PRR said:
"" <50RH will keep mold and dust mites, and other nasty stuff in check.
?? My goal in the cellar is 65%. Mold becomes a real no-problem. Have not inventoried my dust-mites, which I agree would be an issue in a living space.
> Indoor humidity seems to be more influenced by weather than I expected.
Indoor air IS outdoor air. Even a Canada-tight house has several air-changes a day.
It doesn't get Maine cold here, but cold enough that I can find the air leaks on cold nights when the wind is blowing.
Bring your house up here. When it's blowing 0 deg, you'll find your air leaks. Actually it takes a while, because each improvement makes a lesser level of leaks dominant.
I have added an extra layer of storm windows (on the inside), so my house is pretty tight (air) but I noticed just the other day that I had left my kitchen door open, relying upon just the storm door to close off the space, The storm door is clearly not air tight and surely allows in moisture.
Here/now is 100% RH outside (at the airport), 35% inside. Same air and water, same absolute humidity, but 35 degrees warmer. Actually, looking at psychrometric chart, that stuff outside could be warmed-up a dozen degrees before it came off 100% RH, but another 25-30 deg F makes it 35% RH.
> mid 40's RH when I retire, to up near 60RH in the morning
What is the temperature drop? If not small, it may just be RH going up as temperature goes down. (But 15 points overnight does sound large.)
Yup, some of that is temperature. The low-mid 40s is after the de-humidifer was running all day. The morning number is significantly cooler but regulated by thermostat. I also notice that the temp of my humidity meter over on my dresser is several degrees cooler than over by the thermostat, so air movement at night is pretty stagnant.
In your situation, more heat may be part of the answer. RH will go down. Heaters are mostly more efficient than freon machines. If there is an alternate to electric heat, the fuel is cheaper too.
Actually my bedroom is part of my ongoing energy saving experiment. I use a TOD thermostat that only heats the bedroom at night when I'm sleeping. Winter daytime temps in the bed room can drop down into the 50s. Similarly I only heat my main room in the day time when I am working in it. I am still waiting for winter to kick in here but about two weeks ago we had an overnight temp down in the 20's and my main room was 59 degrees that morning before I turned on the heat.
I have mold pretty much under control now after pulling so much water out of the space, but now I am concerned about the pattern of heating my bedroom at night, so the air holds more moisture, then supplying that moisture by breathing and being human. When the heat turns off in the morning, that 55-60- morning RH would shoot up as temperature drops perhaps dropping below that rooms local dew point, allowing moisture to precipitate out and support new mold growth.
I am still waiting for winter temps to set in so I can determine the new pattern. On cold mornings, the morning bedroom RH is lower (<50), my suspicion is on cold nights I do not perspire like during warm weather. Even a 50RH at 70' could be too high after room cools down to 50'.
I suspect most cost effective would be to run the dehumidifier with the control set to maintain <50 RH. It will do this by a combination of heating the air and pulling moisture from the air. I suspect this will be cheaper that heating the room to 70' 24x7.
In my situation, the temperature would be very-nice if it were not so dry. We live 1/8" off the edge of the Comfort Zone on a temp/humid chart. We do have water pans at most hot air registers, but without BIG input of BTU they are only good for a few points. Humidifier on the furnace is common, and commonly failed; and always increases the BTU consumption to get in the Zone (no free lunch, no free warm room).
How cheap and how cool is your tap water? My well water can be frigid. Pipes sweat (not much with PEX). If I ran the well water through a radiator and blew it, I think it would be a fair dehumidifier. However well water is ultimately expensive. And most towns gave up unmetered water a long time ago.
Yup I pay for my water... I don't use enough normally to pull much heat/cold from it 600-900 gallons a month.
I am optimistic that drier winter months may result in only needing to run the de-humidifiers a few hours a day... still figuring out my personal patterns/cycles for this.. When outdoor temps fall to the 40-50s the outdoor air will be a bunch drier.
JR
Probably enough to brew beer with... Hickory air stout. 8) ewww. 
