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I've got an electric dryer currently vented outside: enter image description here

Behind the door to the right is our heat pump furnace: picture from other side

I was thinking about, during the winter, venting the dryer (properly filtered for lint) into the furnace return (which would then be filtered again by the furnace filter) instead of outside.

My thinking is that it will act somewhat as a humidifier (the house is very dry in the winter) as well as conserving a bit (not much) of heat from the dryer output.

Can this setup work? What dangers would I need to address? And of course will I actually be doing any good at all?

3 Answers 3

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Can you do this? ABSOLUTELY NOT!!

Aside from (likely multiple) code violations you will never filter the lint out and will create a dangerous situation inside the furnace plenum, aside from clogging the furnace filter probably weekly.

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    Yeah, you'll at least wreck your heat pump with all that lint, not to mention a dozen other reasons why this is a bad and dangerous idea.
    – iLikeDirt
    Commented Feb 6, 2015 at 16:33
  • Well in my defense, I did say properly vented to remove the lint before it enters the return. I haven't really done much research to see if that is feasibly possible but if you guys are saying that it isn't I'll trust your judgement. Btw, what code violations would there be? I don't see this as much different than one of those inside vents for a dryer they sell at Home Depot and/or a humidifier hooked to the return (which my hvac guys said was standard when they installed the unit but I opted not to get it for now)
    – kinar
    Commented Feb 6, 2015 at 17:24
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    The code violations are 'dryers must vent outside' because of the significant fire hazard presented. The indoor vents they sell at Home Depot are probably not legal to use either, but if they are, the difference is that the kit has passed some testing standard and does not potentially exhaust highly combustible material into the furnace.
    – Zhentar
    Commented Feb 6, 2015 at 19:53
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    Fair enough. I suppose even if I could do it safely, the cost involved to put in the necessary failsafe checks would probably outweigh any potential gains. Plus once those failsafe checks are in place, that may well remove the efficiency gains as well.
    – kinar
    Commented Feb 6, 2015 at 22:32
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You could build (or possibly buy) a heat exchanger for the dryer exhaust. This way you could extract the heat without mixing the air (and lint). This wouldn't be as efficient as what you wanted to do, and you wouldn't get the humidifier effect. But it would be safer.

DIY Example: http://www.alkeng.com/AL/dryer

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I wouldn’t exhaust it into your furnace. That would definitely be a fire hazard. Just disconnect the hose, cover the hole to outside and put something to filter the excess lent and use a fan pointed outside of the laundry room so it doesn’t get overly humid in your laundry room. Also, don’t do this with a gas dryer. Only do this with electric. The heat energy recycled would be approximately 3,400 BTU/kWh used for the dryer.

One other advantage, and this would mitigate issues with excess humidity. (In case it’s not super-dry, but cold enough you need to heat your home. If you have a dehumidifier, set it to about 40%, so that any humidity in excess of 40% will get removed anyway. Don’t concern yourself with the extra electricity that a dehumidifier uses because of the dehumidification (condensation) process will release latent heat. This amounts to an additional 1840 BTU/pint. A small 20 pint/day dehumidifier uses about 400 W, which adds another 1340 BTU/hr. So for every hour the dehumidifier runs you’re getting 3000 BTU for 0.4 kWh. (7500 BTU per kWh) This does not count the electricity useage and heat released from the dryer.

And yes, I realize that latent heat will get absorbed by the evaporation process (latent heat of vaporization for drying your clothes). This is lost anyway, so the net effect would be negligible.

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