PosiTemp Shower and Hot Water Recirculating Line Will NOT Play Nice!

I'll apologize in advance for the long post, but I've already done a lot of trial and error experimentation that might be helpful to solving my issue.

I have a Moen PosiTemp Shower in the master bathroom. Connected to the hot water supply (just before the PosiTemp) is a hot water return line. The original construction of the house had a straight line for return (no backflow prevention valves, no recirculating pump). At that time, the hot water recirculation worked great, but the PosiTemp shower had intermittent hot water. It was almost as if the PosiTemp couldn't decide if it should pull from the hot water supply line or the recirc line. Closing the shutoff valves in the recirc line eliminated the problem, but left me waiting for hot water everywhere in the house.

The next step was to install a swing valve to prevent backflow in the recirculating line and, of course, open the recirc line shut-off valves. This seemed to work, as the PosiTemp shower now functioned properly, however the swing valve was too restricting for the gravity-fed recirculation...so I was still waiting for hot water at all points in the home.

The next move was to add a recirculating pump, to overcome the restriction of the swing valve. I've installed a Grundfos Auto PM pump just above the water heater on the recirc line. Pump works great. I have instant hot water at all locations, however now I get constant warm water through the PosiTemp shower. Turning the handle to the full hot position yields only lukewarm water. I've again tested by closing the shutoff valves in the recirc line and everything is fine (if you don't mind waiting for hot water, of course).

The other thing that might add some complexity to this is that I have dual 40gal water heaters that are connected in an unorthodox manner. The link below has three pics to show how the water heaters are connected, where the recirculating line comes in and where the far end of the recirculating line is connected (just below the PosiTemp valve).

http://1drv.ms/1vt6UQJ

Any insight would be GREATLY appreciated!

I think your issues are with the way the water heaters and return line are run. Should look like this.

Not more yellow ruled paper!

(sorry, it's an inside joke!)

thatguy said:

I think your issues are with the way the water heaters and return line are run. Should look like this.

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That is piped in series, and in most residential applications is the wrong way to pipe double water heaters.

phishfood said:

That is piped in series, and in most residential applications is the wrong way to pipe double water heaters.

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It depends on what you are trying to achieve. Volume or recovery time. Series or parallel, neither is wrong. If he wants to keep it in parallel, at least the inlet and outlet piping of each w/h should be equal length with the same fittings before feeding the house. To the OP, here's a diagram from AO if you want to clean up what you have. http://www.hotwater.com/lit/wiring/315268-000.pdf I'm not saying your issues will be solved just by repiping your w/h, but it should at least be right either way you want to do it.

Same fittings and pipe length are for a balanced parallel setup. What the OP has is first in/last out parallel piping, which is a lot more forgiving on fittings and pipe length differences.

I don't see the check valve in the pictures. Can you post a picture of it?

Like I said, I've only been plumbing for 12 years, so I don't know everything since there is so much to know in plumbing. I've been explained the differences somewhat, but that was from one person, my boss. I am always up for learning new ideas and ways of doing things and reasons for doing things from new people. If you dont mind, i would like to know why you said series was wrong. I've installed double w/h in series and balanced parallel, but not first in last out. I did a little looking at how that works and do see how it would be easier to plumb. How much better is the performance and efficiency on the 1st in last out method over balanced or is it mainly for ease of plumbing?

thatguy said:

Like I said, I've only been plumbing for 12 years, so I don't know everything since there is so much to know in plumbing. I've been explained the differences somewhat, but that was from one person, my boss. I am always up for learning new ideas and ways of doing things and reasons for doing things from new people. If you dont mind, i would like to know why you said series was wrong. I've installed double w/h in series and balanced parallel, but not first in last out. I did a little looking at how that works and do see how it would be easier to plumb. How much better is the performance and efficiency on the 1st in last out method over balanced or is it mainly for ease of plumbing?

Click to expand...

Dude, I have only been in this awesome trade for 20 years, and I am continuously frustrated (and inspired) by how much I have to learn. I just recently quit my day job in multi family new construction (apartments) to focus on my own remodel/service business, and every day is a learning experience.

Parallel balanced piping works well, but it is very critical that the length of pipe and fittings are the same for both heaters. First in/last out is another method of parallel piping that evens out the usage between the two heaters, but doesn't require such exacting work on distances and fittings. The picture the OP shared really is a pretty good example of it.

Series piping in most residential applications tends to overuse the first unit in the series relative to the second unit. You could alleviate this somewhat by setting the first one to heat roughly halfway between incoming temp and final temp, and the second one to take it from that to final temp. But this wouldn't hold up in a situation where you had a volume demand that exceeded the second unit's ability to heat, once it's initial volume was depleted, there would be a temperature drop. Whereas with parallel units, you will have roughly the volume of both units available before you experience a temperature drop. And with parallel units, the load is more evenly distributed between both.

Does not look balanced though.
Cold risers are longer than the hot so there is more pipe between the TEE's on the cold that on the hot.
The return line is not balance to the 2 heaters. Looks like it will move more water through the right heater if both valves are opened
You could recirc it through 1 heater but that heater will work a little harder to keep the return line hot.
.

phishfood said:

I don't see the check valve in the pictures. Can you post a picture of it?

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Not quite sure how to post pics directly in the thread, but I've added a couple to my original link. One shows the location of the check valve in the recirc line and the other is a close-up of the valve.

I also want to thank everyone who has posted so far. I can tell that there is a wealth of knowledge on this forum and am appreciative that you are interested in helping me solve this puzzle.

Either the check valve is not sealing, or the hot water is crossing over to the cold somewhere else.

I asked for pictures because you mentioned swing check, and I wanted to make sure it was oriented correctly.

phishfood said:

Either the check valve is not sealing, or the hot water is crossing over to the cold somewhere else.

I asked for pictures because you mentioned swing check, and I wanted to make sure it was oriented correctly.

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Were you able to see the pictures I posted in the original link? Also, if the hot was crossing to the cold somewhere else, wouldn't it also cross to the cold when I close off the recirc line?

wisconsinben said:

Were you able to see the pictures I posted in the original link? Also, if the hot was crossing to the cold somewhere else, wouldn't it also cross to the cold when I close off the recirc line?

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Yes, to the first, and yes, to the second. I would "check" the check valve.

phishfood said:

Yes, to the first, and yes, to the second. I would "check" the check valve.

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Thanks for your help. I don't suppose there's any special way to check the valve, other than removing it from the line and inspecting/replacing it?

You could try coming up with a way to apply back pressure to it, but a new spring check with a couple of PEX threaded adapters is cheap enough that I would just try that.

Though if you have a tall ladder, you could try disassembling the swing check and seeing if there is some solder or debris stuck in the seat.