Balancing Pressure with Recirculation Line?

[Squeak]

Tankless water heater with dedicated recirculation line and pump (Rinnai). Water line from heater to farthest faucet (kitchen) is 3/4" CPVC, and then the recirculation line is a straight run of 1/2" PEX back (about 40-50').

Heater is set at 130 deg.

The symptom I am seeing is our master bathroom shower never seems to get above about 115 degree (taken a thermometer) when the recirc line is open, but if I close it down, it quickly climbs up to 125 degree.

There is a check-valve on the recirc line (swing gate right after it splits off the CPVC), and then it makes its long run directly into the cold water source. About 2 feet upstream from where the recirc line comes in is another check valve (spring) protecting the cold water line.

Originally, my MBR shower was plumbed not on the main recirc loop, and I thought that was the issue, so I took some time yesterday and redid it where the first fixture now after the heater is the shower (about a 4' 1/2" branch to the shower from the main trunk).

That did not seem to fix it.

The tankless will also show me the waterflow through it, and when the shower is on, and the recirc line is open, it is 1.5GPM, but when I close the line (and the temp at the shower jumps), the waterflow through the heater increases to 1.8 GPM.

So, my theory is that the recirculation line is giving a path of very little resistance and hot water is being diverted from my shower and into the loop -- such that the tankless is drawing a decent portion of its "source" water from the recirculation line, and not from the cold water line (by my calculations, about 30%).

Originally my plumber had a swing gate check valve on the cold water line by the heater, but it was fairly loud, so he replaced it with the spring one. I don't remember this being an issue before the change, but I can't be positive, because I just started measuring the water temp exactly.

Is it possible that the resistance on pulling from cold water line is higher now due to the spring valve, and that is allowing for more water to be pulled from the recirc line, causing the symptom?

My next step was to install another spring check valve on the recirculation line, but as close as possible to where it connects into the cold water line, with the hope that it will "balance" the resistance of both lines, and reduce the draw away from the shower.

Or am I thinking about this wrong?

 

[Smooky]

It sounds like cold water is backing up through the recirc line. A swing check should not be installed in a vertical line if normal flow is from top to bottom. The valve will hang open in this position and allow the cold water to pass the valve.

 

[Sponsor]

 

[Squeak]

Check valve by the shutoff of the recirc line is in a horizontal position.

I just did another experiment, and with the shower on, and the recirc line valve closed, the heater says 1.8GPM flow. If I open the valve even just the slightest, the GPM drop down to 1.5GPM. It is not like I can balance it with the shutoff valve.

 

[Squeak]

I have a Sharkbite check valve on order (http://www.sharkbite.com/product/check-valves/), and should be here tomorrow, and will try and put it on the recirc line close to the cold water connection.

 

[Squeak]

It sounds like cold water is backing up through the recirc line. A swing check should not be installed in a vertical line if normal flow is from top to bottom. The valve will hang open in this position and allow the cold water to pass the valve.

Thank you! Thank you! Thank you!

While it doesn't make sense to me 100% yet why it is failing, but your post sparked me to do some more investigating, and it turns out you are 100% correct: the check valve is letting cold water back in.

By the grace of god in the way the recirc line was plumbed in to a hook up for a basement bathroom I can directly connect and isolate the recirc line to the hot water side of the bathroom vanity. I shut off the hot water to the bathroom, and shutoff the recirc valve and opened the hot water side of the bathroom sink, and no water.

But open up the recirc valve, and a slow, cold trickle of water comes flowing out. After measuring it, works out to be about 6 cups a minutes (or a little more than the 1/3 of a gallon I was seeing). Cold water is working back up through the check valve! You are so smart!

Real test will be after installing the new check valve.

 

[Smooky]

Glad I could help you figure it out Squeak. Good job and let us know how it does with your new spring check.

 

[Squeak]

Well, that was it. New spring check valve removed the back-leaking into the hot water line. Now the GPM to the shower is consistent if the recirc line is on or off.

Whew! Thanks again!

(Got lucky as well, ended up cracking the plastic coupling piece inside the sharkbite valve due to inserting the PEX wrong and causing too much pressure, but not bad enough that it leaks -- for now. New one is on order, but at least I am back up and running for today).