Plumbing – No hot water if hot water recirculator pump is shut off

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