Finally I found the problem and fixed it. My humidifier's transformer was hard-wired into junction box of furnace and to humidifier directly, NO relay sensor, of cause water will keep flowing to humidifier unless control stat is turned down. That was installed by "professional contractor' from Air Group in NJ in the first place years ago, I hadn't had it working for years because of saddle valve blocked. This year water supply was re-routed with a real valve. Can we trust any contractors in these days??

I think Matt's HVAC superstores posts are bordering on SPAM. I think it might be possible but with some energy or other expense. The simplest way would be to have a thermostatic heat tape attached to the water line, which itself should be well-insulated. The other method would be to perhaps run the water line inside the hvac duct, if possible - of course I don't know if that would be strictly legal. Another precaution you could take would be to use a humidifier that used a solenoid (electric) valve to control the water, again, you'd want heat tape and insulation on the line, but the valve could be inside the warm air in the house, pair that with a water leak detector outside, to give you a bit of flood safety. Just a couple of ideas.

Sorry for delay and thank you for your interest brickeyee. Aprilaire Humidifier model 760 flow through system. When HVAC was here in September doing fall furnace check he installed a new AA feeder tub ("NP OTP" on bill), saying the old rubber tube may have weakened and splayed at its end over the years and pose a water leak risk. He also put in a new water panel and #35 pad. There was no discussion of the Solenoid valve assembly nor of the water inlet orifice. No problem was noted on testing the humidifier except for water leakage from the gas water heater branch supply: the "saddle" valve controlling hot water supply to the dedicated humidifier line leaked. All was turned off until further investigation of this saddle valve. Below are pictures of the water valve above the gas water heater: some say it is a saddle valve (which, as all or nothing" is not allowed I gather) and others, including the company's plumber say it is a turn valve. The company plumber tightened up the nut a bit on Friday and the leak seemed to stop. The system was turned on for test, but after the plumber left I noted the continuous water running to the humidifier, heard it spraying into the box and saw the constant stream from the outlet to the sump pump so I turned it all off again. Here are key points Aprilaire owner's manual makes on the 700 models: "The correct water flow is determined by an orifice equipped with each drain type humidifier. When the humidifier is operating, there will be a small, steady stream of water to drain, which flushes away most of the trouble-causing minerals. Do not use the saddle valve to regulate the water flow. It is designed to be completely open or closed." Here are some photos-sorry can't seem to resize them on PB: http://www.aprilaire.com/themes/aa/en/manuals/Owners_%20Manua_Model_400_%20500_%20600_%20700_10006899.pdf to be completely open or closed." Basically, the Aprilaire Humidifier looks like this and has separate humidistat on return. Thanks for any advice.

Ok those pictures tell a lot. This furnace was probably one of the first condensing furnaces made, LOL. So your furnace is a condensing furnace. So that relieves the concerns of the flue pipe. It still isn't a bad idea to get this furnace inspected though, that inducer motor has got a lot of fur babies on it. The return cutout definitely needs to be addressed, I would have that sealed up and if your system needs additional return air make the return so it draws air from outside this closet away from furnace area. Code may also require you to bring in out door air to this closet for combustion air for furnace. That is the proper way to do it to ensure the furnace never goes into a mode of 'incomplete combustion' due to lack of combustion air. This is typically what results in the production of CO (carbon monoxide) as normal combustion results in CO2 (carbon dioxide) Even if this furnace is not producing CO, but a crack in heat exchanger is found the furnace should be replaced. You should do these things before considering adding a humidifier to this system.