View Full Version : Steam humidifier in a multi zoned attic with gas furnace.

01-26-2012, 05:20 AM
I am considering a residential steam humidifier ( april air 800) to pump humidity into my hvac system that is in my unheated attic. Can't use an evaporative system there because of freezing risk.

This unit heats 4 bedrooms which I am hoping to retrofit and zone individually with a thermostat in each room and a mechanical damper to each room.

Manufactured explained that the 800 system humidity controller is linked to the fan/blower because a normal heat cycle may not be long enough to generate the set steam humidity level. So the humidifier can stay on with the blower until the humidity level is reached even if the heat is not firing.

Question is : What if the system is zoned and dampers are closed or not calling for heat ? Has anyone installed this type of system in an area of multiple zones and does it integrate with dampers? Do I need to keep one zone always open for the humidifier?


01-26-2012, 06:49 AM
I think you may have problems unless you can control the dampers to all open when none of the rooms are calling for heat...then, the humidification can take place regardless. By blowing air through all rooms, you'll also provide more even heating. This will put a larger duty cycle on them - I'm not sure what that would do for longevity. See what the pros have to say about it.

01-26-2012, 03:34 PM
thanks for your input.

most appreciated

01-27-2012, 02:27 PM
I can't imagine where in WA the wintertime dew points are so low that you'd need or want to add humidity if you have a house that's reasonably tight. Dry air in winter in any place I've lived in WA (Columbia Basin, Colville res, or west of the Cascades) would be an indication of excessive ventilation or infiltration rates. (There's no need to humidify the air in my current residence, where the wintertime dew point averages are lower than in most of the PNW, or at least most years they are- the weather averages have been crazy-warm this winter- feels almost like Seattle some days, take TODAY f'rinstance!.)

Any time you add humidity to the air in winter you're increasing the risk of condensation/mold/rot along the exfiltration path during colder weather. Rather than adding humidity, better solution to winter dry air is to tighten up the place, and control the humidity by controlling the ventilation rates. Ideally for comfort & health, and the longevity of the structural wood you'd run the wintertime interior RH around 35%, which doesn't take an ultra-tight house to achieve simply by reducing infiltration, unless it's 1- person household that's unoccupied for 10-12 hours/day. Letting it rise to ~50% is just fine in the warmer months, when all of the wood is above the dew point of the interior air.

HVAC systems in the attic can sometimes be the driver of unwanted and unintentional ventilation due to pressure differences caused by air leakage on the ducts or air handlers, or if there are doored-off rooms without returns, creating imbalances in the system. Air sealing both the ducts and the house would make a difference, but start with the ducts & air handlers first. (Putting any part of the mechanicals outside of conditioned space is a bad idea in general, but is very common with slab-on-grade construction.) Proving return paths inside of conditioned space for any of the doored off rooms is also an important first-step.

Making the house super-tight and spending the money on a heat recovery ventilation system (HRV) with it's own tiny duct system (4-6" trunks are common) rather than a humidifier is likely going to be a better investment in health, comfort, and utility costs. Making it super-tight would require first fixing all the leaks you know about. The attic penetrations & foundation/basement/crawlspace penetrations come first to kill stack-effect drives, paying attention that could be an unfettered air channel between the lowest part of the house and the attic- worry about the windows & doors later. Then hire an air-sealing company that uses calibrated blower doors and infra-red imaging to find the other 500 square inches you never thought could possibly exist, and fix the most readily-addressable bits. If you can get the blower door test down to even 5 air exchanges per hour at 50 pascals pressure you won't have any dry-air issues unless your HVAC leakage is driving infiltration like hell due to leaks or system imbalance.

It's your house, your money, but that's where I'd be looking...

01-28-2012, 04:35 AM
Thanks for your input.

We are in washington D.C. not washington state and the winter air is quite dry. The house is tight and well insulated.

It is common here to have hvac units in the attic, but not humidifiers because of the risk of freezing pipes. A steam humidifier could live in my utility room in the heated area below the attic I am just note sure how to integrate it with a system that has dampers. Maybe there is a controller that runs the fan and opens all dampers for humidification only. Just not sure.


01-30-2012, 12:05 PM
Fooled by the flag-avatar!

The average wintertime dew point of outdoor air in Washington D.C. is about +24F, (comparable to Spokane WA, or the Colville res.) which has considerably more moisture content than in central MA where I live, with average dew points of about +14F. If my neighbors & I can keep the interior relative humidity above 30% @ 70F (dew point, of about +37F) solely by tightening up the house and controlling the ventilation rates, surely you can with your (comparatively) moisture-rich ventilation air.

Air with a dew point of 24F has about 26 grains of water per lb of air, whereas +14F dew point air has ~16 grains/lb. At ~40F dew point air (mid 30s RH @ 70F) there has ~40grains/lb, so your humidity shortfall on ventilation air is only 14 grains/lb to my 24grains.

The insulation levels of the house mean nothing from a humidity point of view- it's all about air-tightness of the building envelope, and the tightness of the ducts & air handler which are otherwise driving infiltration by creating pressure differences that get short circuited by using the great-outdoors as the return path. It really takes tightness on both, especially with ducts & air handlers in a ventilated attic, which is outside the pressure boundary of the house.

If you think the house is as tight as you can get it, it's time to hire a pro with the blower doors & IR cameras to nail down the rest of the low-hanging fruit. It will lower your heating energy use, and in muggy-D.C. it makes a substantial difference on the latent cooling load as well, making it a more comfortable environment year-round.

01-30-2012, 03:52 PM
thanks so much !