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?

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

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.

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.