Hydronic-hot water tank-forced air

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jean55

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We have a First Co. hydronic HVAC system that was installed in 2016. In the air handler unit, the evaporator coil is at the bottom, with the blower and heating coils above it. There's a loop for the space heating with the return flowing back into the hot water tank, and a mixing valve for the water supplied to all the taps. During the cooling season the HVAC technicians advise turning off the space heating supply and return valves, either above the air handler or at the hot water tank. The pump normally runs every 6 hours, even with the system off; we've read that is to prevent bacterial growth. We keep the 75-gal Bradford White Hydrojet tank temp at 135-140F. We haven't checked the chlorine content, but at times we notice a strong chlorine odor.

1. If we leave the heating supply valves closed for 5-6 mos (in VA) could Legionella grow in the stagnant water in the pipes?
2. If we leave the heating supply valves closed, could an air gap develop requiring flushing of the system when heat is turned on?
3. Would stagnant water in the heating coils increase the likelihood of coil corrosion?
4. If we leave the heating supply valves open, and let the pump run every 6 hours for one minute, would the 50-60 degree cooled air passing over the warm coils (max 140 degrees) above the evaporator coil cause enough condensation on the heating coils to cause a problem?
Thanks for sharing your thoughts/advice!
 

John Gayewski

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Your space heating water and your domestic hot water should be separate 100 percent. It's as simple as that. No other way will be sanitary. These systems that mix the two aren't legal (although I'd have to do some more research to be sure about that). Use a heat exchanger and separate the systems.
 

Fitter30

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For the heating coil to sweat the water temp would have to drop below dew point. Heating coil will not get that cold to sweat. You have a open heating system thats using the same water as the dhw which reall recommended as for what you stated with water stagnant. A heat exchanger, expansion tank, pump along with a little wiring would make the heating loop independent.
 
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jean55

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Thanks for the replies. Because we are in a condo with rules to follow and limited space in the utility closet, I'm not sure if we could separate the systems, but I'll call our HVAC company to see if it would be feasible.

Now we thought of another problem: If to prevent stagnation we do NOT close the supply and return valves to the space heating lines, and the circulating pump does its one-minute run right after the A/C runs, the pipes near the air handler undergo an abrupt change in temperature, from 55/60 to 135/140 degrees. That can't be good for the pipes.
 

jean55

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We've got about 12" of copper extending up from the air handler sheathing/cover, and 8" of copper attached to the water heater. The 25' of piping in between, running above our ceiling, is all CPVC. Can the CPVC connected to the copper pipe take the abrupt temp change?
 

jadnashua

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In the middle of winter, in your hot water lines to say the shower, after sitting all night, the lines will have cooled to at least ambient, and maybe more, depending on how well things are insulated and if they're in an outside wall. Then you turn the shower on, and the hot water arrives...you'll get a similar thermal shock to the pipes. As long as the temperature extremes are within the limits of the pipe involved, it should not be an issue.
 

jean55

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Thank you for the replies. When I spoke with a tech at First Co., I was told that, except for servicing, the supply valves should always remain open to prevent stagnation. I still need to look into adding a heat exchanger to separate the space heating from the dhw.
 
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