Jadnashua
Retired Defense Industry Engineer xxx
Mine is probably 15+ years old, but is similar to the model 500. It is an AprilAire unit. It works just fine on the cold supply. You might end up using slightly more water if you use cold, but I think the major reason to use warm water is that it might not cool the exhaust stream quite as much. The extra heat from the hot water supplied does not really make that much difference on how much evaporates, but helps a little by not cooling the air from the evaporation process as much. Just like sweating cools you, evaporating the water from the pad will cool the air. How much is evaporated is MUCH more dependent on how hot the air is and its relative humidity level than the inlet water temperature. I've not had any issues. Rather than having issues with stratification in the WH, and then going to the expense and hassle of adding a pump and control circuits, I'd just swap it to cold. BTW, it calls for a MAXIMUM of 140-degree water, not that it must be that hot. Most WH are not set that high. I look at it this way...the thing doesn't usually run all that long, and doesn't use a lot of volume. The water in the pipes will have a chance to warm up to the ambient air temperature, and depending on where you run it from, it might actually be hot based on what it is running next to, so running dedicated hot water to it is IMHO, a waste. Now, I've not tried both ways, but mine does run successfully and keeps the humidity where I want it while feeding it cold and not impacting the hot air output. My supply line does run part way against the outlet ducting, so probably absorbs some heat from it, but I doubt it is truly hot from the cold supply. If it ran constantly and never reached the desired humidity levels, I'd reconsider, but it works fine as installed. Your results may differ. If the supplied water was near boiling, or was a steam based system, it would be a different story since the on-board heater may not be big enough to raise the temp from what could be near freezing, but an evaporative, bypass system, I don't see it. Cold water evaporates, too, and the hot air moving through has a great affinity for absorbing it. Just walk outside with one hand wet and one dry in the winter, see which one gets colder. That happens because one is evaporating. It's also pulling heat from your hand, but the outlet of the furnace is hot enough so you'd probably never notice.