Debra’s new house in southwestern Virginia will be a one-story design of 1,344 square feet with half the space devoted to a single, open room and the remaining area divided into two bedrooms, two baths, and a utility room. The main source of heat will be in the open room, and in the absence of a conventional forced air heating system, Debra’s quandary is how to distribute the heat evenly.
Heating loads for this Climate Zone 4A house are relatively modest — about 14,000 Btu per hour. Between small baseboard heaters and leaving doors open, the bathrooms and utility room should have enough heat. It’s getting heat to the bedrooms that’s looking more problematic.
In a post at the Q&A forum, Debra wonders whether low-output bathroom fans can be used for heat distribution.
“We are interested in using a couple of Panasonic bathroom fans to send warm air from the main room to each bedroom, using 6-inch metal ducts going straight about 15 feet (within the building envelope),” she writes. “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,” she continues. “I don’t care if the bedrooms are a few degrees cooler, though.”
This simplified heat distribution system would let Debra skip baseboard heaters and programmable thermostats for the bedrooms. But will it work?
That’s the topic for this Q&A Spotlight.
Sorry, the plan won’t work
GBA senior editor Martin Holladay gives Debra two reasons why this is not a good idea.
“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,” Holladay writes. “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.”
There’s another reason, he adds, and that’s the relatively low delta-T (the difference in air temperature between the main room and the bedrooms). Holladay explains it this way:
“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.” Plus, a furnace would have a much more powerful fan, one capable of moving 800 cfm, not the 150 cfm that’s possible with a bathroom fan.
“The bottom line: Install a ductless minisplit in the main living area,” Holladay says. “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.”
Still, the fans would help
Holladay’s formula suggests to Debra that bathroom fans, while maybe not a perfect solution, would certainly help. She thinks they could provide at least half the heat the bedrooms would need when doors are closed at night (“one of us snores loudly,” she explains).
“So, the fans would reduce how cold the rooms will get,” she says. “And the baseboard heaters would supplement that, if needed. I might not mind bedroom temperatures down to 60°F.”
“We’ll leave the bedroom doors open during the day,” she says. “We can’t afford a second minisplit for the bedrooms, but are seriously considering using one for the main living area.”
Jon R adds that a 150 cfm fan will move about one-half the heat provided by an open door. If Debra does decide to use fans, he suggests installing them in pairs — one for supply and one for exhaust.
Could bath fans help keep air fresher?
If using low-volume bathroom fans won’t solve the heat distribution problem, Bill Dietze wonders, wouldn’t they at least help keep air in the bedrooms seem 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,” he writes.
If fresh air is the goal, Holladay replies, the best approach would be a mechanical ventilation system — ideally a balanced system that both exhausts stale air and blows in fresh air from the outside. Either a heat recovery or energy recovery ventilator would work.
“If you have odors,” he says, “the idea is to remove stale air from the smelly rooms, and introduce some fresh outdoor air.”
The ducted minisplit solution
Unlike a wall-mounted ductless minisplit head, a ducted minisplit distributes conditioned air to nearby spaces via a fan and dedicated ducts. But the units are not nearly as powerful as a conventional air handler, and duct layout is crucial.
“The layout matters, and don’t put the ducts up in an unconditioned attic,” Dorsett says. “The miniduct 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 and service pretty easy.
“These things aren’t nearly as powerful as bigger-deal air handlers, and these systems take some amount of careful design and installation competence, but it seems like the ‘right’ solution to your house,” Dorsett adds. “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.”
A vertically mounted ducted unit was described in a similar thread last year by John Semmelhack, Dorsett notes. In that thread, Dorsett cautioned that finding an HVAC installer who could design ducts for such a system might prove tough.
In Debra’s case, Dorsett says that assuming heating loads in the rooms that will be closed off from the rest of the house are very low, they could still be kept comfortable with the heat provided by a wall-mounted minisplit in the main area. “But,” he adds, “that usually involves limiting the window sizing in the bedrooms and using triple-pane glazing in those rooms.”
Where to run the ducts
Semmelhack agrees that a Fujitsu system would be a good fit for Debra’s new house, as long as the air handler and the ducts are located entirely within a conditioned space. He could imagine several ways of accomplishing this in a one-story house in Virginia, including putting the equipment in a conditioned crawl space.
Debra is planning on a vented attic and a conditioned crawl space, but she doesn’t think that putting any equipment in the space below the house is a good idea.
“I’ll have a vented attic, and a closed conditioned crawl space,” she writes. “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.”
A dropped ceiling in a 15-foot-long interior hallway might provide enough room for the equipment, she says, but it would require 8-foot duct runs to reach both the bedrooms and the main living area. “Not exactly very short runs,” she says.
That’s a question a Manual D calculation would answer, Jon R replies, although he imagines that the Fujitsu could handle duct runs much longer than what Debra is considering.
Our expert’s opinion
GBA technical director Peter Yost adds this:
I wish I could say I have a conclusive answer to this question: Can you use exhaust fans to create adequate distribution and mixing in bedrooms that are not served by a central ducted system?
My own perspective: By the time you purchase two quality fans that are quiet enough, energy-efficient enough, and easily adjusted, you might as well go with a dropped-soffit, ducted hallway solution. Additionally, we need to let go of concerns around installing equipment in properly detailed, conditioned crawls and attics; it’s an irrational bias based on our history of thoroughly confused and dysfunctional “vented” crawls and attics.
But I am no HVAC expert, so as I usually do, I checked in with, first, a local HVAC expert — Mark Russwick of ARC Mechanical and then with someone I respect who has actually deployed this type of distribution solution — Carl Seville at SK Collaborative.
First, Mark Russwick:
“I’m not a fan, no pun intended, [but] will this work in Virginia? Yes, most likely, but the bedrooms are slaves to whatever is happening in the main room and I find bath fans noisy. It’s too bad that someone doesn’t make a single-zone, down-flow air handler, heat pump split system.”
Second, Carl Seville (based on comment #18 in the original Q&A thread and this GBA blog of Carl’s):
“It works quite well. When there are no guests we leave all upstairs bedrooms doors open and the [minisplit] head in the hallway conditions the upstairs fine including the master suite — we don’t even turn on the head in the master under those conditions.
“When we have guests and close the bedroom doors, we use a 190 cfm exhaust fan located above the hallway minisplit that is ducted into the two front bedrooms. It is on a relay that turns it on whenever the minisplit is on with an override switch that allows us to turn if off when we don’t want to use it. (We keep it off most of the time.) We will also turn on master bedroom head as well when doors are closed.
“The front bedrooms have jumper ducts to the hallway to equalize pressure when the doors are closed and the fan is running. We have found that without the fan, the bedrooms will be noticeably warmer or cooler or more humid than the rest of the house if doors are closed. This summer my wife was ironing in one room with the door closed and the fan off and it was about 10 degrees hotter than the rest of the house. With that tight an envelope (0.88 ach50) even an iron can heat up a room.”
My parting perspective: Carl’s home, its performance, and the configuration of his exhaust fan solution all are significantly more honed than the dual exhaust fan approach that Debra is proposing. Carl’s solution isn’t really evidence that contradicts my or Mark’s concerns; rather, I think it supports the need for a more sophisticated approach.