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MIXING VALVES QUE VEUX DIRE "MIXING VALVE"? Ok. We have a combination furnace/water heater system. There isn't (really) a hot water tank; instead we have a small tank hooked into the furnace (the way I think of it, the furnace has four zones; the three floors of the house & the hot water tank). We just had the gas company folks in here for some odd number of hours trying to figure out why we don't get hot water for our shower, and after some playing around they concluded that it was the mixing valve on the outflow of the hot water mini-tank. Could someone quote me chapter and verse on what mixing valves are about? They said it mixes the newly heated hot water coming out of the tank with cold water before sending it into the house system; why? I have some guesses, but I'm getting a little tired of trying to do the intelligence for experience substitution. If I can manage it, I want to understand our heating system so that I can talk intelligently with the folks who work on it--we've had too many problems with it. Thanks in advance .... -- Randy OK, mixing valves.... You have a system similar to what I have- a tank of VERY hot water (heated by the same furnace that heats the house)... the water is so hot that it would scald anyone taking a shower. The mixing valve is a small thermostatic valve that has three ports and an internal feedback loop. Scalding-hot water goes in on one port, cold water goes in on another port, and mixed (temperature-controlled) water comes out the third port. The device looks like a small faucet knob on top of a T with too many legs. The internal valving uses a thermoexpansive device. If the output water is too cold, it opens the hot-side valve more. If the output water is too hot, it opens the cold-side valve more. The little black knob on top define "too hot" and "too cold". Common failure modes are sticking (in full-hot, full-cold, and in between positions), and corrosion that jams the knob. The fix in all three cases is to replace the valve. Sometimes you can replace just the internal core; if you can buy the same valve at Home Debit the core unscrews and you can swap cores. If you can't unscrew the core (TURN OFF the water first!) then you need to replace the whole valve, which is a torch job. -Crash If I had to guess (and I do), I'd say that the mixing valve is there so the furnace can generate a relatively small quantity of hotter water, and mix it down with cold water so the output into your house hot-water system is close to 120F (i.e. not hot enough to parboil you in the shower if you mess up and turn off the cold). The best thing, however, is to go do a little research -- look for a *real* plumbing textbook (i.e. what the trade schools use) in the library. This should have more detail about contemporary types of mixing valves, i.e. whether they are just mechanically set or thermostatically controled, etc. This ought to help you recognize what they've done to your house. I've lived with "tankless" h/w like this, and a thermostatic mixer would have been nice -- the hot water temp would fluctuate as the furnace got the signal to fire up (after you started drawing down the mini-tank). The mixer would at least put an upper limit to the temperature. -- Larry At least, changing the core worked for me. :) See also: Water Heaters |
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