Powered vs Bypass Humidifier?

[Jim Miller]

I've reviewed the Aprilaire and Honeywell humidifier specs and the only thing I can find as benefits to the powered is about 5% (4200 vs 4000 sqft) greater coverage and no ducting to the return plenum. In return for that is a somewhat larger hole for the fan powered unit and the need to hook up 110v if an outlet is not nearby.

The AA600 has a builtin damper on the bypass and is cheaper than the bypass Honeywell.

The Honeywell claims to learn the on/off patterns of the furnace over time as a replacement for a external temperature sensor: more on time means it's colder outside.

I'm tempted to just get the AA600 for simplicity sake; one less fan to go south.

OTOH, I'm at 4200sqft which is at the highend for any of these. I'm just north of Baltimore so the temperature and relative humidity isn't as bad as when we lived in NH. There we always got shocked in the winter. Here we seldom do but it's certainly dry enough to cause nasal discomfort and some wood shrinkage.

Can anyone really tell the difference between the 700 and 600 given that the output capability only differs by 5%?

tnx jtm

[nicksanspam]

Time for more air-sealing, with a blower door test?

Nick

[Jim Miller]

Nick

I'm reviewing their specs and don't see much other than 5% between powered and bypass. I don't understand your response.

jtm

Time for more air-sealing, with a blower door test?

Nick

[nicksanspam]

Air-leaky houses need more humidification. I was suggesting a deliberate effort to better air-seal and caulk your house before undertaking winter humidification. Air sealing can dramatically lower the required humidifier capacity and save vs waste heating energy. There are probably "blower door test" people in your area who can measure and reduce your house air leakage.

Nick

[Chet Hayes]

The two issues I thought of when I went with a powered model were:

1 - Bypass units decrease some of the blower capacity by recirculating a portion of the air

2 - They also send hot very moist air right back through the heat exchanger. I would think that could lead to rust and a shortened heat exchanger life, but I have no hard evidence of it.

[Zypher]

Chet is right;

The by-pass models have a tendency to shorten the life of some 'iron' heat exchangers. The newer furnaces today though have aluminized heat exchangers and aren't subject to rusting like the older heaters. Some manufactures use tubular heat exchangers too. You might check to see which one you have.

Powered humidifiers inject the moisture directly to the supply plenum. Also, there are models that 'spray' water on the cooling coil and use the existing drain pan for excess run off. These also do not use bypass methods.

As for energy use, when the moisture level is kept around 50% - 60% during the winter, the we use less energy (less cycling) since we feel more comfortable at lower temperatures than if the air was warmer and dry. If the air is dryer, we feel the need for more heat. Also, the air is more receptive to heating when moisture is present.

[nicksanspam]

You seem confused. "Cycling" has little to do with energy use.

Not much lower... The ASHRAE 55-2004 comfort standard says a 48x48x8' house with R20 walls and ceiling would be equally comfy at 69.4 F and

20% RH or 68 F and 50% RH. If it's very tight, with 0.5 air changes per hour, would humidification to 50% save energy?

G = 48'x48'/R20 + 48x4x8/R20 = 192 Btu/h-F, so dropping the room temp from 69.4 to 68 F saves 1.4x192 = 269 Btu/h. At 69.4 F and 20% RH, Pd = 0.2e^(17.863-9621/(460+69.4)) = 0.1466 "Hg, approximately, with wd = 0.62198/(29.921/0.1466-1) = 0.003063. Air at 68 F and 50% RH has wh = 0.007347. With 0.5x48x48x8/60 = 154 cfm of air leakage, humidifying from wd to wh requires evaporating 154x60x0.075(wh-wd) = 2.96 pounds of water per hour, which requires about 1000x2.96 = 2960 Btu/h of energy, so the net "savings" is 2960-269 = -2691 Btu/h, or minus 64.6K Btu/day, costing about $1/day more with oil heat or $2 per day with electric heat.

People tend to forget that evaporating water takes heat energy, even if the "humidifier" uses little energy by itself, and that heat energy has to come from somewhere. And we often get into discussions about health and furniture, vs energy, and forget that caulking (vs humidification) can raise the indoor RH while SAVING vs wasting more heating fuel.

Heat and temperature are different.

You have an unusual grasp of physics :-)

Nick