from the center of the hall, it would still need nearly 8 foot ducts to reach the bedrooms and main living area. (NOT exactly very short runs.)

Still being in the planning phase, I can make changes to our house plan, including changing the bedroom windows to triple pane, if necessary. We only have one window in each bedroom, but it is somewhat large due to egress requirements for a double-hung window.

16. | | #16

Whoops! Just spotted the recommendations for professionals in a link to a different thread suggested above.

17. | | #17

I see no problems with the Fujitsu and duct runs much longer than you are considering. But get a proper Manual D.

18. | | #18

Carl Seville posted about installing bath fans to more air in his new house. He posted he would test it out when he had guests using those rooms, but so far no update. I'm going to post on that entry to see if we can get an update on how it is or is not working in his house...