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I'm a newbie to the site and posted a question last week regarding help with wiring a Honeywell L6006C Aquasta to a Taco 006-BC4 Hot water Circulator Pump. That question was answered quickly thanks to @JPhi1618. Thank you @JPhi1618!

All worked out well with the wiring and the aquastat installation. However, I stumbled across an issue after installation. The following are my setup details and my current issue. I saw some familiar posts but they did not actually match my issue. Hopefully this is not a duplicate issue.

I have a 3-year-old recirculating hot water loop that was installed as part of the new house. It appears to be a fairly typical installation. The circulator pump is a Taco 006-BC4 and it was hooked up to a simple mechanical timer to shut off for a few early morning hours. I decided to eliminate the timer and installed a Honeywell L6006C strap-on aquastat to control the hot water recirculating loop pump. I strapped the aquastat to the hot water outlet line and I set it up to turn the pump on when the hot water temperature dropped. It appears to be working fine. It turns the pump on when the temp drops (temp setting minus differential) and shuts the pump off when it rises to the set temperature. Also, there’s a check valve installed between the circulator pump and the cold water inlet to the hot water tank. The Aquastat and circulator pump appear to work correctly being I get plenty of hot water to any faucet and get it quickly. I think it also indicates that the check valve is installed correctly being the hot water recirculating return pipe coming from the pump and into the cold water inlet pipe at the hot water tank is hot when running.

Sounds all good so far but here is the problem I stumbled across. Whenever the pump is shut off and you pull hot water, it immediately allows cold water from the cold water inlet pipe to come back into the hot water recirculating return pipe right through the check valve and through the circulator pump. The hot water recirculating return pipe becomes cold and continues to stay cold if the circulator pump is off and you pull hot water. Also, the master bathroom, which is the furthest away and the end of the hot water recirculating loop quickly has no hot water. The kitchen and bathrooms still have hot water being there is plenty of hot water in the tank and it’s being replenished.

Once the system is in this state, the aquastat does not turn the pump back on being the water coming through the hot water outlet is hotter than the aquastat set point minus differential. With the circulator pump not running, the master bathroom quickly has no hot water and stays that way until you manually set the aquastat to a higher set point temperature which then turns on the circulator pump. As soon as the pump is back on and recirculating the loop, the master bathroom immediately has plenty of hot water.

I have replaced the check valve even though it appeared to be working and I inspected the one I took out. As I expected it was in good working order.

I can replicate the issue at any time by shutting off the circulator pump and pulling hot water from anywhere in the house. Also, if I isolate the recirculating loop pump but turning off the pump and shutting off the shutoff valves in front and in the back of the pump, there is plenty of hot water everywhere including the master bathroom. However, it takes longer to get hot water to where you are.

I believe this issue may have been here all along, being the circulator pump was actually never shut off when it was hooked up to the timer, except for the few early morning hours when no one was pulling hot water.

Any advice or suggestions on why this may be occurring is greatly appreciated. Thanks.

Photos: https://photos.app.goo.gl/cWiYoxCePMN5UAoR9 https://photos.app.goo.gl/gA6fvyWZfnS4d8fP8

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  • You say come back into the hot water recirculating return pipe right through the check valve and through the circulator pump, but isn't that check valve supposed to stop that cold water? Or maybe there is a missing check valve that should be somewhere else?
    – JPhi1618
    Mar 30 '20 at 16:16
  • Agree, the check valve should prevent the cold water from backflowing into the loop so it was the first thing I replaced. Interesting thing was when I inspected the old one it appeared to be good. This did not fix the issue. Just moved the aquastat to the hot water circulator retun by the pump. Appewars to be working okay. I will update all after a day or two of running.
    – Nickrnm
    Mar 30 '20 at 18:48
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I received the Caleffi NA51249 1/2" spring-loaded check valve today and installed it this afternoon. Great news! -it fixed the cold water backflow into the hot water recirculating loop whenever pulling hot water. I tested it a few times with the pump shut off and all appears to be working as it should. So, the final fix was replacing the swing type check valve, located between the hot water recirculation pump and the cold water inlet, with a spring-loaded check valve.

I would like to "Thank" all of you for your recommendations and assistance! Greatly appreciated and Thank you for all your help!

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It looks like your cold water inlet is teed into the hot water return just before it goes into the tank. This is fine except I don't see a check valve on the hot water return line between the Taco pump and the tank. When you call for hot water without the Taco running I think your getting backflow from the cold water supply through the hot water return.
Maybe it's the picture but not sure if that's a check valve on the right? Doesn't look like one. You might need a check valve in there.

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  • Yes, there is a check valve between the pump and the cold water inlet. I had already replaced it. I received a suggestion from Steve from CA on another communication, to mount the aquastat on the return pipe by the pump. I just quickly move the aquastat to the return side of the pump. Initial results, it appears to be working as it should. I will update with additional results after a day or two of running it. Thank you for the input and suggestions!
    – Nickrnm
    Mar 30 '20 at 18:37
  • Hope it works for you.
    – HoneyDo
    Mar 30 '20 at 18:42
  • After placing the aquastat on the return recirculating pipe by the pump it fixed the No hot water in the master bathroom issue but it causes the pump to short cycle. Once aquastat senses low temp (set-point minus variance) it starts the pump. It runs for about 30sec, senses the hot water set-point and shuts the pump off. It repeats this cycle for as long as you are pulling hot water causing the pump to turn on and off every 30sec or so.
    – Nickrnm
    Mar 30 '20 at 21:17
  • In my case, moving the aquastat near the pump, is only masking my underlying problem as to why cold water back-flows into the hot water recirculating loop even though I have a good check valve between the pump and the cold water tank inlet?
    – Nickrnm
    Mar 30 '20 at 21:19
  • I attached a better photo of the hot water recirculating loop system afer moving the aquastat. I believe the check valve is in the correct location? I tested the new check valve after installing it by turning on the shutoff valve that is installed between it and the cold water inlet. No water came out of the check valve end that the pump is attached to. Photo: photos.app.goo.gl/AUGECU4mjsYyjthg8
    – Nickrnm
    Mar 30 '20 at 22:07

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