Yes if you have a higher pressure it is easer to push the air out. its the same way to circulate, if you pressure is to low it will not want to flow to the upper story, especial if the pump is too small. If you go to the hi velocity pump,and thats what i always use in a three story house, you will have one little thing you may not like. On a zone system , when only one zone is open you might hear a little water movement noise depending on you piping system. Engineering will vary from one engineer to another a little. But i am going to give you some facts. normal hot water system no more than 30' to 40' of radiation on one 3/4 in. loop or zone and standard zone values 3 to 3 1/2 gal.s per min. . Now if you have too long of loop you will slow flow down and lose too much heat, same if you have too much radiation on a loop. You said three story two zones. this sounds like long loops which means hi head pressure, and maybe too much radiation per zone. some times when i run into this i advice the owner that we will have to switch the zone values to a full flow zone value, they go up to about 7 gal.s per min. flow, this along with the hi head pump will take care of too much heat loss,and too much head pressure. I hope all this is not confusing you too much, But hot water in a older home can be a little complicated. and a lot of guys do not understand this. You can check a zone out one way by checking the temp leaving the boiler and then check the temp. coming back from the zone. no more than 15 degrees drop.. Later Paulbm

Ok, further reading on this problem suggests the constant water running may be due to a couple of things from what I see: a. even with furnace off, the humidistat senses ambient humidity below what it is set at and so allows water in through the water inlet orifice and inlet valve assembly, controlled through a solenoid which itself is controlled by the humidistat. I guess the humidifier is mounted directly to the hot air supply duct as I see no separate humidifier supply takeoff duct on my system. Any unevaporated water from the flow through evaporator pad runs off into the drain pan and flows through to a household drain, which is what is coming out in force. Perhaps this is normal (humidistat sensing low humidity when set to 25 hence pulling water in) then. b. perhaps this is a solenoid valve assembly is not working correctly. Any suggestions on how to test which is the case? Water is very expensive in my town (12% additional company increase just asked for, and 6% increase from sanitation company). I conserve water.

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.

Thanks for the advice. I'm still not sure what caused the humidity level to drop that low. The thermostat controller was set for 40%, but maybe the window saving feature dialed that down without telling me because the outdoor temperature recently had a severe drop? I've raised the windows protection setting to the max since, and it's back up to 34% now. We had our HVAC company's Energy Auditor / Sales person out last week with a proposal for the new house, and he said that there's something wrong with the pump and they'll replace it for us (provided we buy our new HVAC system from them, of course). It's just disappointing that the service tech comes and says, "Yeah, that's normal, it's supposed to make that sound all the time. That'll be $90", and it takes getting a sales guy out and the prospect of new business for them to say, "Yeah, it shouldn't do that. It needs to be replaced." I mean, what's the point in spending $10K on a high-end, ultra-quiet HVAC system when it's undone by the noise from a $70 pump?