Slightly tricky plumbing question
We are in the middle of a major addition to our house, which includes a
whole second floor and new HVAC, including hot water. The question is about the hot water system. The new hot water system uses the main boiler to heat the water (through a heat exchanger) and store it in an insulated tank. The tank therefore has three fittings: 1) Output to the rest of the house (on top of the tank); 2) Output to the heat exchanger (on top of the tank); 3) Input from the heat exchanger (on the bottom of the tank). The cold water supply merges with (3) to supply either the tank or the heat exchanger as needed. The upstairs bathroom is 50 feet away from the hot-water supply. Moreover, because the bathroom includes a whirlpool bath, and we would like to be able to fill it quickly, the pipes from the hot-water supply to the bathroom are 1 inch in diameter. Therefore, it would ordinarily take a long time for the water to the bathroom to become hot--many minutes, for example, if one were just running water in the sink. To avoid this problem, there is another line that recirculates hot water from the bathroom back to the storage tank. It connects to the hot-water line in the bathroom, near the sink, and to the input from the heat exchanger at the storage tank. It relies on the fact that hot water is less dense than cold water, using convection to circulate water from the tank, up the hot-water line, back down through the recirculating line, and back to the tank. That's the theory, anyway. In practice, it works just fine, except for two problems. We have solved the first problem, but in doing so, we have created the second problem, which we now need to know how to solve. Here's the first problem: When you turn on the hot water in the bathroom, it draws water through both pipes: the hot-water pipe at the top of the tank, and the recirculating pipe at the bottom of the tank. This means that cold water comes in from the cold-water supply, and instead of going into the heat exchanger, it flows up the recirculating line, diluting the hot water. The effect is to make it impossible to get adequately hot water in the bathroom. There turns out to be a standard solution to this problem: Put a one-way valve in the recirculating line. This valve allows water to flow only toward the tank, so it prevents cold water from being drawn back upstairs. This solution works fine--except for one snag: If air ever gets into the recirculating line, such as might happen during maintenance, there is no way to get it out! The trouble is that the one-way valve allows water to flow only downward, and there is no place at the bottom of the pipe for the air to go. So there needs to be some kind of bleed valve in the system. Here, then, is the question: I can think of two different ways of putting in that bleed valve. Which one should I do? Right now, the check valve is near the storage tank, and has a shutoff valve on either side of it, which makes a bleed valve easy to install. Here are the alternatives: 1) Put a bleed valve downstream from the one-way valve, the output from which I can exhaust into a bucket when I want to bleed the recirculating line. 2) Put a bypass around the one-way valve, with a shutoff valve in the bypass. If I do (1), then I would bleed the system as follows: 1a) Close the shutoff valve that is downstream from the one-way valve. 1b) Open the bleed valve and let it run until there is no more air. 1c) Close the bleed valve. Close the shutoff valve that is UPstream from the one-way valve. Open the shutoff valve that is downstream from the one-way valve. 1d) Open the bleed valve again and let it run until there is no more air. This removes any air that is trapped downstream from the bleed valve. 1e) Close the bleed valve. Open the shutoff valve. If I do (2), then I would bleed the system as follows: 2a) Close the shutoff valve in the main hot-water line going to the bathroom. 2b) Open the bypass valve around the one-way valve. Now the only path for hot water to get upstairs is through the recirculating line. 2c) Go upstairs and run hot water from the sink until there is no more air. 2d) Go back downstairs, open the shutoff valve for the main hot-water line, and close the bypass valve around the one-way valve. It seems to me that each solution has advantages over the other: If I do (1), I can bleed the recirculating line from one place, and can remove all air that might be trapped anywhere in the system. If I do (2), I have to go upstairs and back down. Moreover, I don't know what to do about air that might be trapped in the small section of pipe that is downstream from the check valve but upstream from where the bypass joins in. On the other hand, I don't have to use a bucket, and have one less valve that if opened, will spew water all over the basement. Moreover, (1) works by forcing trapped air *downward* through the recirculating line; (2) works by letting it flow upward. That suggests to me that perhaps (1) might not even work. I suppose there is one other possibility, and that is to do both (1) and (2). I am quite sure that would work--but would it be overkill? |
Slightly tricky plumbing question
On Tue, 04 Nov 2003 15:27:25 GMT, "Andrew Koenig" wrote:
long explanation of trapped air problem snipped Think about adding an automatic air venting valve. Go to mcmaster.com and search for 4928K3 If you can add this to the high spot of the section that can trap air, it should do the trick with no problem. They can dribble a few drops now and then when it is venting, but you could put it under a sink or somewhere else safe. Also, think about whether or not moving the check valve to the other end of the line it's in would help the situation. It shouldn't matter where in the line it's located as long as it faces the correct direction. HTH, Paul |
Slightly tricky plumbing question
Think about adding an automatic air venting valve.
If you can add this to the high spot of the section that can trap air, it should do the trick with no problem. They can dribble a few drops now and then when it is venting, but you could put it under a sink or somewhere else safe. Not possible, I'm afraid. I don't know if there are any local maxima in the line, but if they are, they are in inaccessible locations because ceilings have been sheet-rocked already, etc. Also, think about whether or not moving the check valve to the other end of the line it's in would help the situation. It shouldn't matter where in the line it's located as long as it faces the correct direction. I can't see how moving the check valve would help -- and anyway, there's only one place where it will be readily accessible. Air shouldn't be trapped in this line in the first place, under normal circumstances -- it's only if the line needs to be drained for some reason that air can get in. |
Slightly tricky plumbing question
On Tue, 04 Nov 2003 18:16:49 GMT, someone wrote:
Air shouldn't be trapped in this line in the first place, under normal circumstances -- it's only if the line needs to be drained for some reason that air can get in. You are making this WAY to complicated to accomodate a rare situation. Think a minute. The return line is under supply line pressure. Shut off the line below the check valve. Now wouldn't it be great if that shutoff valve had a bleed or drainoff built right into it, as many valves do. Open that bleed and the 60psi line pressure will blow that line right out (it can't take supply 'backwards' from the storage tank side as you have shut off the valve in that direction). Either that or if you must have a bleed, yeah, put one in upstairs. Sheetrocked already? Too bad. It aint the ceiling of the Sistine Chapel. Just punch (okay cut) a hole in it. It is not a big deal. Gee, a sheetrock patch. The lengths people will go to, to avoid cutting a piece of SHEETROCK, my god you'd think the stuff was sacred. Just buy a curved "knife" for spackling, one about a foot long, not a little 4" wide 'putty knife' and then wonder why you can still see the seams. I have DIY sheetrock patches from access holesand teenaged mishaps in my house, that you can stand right in front of at eye level and look at in bright light and you cannot detect them. Sheetrock is the tail not the dog. Go ahead and cut sheetrock if it serves a purpose. -v. |
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