I am in the process of replacing my aging system which is currently an oil furnace & outside AC unit. I am installing a heat pump to provide cooling in the summer and heat on moderate days & a new oil furnace for the colder days.
I like the results of a humidifier but I'm not sure about using one with the heat pump. Does the heat pump produce air hot enough to use a by-pass humidifier, or do I need something else?
I can't give you a yes or no answer, I can give you a little theory though,
since the heat pump will be used in mild weather, your humidity demand will theoretically not be as high as it is in the winter when you are running the oil furnace, my guess and its only a guess, is that if that humidifier is sized properly for the home during the coldest weather , then it will suffice for the mild weather also even when you are running the heat pump.
Even though you will evaporate less water into the air with the heatpump, you will also get a longer run cycle out of it than you would if you were running the oil furnace in the same condition, so I don't think you will see a problem.
I have a bypass humidifier on my dual fuel system, and it does fine. I do have it connected to hot water, though.
You might tend toward one of the larger units, though- maybe the 17 gallon per day model instead of the 12, for example. My experience agrees with gruvn; it won't work as well when you're heating with the heat pump, but you don't need as much humidity output then anyway.
Make sure not to oversize the furnace, though. No evaporative humidifier works well when used in conjunction with a furnace doesn't work very hard. I seem to remember that the humidifier manufacturers rate their gallons per day output based on the assumption that the furnace runs *75%* of the time. A furnace that's too big for the job will only run for a few minutes here and there, even on the coldest days.
Thanks. It seems as though most of these are made by one manufacturer (Nordyne?) and they all have the same manual and specs. They all list specifications based on a 120 degree plenum temp. Do heat pumps have a target range for this or are there too many variables involved? What is the plenum temp on an oil fired furnace? The comment about not needing it as much when running the heat pump makes sense since the temp is lower.
No, there are several manufacturers of the humidifiers- though there is a great deal of rebranding, too. The main manufacturers are Honeywell, General, and Aprilaire. Most of the units that are branded by the big furnace/AC manufacturers (Carrier, Trane, Lennox, etc.) are made by Honeywell and Aprilaire. They just have a different brand name stamped on them. Across the all the brands, the design and specs are usually pretty similar, though.
They assume a 120 degree plenum temp, which is more typical of fossil fuel systems (gas, LP, or oil). Most furnaces are rated for a 30-70 degree temperature rise.Heat pumps, in dual fuel systems, will typically have plenum temperatures more like 90-100 degrees (a rise of 20-30 degrees), depending on outdoor temp and some other variables. Since the plenum isn't as hot, the water doesn't evaporate as readily from the humidifier, and that's why the humidifier output won't be as much when you're in heat pump mode. That's why I suggested aiming for a larger humidifier unit than you might use otherwise; the extra capacity will help a bit there.
The compensating factor is that the heat pump will run a heck of a lot (in temps in the 40s and 30s, probably nonstop). That means that the humidifier can run nearly nonstop, too; the added operating time helps to compensate for the lower plenum temps.
My experience with a bypass humidifier in a dual fuel system (my own home) is that the humidifier runs pretty much all the time that the heat pump runs. When it gets really cold and the furnace takes over, the humidifier actually runs a bit less- maybe only half of the time that the furnace is running. So or my two ton/54k btu output system in a moderately tight house with a 33k heat loss, a 12-gallon capacity bypass unit connected to 130 degree domestic hot water has had just enough output that it can keep up in heat pump mode, but it's plenty of capacity when it gets cold enough to burn fossil fuel.
Keep in mind that these systems can use quite a bit of water. Most consume 6 gallons per hour of operating time (most of the water goes down the drain, so that's why the output is 12-17 gallons per day). If a 6 gph humidifier runs for 18 hours a day, you are looking at 3200 gallons per month. Running one with a heat pump lowers its productivity, so it will run more of the time than it would with fossil fuel heat alone. More runtime equals more water consumption.
Some will advocate the use of steam humidifiers with heat pumps. Those are basically the same idea as boiling a pot of water on an electric stove. Instead of using the heat from the heating system itself to make the water evaporate, they boil it using a heating element. They are fairly water efficient in comparison but can burn through a huge amount of electricity in boiling all of that water. They are also relatively maintenance intensive compared to the evaporative flow-through humidifiers, especially in areas with hard water. Finally, they're at least twice the price of bypass units.