stop gravity loop in domestic hot water recirculation system? (dedicated return line)

[iconoclast hero]

There's a 2012 GE HW tank in the basement, somewhat central to the original ranch structure.
Adding bath to existing addition with potential future expansion f/mud room & garage.
Ran 1" PEX to bath while picking up existing kitchen hot along the way.
A 1/2" PEX dedicated return line connects up with the 1" PEX at the end of the run (new bath) as well as at a bypass valve located past the kitchen where the addition shutoffs are located.

I followed the right side of this this diagram from Bell & Gossett, but considering the age of the HW tank and the likelihood that the replacement tankless water heater will have a built in pump, the HO and I decided to skimp on the pump ($35 Vevor pump from Amazon Warehouse—THD and Lowe's both sell that pump btw...):

(I'm unsure whether it is worth cutting in an 1/8" MPT radiator valve on a tee, and if so, where, but it is ready to go.)

Here's the string of brass and the pump:

(That bleeder valve's on the floor in the top left of the picture.)

Anyway, 1" PEX out, 1/2" PEX back. Effectively the bypass has been open and the addition off as the latter's still under construction. I figured out yesterday that I have a gravity loop I had not intended ... which I want to stop before summer arrives.

How?

 

[iconoclast hero]

So the OP was posted before I intended. Anyway, the problem is this: I have a gravity loop that is moving hot water without the pump being on. Great — in the winter when the waste heat is being used... not so much in the summer when it is working against the AC. The 1" HW line and the 1/2 " return line are both insulated for the most part, and as shown the return is plumbed into the HW tank drain line for a variety of reasons. (Regardless of being insulated, the 1/2" line is always going to be colder faster because of surface area/volume ratio vs. 1".)

The question is really this: How can I turn the gravity loop off in the summer easily and cheaply while still maintaining the ability to have simple timer control over the pump?

Some more data:
From a wifi temp monitor on the 1" line under the kitchen:



Left: return line shutoff open at pump/HW tank.
Right: return line shutoff closet at pump/HW tank, ca. 10 PM. 2024.01.26

 

[Sponsor]

 

[iconoclast hero]

Objections to/problems with plumbing into the CW line above HW tank:
1. As mentioned, it's a 12 year old tank so anything I do is going to be somewhat temporary on the order of not more than a couple dozen months.
2. There're space considerations. There's a line splitting off to the boiler fill that is an issue and otherwise, this was just easier to connect for proof-of-concept and hot water for coffee in the morning.
3. I'd have to get everything in sweat and return all the brass.
4. While the bucket isn't the greatest support, it satisfices for the moment; I'd have to support the pump by another means if it were higher.
5. I'd have to sweat onto a wet line in the basement.

 

[iconoclast hero]

On the surface of it, a 1/2" solenoid valve (e.g., https://www.amazon.com/gp/product/B07N64J7ZF) on the same switch as the pump would stop the the gravity loop and work with whatever electronic control I put in. I'm a bit concerned that I would need a delay relay on the pump to give the valve time to open, but none of this is hard to add on as I go along.

 

[John Gayewski]

You could try a spring check vs a flapper. Find one with a strong spring in it.

 

[Fitter30]

Is there away to put a inverted loop at the beginning of the loop off the 1" like in the floor joist for a heat trap minimum 6"

 

[iconoclast hero]

Is there a way to put an inverted loop at the beginning of the loop off the 1" like in the floor joist for a heat trap minimum 6"

I believe that I could do that but it would have to be with (4) 1" PEX elbows...

Actually, I guess if I want to have the gravity loop available at times (e.g., in the winter) I would have a pair of tees, a pair of elbows, and [at least] a shutoff on one of the two paths...I assume the existing one.

 

[iconoclast hero]

I installed a cheap 1/2" × 1/2" spring check valve from Amazon along with some 1/2 in hex nipples behind the existing 3/4" swing check valve.* [The connections on these aren't the greatest and are a bit leaky but...] This has stopped the gravity loop.

The homeowner would like to have the gravity loop running in the winter when the waste heat is useful and then shut off in the summer. As Delaware is in the northern hemisphere it is winter now so I have some time to source another spring check vavle.

Remaining questions:
1. Can I move the spring check valve to a spot behind the pump with a bypass so that I can gravity loop for the heating season and then pump during the cooling season?
2. With the above: Is there an issue with having the swing check valve after the pump (i.e., between the pump and the HW tank drain to prevent debris from damaging the pump) and the spring check valve before it? Two check valves (or perhaps a GHT vacuum breaker at the HW tank?!?) seem to be required to have option gravity loop whilist protecting the pump.
3. Does a GHT vacuum breaker work as a check valve in this scenario?


* so the order on the return the HW tank drain is:
1/2 in PEX > shutoff > pump > hose bib > spring check > 3/4 bushing > 3/4 swing check valve > GHT swivel adapter > GHT Y > HW tank. (That swivel adapter is quite helpful.)

 

[John Gayewski]

Any check valve should be on the outlet side of the pump not the suction side.

Building a bypass might negate the flow all together. So if I were going to try to do this I would have the spring check in line and the swing check in the bypass, but make sure you have straight runs off pipe on both sides of everything. A chattering check valve isn't great.

 

[iconoclast hero]

Any check valve should be on the outlet side of the pump not the suction side.

That's the question I should have asked directly, thank you.

Building a bypass might negate the flow all together.

The gravity flow? Well, it would be nice to take advantage of the passive loop, but it isn't essential

A chattering check valve isn't great.

...because it is annoying or because it is damaging to the valve/system?

 

[John Gayewski]

That's the question I should have asked directly, thank you.


The gravity flow? Well, it would be nice to take advantage of the passive loop, but it isn't essential


...because it is annoying or because it is damaging to the valve/system?

A chattering check valve will probably break after a short time.

 

[iconoclast hero]

I think I'm going to take out the spring check valve and return it while the Amazon window is open. It leaks and I know that it works to stop the gravity loop. I should be able to replicate that without issue with a better spring CV.

This will leave the 3/4" swing CV to protect the outlet of the pump from debris in backflow. On the inlet side I'll put in a PEX T with two parallel branches. One has a manual ball valve, one has a NC 120 VAC solenoid ball valve. Manual closed when not heating season, solenoid and pump on same switch, whatever that turns out to be...solenoid could be left unpowered during heating season.

This will hopefully provide a recirc system long enough for the 2012 HW tank to fail, otherwise, if the solenoid doesn't last, get a replacement spring CV for the swing CV and call it a day...