I think that the goal of running humidity "as high as you can to keep plants happy" is perhaps not the best approach to providing a growing environment for your plants. The 60% I mentioned above is *not* a target, it's the maximum allowed. If it stayed that high all the time, there would be problems with mold and peeling paint. At the moment, with some sensitive plants as temporary houseguests, I feel much better when it's around 50-55% for several hours a day, with the aforementioned dry-off period every night. I tried to mimic, if not duplicate, greenhouse conditions in my grow room for the first few years. I was convinced that with my HID light that my plants would just shrivel up if they weren't getting constant high humidity. But you know what? Finally I got so sick of cleaning my fogger that I tossed it out and thought, if the orchids don't like it, let 'em die. Guess what? They went right on, and I'm having more flowers now than before because I'm buying plants that are appropriate for the conditions that I can apply without breaking a sweat, and all that attention to humidity has been directed to other considerations. At the moment, I have three sensitive plants that I want to bring into bloom, enjoy, and move on so I'm paying more attention to humidity. After they're gone, I MAY run one humidifier in there to keep things above desert conditions while the heat is on - I'd be happy with above 40%. In the spring, I'm just going to ventilate the room and forget about humidity. J

watertite would block it for the most part, providing the humidity is coming from the basement walls. Besides, why did it work for 43 years without watertite, and suddenly it's absolutely needed? None of the immediate neighbors have regraded their yard, nor did my mother. They dug a trench to replace the root-clogged sewer pipe, and it was backfilled flush with the surrounding soil. No changes were made to the downspouts, they still discharge about 3 feet away from the foundation walls. The interior concrete feels dry to the touch, and there is no white efflorescence anywhere. There is no sump pump, and of course the sewer pipe from the main stack to the street is all brand new PVC. All toilets, sinks, tub drain properly with a quick swirling action. There was never more than 2 inches of water in the basement, and it was addressed immediately. Everything higher than perhaps 3 feet up to the ceiling was not damaged and left untouched. A hygrometer http://www.amazon.ca/General-Tools-Instruments-MMD4E-Moisture/dp/B00275F5O2/ref=sr_1_1/179-1872848-1950766?ie=UTF8&qid=1445032321&sr=8-1&keywords=general+moisture+meter shows between 5 and 8% humidity in the wood. Could there be another possibility for the increased humidity levels? I am no expert, but it seems to me that enough moisture coming from the basement to cause a 20% increase in RH levels would be noticeable with the naked eye. We bought two hygrometers, one analog and one digital, and I brought a third one from my house so I know it's not just a faulty hygrometer. I am not discrediting you or saying that you are wrong, or saying that I know better than you, I am just trying to understand where the excessive moisture would be coming from. Thank you for taking your time to help me.

When there is little to no cooling to be done, there can't be any dehumidification without overcooling and reheating if you are working with conventional refrigeration cooling like this. Residential systems are not set up to do that unless you have a dedicated dehumidifier. Variable speed compressors and blowers help a lot, extending the dehumidification activity by biasing the system to humidity removal and away from cooling. They can only go so far. At night, and in the spring and fall, when heat load is low, you are going to fall out of the ideal comfort zone more frequently that in the blazing daytime in August. You might find that with your vastly different types of systems, upstairs and down, that is at different points. (On the other hand, other characteristics differ upstairs and down so pinning it on the AC would be difficult.) When I get home in the evening, the humidity is at its lowest in the house. By the morning, it has climbed, but still below 50% Since I have seven indoor units and I shut most of them off at night, I have better capacity staging than most and I assume that helps. That is all very different from what you are seeing now. I honestly think that you should be able to attain low 40s in the daytime/evening. Above 50 is too high for a fully variable system like that. When I wrote distribution problem, I meant leaking ducts that cause humidity to be drawn into the house due to depressurization of the house. You describe a different kind of distribution problem. The air flow to individual rooms needs to be adjusted, lower in the cold areas or higher in the warmer areas. Looking to your future, I think that the dehumidify setting works this way. In some systems you get to set a temperature and others (mine) you don't. The latter type system just uses the current temperature as your desired temperature. After a temperature is chosen and it tries to meet it, it is allowed to overcool below that temperature. Depending on your system, you can choose that overcool amount or it might be fixed. That looks like a pretty typical modcon boiler. If it is not oversized, you should get greatly improved efficiency if the old one was an old 80%ish efficiency boiler. Are you using an indirect tank for DHW? Maybe not since you mentioned turning off the boiler. You are lucky to have CI rads, they play well with the lower water temp desirable for highest efficiency. Lots of original rads in a home with significant air sealing and insulation improvements helps a lot too. With Modcons it would be hard to have too much radiation in a retrofit situate. That allows you to keep water temperature down. Don't feel bad about having only one zone for at least two reasons. First, if a home is well insulated and sealed, logic alone tells you that heat (and humidity) moves from room to room much faster than inside-outside so economy from zoning is often not realized. Second, smaller zones can be a problem for a modulating boiler. If it can not modulate down to an output that is appropriate for the smallest zone in low heat demand times, it will cycle a lot and that cuts into efficiency. A buffer tank can be added, but who wants that? There are some homes that went to multiple zones in the 70s and after (when fuel prices climbed rapidly) where hydronics designers recombined zones in to make them work with a new mod con! Mod con geeks/chauvinists like to see their boilers modulating up and down but rarely shutting off in the heating season. With an ideal system and careful boiler set-up, it can be done. An upstairs and a downstairs zones might be nice and might not be that hard to do, but be sure someone does not adversely affect boiler operation if you go that way. Calculations ought to be able to tell you what to expect wrt cost heating with the heat pumps vs. boiler. It might be a little tricky with the unknowns of an old house with unknown insulation and air leakage rates, but it should be possible to come close. Good luck with the AC guys!

I realize that this is an old thread, but folks may review it in the future and benefit from the discussion. Indeed, there is another, recent, similar thread on low indoor relative humidity in a Minnesota home in winter, so it worth correcting some of the misinformation on this thread. "Again and hopefully for the last time you can not dehumidify with a single stage AC to any large extent. It will always over cool, long before it dehumidifies to any large extent." A properly-sized air conditioning system-- including single-stage systems-- will dehumidify air whenever the air temperature leaving the coil is at or lower than the dewpoint temperature. In the summer in my (mixed-humid) climate zone, that's most of the time. I have a single-stage air conditioner. All of the water being drained at the end of the condensate line is ample evidence it's dehumidifying. In fact, it creates a wet spot in the lawn that I need to reseed every fall. When my air conditioner doesn't do an adequate job of dehumidification is during so-called "shoulder seasons" when there is no real demand for cooling (you can't dehumidify if it's not operating.) We get shoulder seasons for three or so weeks each spring and fall. For shoulder seasons, I've found dehumidifiers to be the right complement to my air conditioner. I have one on each level of my home integrated into the HVAC system. They enhance comfort, and they reduce the potential for mold/mildew which is very common in our climate zone. They can also be configured to introduce fresh air into a home, which is a plus, year 'round. "Just because an AC can't dehumidify a high humid situation, doesn't ever mean it is over sized... because the single stage AC wasn't designed to dehumidify. It was designed to COOL" Not so fast. An air conditioner, as the name implies, is part of a system which maintains comfortable indoor conditions for the occupants. It cools, dehumidifies, and moves air throughout the home. The dehumidification part is stripping moisture from the air. Indeed, when designing an air conditioning system, we need to consider both the sensible cooling load and the latent (moisture removal) loads. A properly-sized single-stage system can dehumidify air. Its ability to do so is affected by run time under particular conditions, which is a sizing consideration.