I have a cold air return for a gas forced air furnace positioned high on the wall above a woodstove. They would like to use the furnace fan to recirculate the heat from the stove, but the duct and or fan is too noisy to run continously.
My thoughts are:
1. replace cheap sheet metal vent cover with something heavier (less prone to vibration) like cast iron grill.
2. put some kind of soundproofing on the inside/outside of the duct (i.e. dynamat.) Expensive, but supposedly works well.
3. Replace fan/motor with something quieter.
4. Put a timer on the furnace fan so it runs intermittently.
Any input or other suggestions?
Thanks, Ian
Replies
Figure a way to tell them it's a bad idea, like the possibility of recirculating carbon monoxide or something. This can't possibly make any appreciable difference in energy conservation for the cost and effort involved. How'd you get sucked into this, anyway?
Well, I got sucked into this because it's my parents house. However, it seems that since they run the woodstove all the time that they might as well be circulating the heat throughout the house.
My HVAC contractor advised me to do pretty much what you've got: I've got a large 8"x48" return duct at the ridge above the wood stove and I hope it makes a big difference. Our furnace (which has yet to run because we're still building) has a 1400 CFM fan that should pump heat from the stove through the house.
It sounds like you haven't yet isolated the source of noise. Once you've done this, you should be able to formulate a strategy.
Scott.
I know that both the fan and the intake grill ( which is about the same size as yours - 8x48) are sources of noise. I am only assuming that doing some soundproofing on the duct would help, but maybe not.
Recirculating the air actually is not a bad idea, since the air in the house will stratify on its own, with the warmer air near the ceiling. This is the reason you can run ceiling fans backwards during the heating season. May not necessarily help with the heating costs, but should make the house more comfortable.
I take it you/they are proposing to run the gas furnace fan without the furnace doing the heating, so the air is pulled in the return and pushed out the supply registers? This will just cause wear and tear on the fan motor, consume electricity out of proportion to the benefit, plus draw in more dust to the inside of the duct and filters, etc., etc.. The noise is another problem you mentioned, that won't be helped all that much by soundproofing the outside of the duct or changing the grille. The noise is either being transmitted from the furnace itself or is due to undersized ducts for the CFM that are being run thru them. Unless the motor is on its last legs, changing it won't make an appreciable difference in noise; what would help is installing a 2-speed motor and running it on the low speed for this purpose.
I'd vote for installing a couple of ceiling fans, as mentioned above. They are also great for comfort in warm weather.
They already have one ceiling fan in the living room (same room as the woodstove). The problem is the layout of the house, which has a back section of two bedrooms, a bath and a laundry room which are connected to the rest of the house (in which the woodstove lives) by one doorway. So, the heat from the woodstove (even with the ceiling fan on) never really gets back to this part of the house.) So, then the furnace kicks on to heat this part of the house. Because of this I think that if we could reduce the noise, or run the fan on a timer, they would use just the fan to recirc. air, reducing heating bills. I like the idea of a two speed motor. Could this be controlled at the thermostat? Or just add a switch somewhere? Also, what about hooking just the furnace fan to a thermostat in the back of the house, so it could just come on as needed?
Offhand, I don't know of a thermostat that would allow you to manually control the fan speed (but they may exist), so I would suggest adding a switch near the t'stat. Yes, you could control the off/on of the blower via a second thermostat located remotely, but there will have to be some interlock between the main t'stat and the remote one. The remote 'stat could be a "line voltage" type (120V as opposed to 24V which is normal for thermostats) for direct connection, or a 24V with a relay. This really gets pretty complicated for a residence, and I'm not saying it can't be done, but unless you know an electronics geek who would be willing to spend some time setting it up, you would be better off just using a manual control for both fan off/on (do this at your existing t'stat) and fan speed (new switch).
Incidentally, you may have to search a bit to find a knowledgable parts man who can set you up with a 2-speed motor, since the run-of-the-mill counterman only knows what his catalog tells him, i.e. if you ask for a 2-speed motor for, say, a Carrier furnace and the cat only lists single speeds, most will be lost.
Neal