When hooking up a desuperheater and buffer tank, should the cold water input line leading up to the T that splits off such that one branch goes to the desuperheater in and the other branch goes to the buffer tank cold water in, have a check valve _before_ that T? I have a GeoStar ground source heat pump in my house, with the desuperheater hooked up including a buffer/preheat tank before the house's main on-demand hot water heater. The problem is that plumbers who did this (I was not in the house then), apparently didn't know what they were doing, and didn't plumb things correctly. The correct plubing of the desuperheater is as per page 10 here ( http://www.geostar-geo.com/literature/IM1585X.pdf ) with the hot water out from the desuperheater coming into the bottom of the buffer tank via the drain valve. Instead, they brought back the hot water out from the desuperheater, and via a T, connected it to the hot water out from the tank. But there is a check valve on the hot water out line from the tank, so no way the hot water from the desuperheater could ever get into the tank. When the system is operating a lot now, in the winter, the result is that my entire cold water supply in the house gets heated up. I get hot water out of my cold water taps for a good 10 minutes before things cool down and I start getting cold water. There is currently no check valve on the cold water line leading up to the buffer tank and desuperheater. In a few days, I am going to have a plumber move the hot water out from the desuperheater so it properly goes into the buffer tank at the bottom as it should. My question is if at the same time I should ask him to add a check valve on the cold water supply leading up to the buffer tank and desuperheater (before the T)? I think it is needed, but the plumber is a bit worried that if he puts it in, the cold water system can not act as a buffer for any thermal expansion that may happen. Any advice? Thanks, Colin

Your plumber is right and it will probably open the relief valve unless you install an expansion tank.

Thanks. I am a bit worried that without the check valve, there may still be thermal conductivity to the cold water system, but hopefully with the buffer tank in there, there is much more mass of water (in the tank) which will have to heat up first.

Not a fan of check valve in this application.
Joe

Your plumber is right. Wrong spot.
We actually now always put a check valve in between the buffer tank and the DSH in (where they have a venting waste valve in the figure on page 10). We had some thermal siphoning in the DSH lines. Keep in mind that a high temperature shut off only shuts of the DSH circulation pump, the DSH is still generating very hot water. So we had scenarios where the safety switch was turning off the DSH pump, but thermal siphoning was still heating the buffer tank, once we had it up to 178 degrees in the tank (189 F) leaving the DSH, during a long cold period with not much water use, with a 6 ton HP brought the 50 gal tank quickly up to temperature. All with the DSH pump turned off. With the heatpump cycling and being off, we then had thermal siphoning from the buffer tank to the water lines between the tank and the DSH. after a gravity check valve went in, the standby heat loss of the buffer tank was reduced by 2/3. None of that would have been recognized without the WEL in place. Now a checkvalve is standard for us in the DSH line.

Doc, this questions regarding a check valve has been coming up often recently. When you get a chance, would you post a drawing on how you plumb a DSH with a buffer tank and indicate the check valve and flow direction? Bergy has a diagram that has been posted many times. If your plumbing is similar, perhaps just inserting a check valve into the drawing would work?

Maybe Joe could insert the pressure relief valve into the DSH plumbing drawing that he mentioned in the past as well.  It would be nice to have a definitive drawing that the pros here agree on.

Thanks Doc. I can see how the check valve in the same area as the venting waste valve on the diagram I referenced can help, as it will stop thermal siphoning back along the DSH's cold water input (and in fact that's exactly the kind of thermal siphoning I am currently getting without the buffer tank actually hooked up properly and being used).

That said, even with that check valve in place there, once the tank is properly hooked up, what is to then stop thermal siphoning from the buffer tank itself out its cold water input line, and onto the whole cold water stack? The check valve would not stop that, as it is not between the buffer tank and the cold water stack, only between the buffer tank and the DSH after the T. And you can obviously not put a check valve between the buffer tank and the T (and must remove any check valve that the tank itself comes with on the cold water input) so that you can get a proper loop going with the DSH and water flowing backwards out that buffer tank cold water line when the DSH pump is on.

Regards,
Colin

Also, if the vent coupling (or venting waste valve) and check valve are both in place side by side on the cold water input going to the DSH, does the order matter at all? The vent coupling is supposed to remove air entrapment, so it seems to me that putting it between the new check valve and the DSH makes the most sense.

Btw, I can attest to the fact that even without the DSH circulating pump going on, the DSH can still heat a lot of water via thermal siphoning. I have tried to turn off the DSH pump until this issue is resolved, and I am still getting lots of heating of the cold water lines.

Posted By geome on 03 Jan 2011 08:48 AM 
It would be nice to have a definitive drawing that the pros here agree on.

IMO this is a "more than one way...." proposition. Therefore we tend to agree on generalities more than specifics.

RE T&P valve, it is required where water has a source of heat and can be isolated by stop valves. Since most manufacturers show "throttling valves" on their diagrams they requirement is created (at least anywhere where the IRC is enforced).
Again I have the MI amended version so the language may or may not be identical, but P2803.1 identifies- appliances and equipment used for heating water or storing hot water shall be protected by: 1. seperate press. relief valve and temp relief valve 2. A combination temp/press relief valve

P2803.6 also mandates the following: A check or shut off valve shall not be installed in the following locations: 1. between a relief valve and ....(its) discharge pipe 2. between relief valve and tank 3. Between relief valve and heating appliances or equipment.

So the buffer tank may have a T&P on it that is not necessary if it is not wired or fired (can't heat water itself) so if one insists on "throttling" or isolation valves that relief valve is available and one must simply T it in between DSH and stop valves.
If only one throttling valve (on DSH piping) is used the T&P on buffer or finish tank is adequate so long as no isolation valve is installed between buffer and finish tank. 

In my mind this is not optional or over enforcement on my part. We have already seen (more than once) on this or other forums, folks who contemplated closing valves to DSH piping........ Once that happens, you have created a sealed vessel of water influenced by a source of heat (aka bomb). Will safety controls shut heat unit off before harm is caused to heat pump or building? Perhaps, but when we talk about safety redundant controls are usually in order and I certainly wouldn't want to trust a $5 snap disc switch to protect me from explosion.
J

Posted By joe.ami on 03 Jan 2011 10:27 AM

Posted By geome on 03 Jan 2011 08:48 AM 
It would be nice to have a definitive drawing that the pros here agree on.

IMO this is a "more than one way...." proposition. Therefore we tend to agree on generalities more than specifics.

RE T&P valve, it is required where water has a source of heat and can be isolated by stop valves. Since most manufacturers show "throttling valves" on their diagrams they requirement is created (at least anywhere where the IRC is enforced).
Again I have the MI amended version so the language may or may not be identical, but P2803.1 identifies- appliances and equipment used for heating water or storing hot water shall be protected by: 1. seperate press. relief valve and temp relief valve 2. A combination temp/press relief valve

P2803.6 also mandates the following: A check or shut off valve shall not be installed in the following locations: 1. between a relief valve and ....(its) discharge pipe 2. between relief valve and tank 3. Between relief valve and heating appliances or equipment.

So the buffer tank may have a T&P on it that is not necessary if it is not wired or fired (can't heat water itself) so if one insists on "throttling" or isolation valves that relief valve is available and one must simply T it in between DSH and stop valves.
If only one throttling valve (on DSH piping) is used the T&P on buffer or finish tank is adequate so long as no isolation valve is installed between buffer and finish tank. 

In my mind this is not optional or over enforcement on my part. We have already seen (more than once) on this or other forums, folks who contemplated closing valves to DSH piping........ Once that happens, you have created a sealed vessel of water influenced by a source of heat (aka bomb). Will safety controls shut heat unit off before harm is caused to heat pump or building? Perhaps, but when we talk about safety redundant controls are usually in order and I certainly wouldn't want to trust a $5 snap disc switch to protect me from explosion.
J

I am with Joe. I had too many customers shutting off the valves accidently. We leave them out, I do not see much purpose. See attached drawing for checkvalve position. Pressure can escape to the buffer tank and to the final conditioning tank, which would have a pressure relief valve We have tested many configurations, this one turns out the best for us.
Thermal siphoning usually does occurs in a loop, the water cannot flow backwards much along the cold water input, it might heat up the line for a few feet, but you do not loose much. The key siphoning occurs along a loop (like DSH line), where water gets heated up somewhere along that line, expands, gets a different density and weight, and then starts circulating on its own.
Also note that we push water into the buffer tank at the bottom via the drain valve, which is the opposite than the recommendations in the manuals. We had sediment building up on the bottom of the tank, and plug up a circulation pump once. So we learned our lesson. Since the dip tube gets the water from the bottom of the tank, there is not much difference thermodynamically, but it avoids sediment getting sucked up into the DSH line.

We continue to install valves simply as a maintenance tool, but only on the bottom of the tank leg as top t is above 50 gals of stored water it stays dry if water to tank is shut off. Allows throttling if we must (manufacturers recomendations technically make it code).
I look forward to the day that Manufacturers do away with throttling valves and unbuffered DSH in their books. I also look forward to the day they add T&P's if they are going to permit isolation valves between a DSH and storage tank.
j

Thanks Doc and Joe. I wish we could add posts as "favorites" under our account for future reference (as we can elsewhere.)

Gosh you'd add my latest T&P tirade to your list of favorites? I'm flattered.
(I know, I know you just wanna hang Docs artwork on the fridge door)

Posted By joe.ami on 04 Jan 2011 08:02 AM
Gosh you'd add my latest T&P tirade to your list of favorites? I'm flattered.

The T&P seems like a very good idea.    I just wish there was a way of saving favorite posts on this forum.  The only thing I don't like about removing the handles is the inability to see how valves are set at a glance.

Posted By joe.ami on 04 Jan 2011 08:02 AM
(I know, I know you just wanna hang Docs artwork on the fridge door)

If a crayon version could be done...