Cold Water gets Hot when Sprinklers are On

FredoSP

Hi All,

Could anyone suggest a starting point for my issue? It seems the cold water coming out of the faucet turned hot at the same time I had my sprinklers on. I never noticed this before because my sprinklers are set to run at 3 AM. This could have been going on for a long time and I noticed it now. I have done this experiment a couple of times with the same result.

I do have a backflow preventer on the 3/4" copper coming into the house from the water district. I'm guessing when the sprinklers go on the overall water pressure drops and the hot water from my tank is pushing back into the cold supply? Maybe I could add an inline check valve, but I would like to understand why this is happening.

The 3/4" cold line coming into the house reduces down to a 1/2" and then splits to feed cold water and cold water feed to the tankless coil and after the tankless it feeds to the water heater (I know weird setup).

Any thoughts?

mattmia2

Is there an expansion tank on the dhw?

FredoSP

https://forum.heatinghelp.com/discussion/comment/1809510#Comment_1809510

Yes, on the cold side of the water heater and it's pressurized to match the static pressure of the water coming into the house, 80 psi, which I know is on the higher side but feels great when you shower!

Larry Weingarten

Hi, @mattmia2 nailed it! With reduced pressure, hot water is being pushed into the cold side by the expansion tank. At least it's a working theory. Can you disconnect the tank from the system, or valve it off and then see if the problem goes away? Also, is there a hot water recirculation system?

Yours, Larry

HomerJSmith

How long does the cold water from the tap remain hot while the sprinklers are on and how hot? Is the sprinkler water hot?

"With reduced pressure, hot water is being pushed into the cold side by the expansion tank." That doesn't make sense to me.

Larry Weingarten

Hi @HomerJSmith , It depends a lot on the plumbing configuration and components. I have seen this problem. It could be a crossover in the plumbing, recirc tied into the plumbing in a way that allows backflow, malfunctioning tempering or check valve, expansion tank placement, or??? It would be best if we could get system photos and a diagram.

Yours, Larry

Doug_7

Bad o-ring in a single-lever tap will leak hot-water into cold, when there is a heavy draw on cold water.

Doug

Robomoo

I have seen this in dairy milk houses. A hose on a hot and cold faucet. Hot and cold on and hose nozzle off. Hot pushes into cold line when cold pressure drops. Need to follow all the piping. Feel the pipes and follow the Hot line

mikespipe

Newer hot water tanks have a check valve on the cold side. so the reduced pressure when running a lot of cold , such as when the sprinkler is on leaves the hot side with the expansion tank at a higher pressure that then back feeds the cold faucets.

FredoSP

Hi All,

Thank you for all of the comments thus far. I've been very busy and haven't had a chance to respond. I took some pictures of my setup to illustrate my situation. I also made a very crude diagram (sorry no Microsoft Visio) as a compliment to my pictures. Would it be as simple as soldering in a check value somewhere once I find where the hot water is coming from? My guess is the water heater but this never happened with the old one? Maybe the old water heater didn't have heat traps?

https://forum.heatinghelp.com/discussion/comment/1810248#Comment_1810248

I know the Bradford White has heat traps on the dielectric nipples, is that what you mean? The water heater is only a couple of months old.

#1 Water coming into the house, 3/4"

#2 Water can go to the sprinklers or house after the backflow preventer. 3/4"

#3 The cold travels another 15 feet and then hits a Tee. One side of the Tee is the domestic cold water for the house (this pipe gets hot to the touch when the sprinklers are on) and the other side reduces down to 1/2" where it can travel one of two ways: boiler for hydronic heating and domestic hot water via tankless coil, water heater or both.

#4 Here's an up close view after the 3/4" cold is reduced down to 1/2" and it goes to the boiler for hydronic heat and domestic hot water.

#5 At this point the cold water can go to the tankless coil or skip the tankless coil by closing the shutoffs. I did this on purpose if I ever wanted to abandon the tankless OR had to replace the water heater.

#6 Here is a wider view of things. I can start to turn shut off values in order to isolate the problem but I think its coming from the water heater. I don't ever remembering this happening with the older water heater.

#7 My drawing of everything 🤐

Larry Weingarten

hi, From your schematic it looks like putting a check valve on the cold supply to the water heater would prevent the water from being able to expand backwards, winding up in the cold side. I'd put the check valve upstream of where the expansion tank ties in.

Yours, Larry

Robertw

I would also suggest shutting off the incoming water supply to your clothes washer as this is a common cross connection source. Aside from shower cartridges

Grallert

Could it be that domestic expansion tank is pushing hot water through that tankless coil into what is the cold side of the domestic system when the water pressure drops do to the high demand of the sprinkler?

I'm also surprise to see no tempering or mixing valve at the tankless.

Big Ed_4

Shut off the water to the hot water heater and then open up a hot water facet . After the thermal expansion tank equalizes the water flow will stop, if not you have a bypass.

FredoSP

As a test I closed the ball valve in front of the expansion tank and turned on the sprinklers. The cold line that would normally get hot did NOT (success). Since I cannot have the ball valve closed during normal operation, I will probably add a check valve on the 3/4" "cold" side of the water heater - does this make sense?

Big Ed_4

Another test you should do is to feel the pipe temperature or the migration of hot water rising .. I assuming you will find a bad shower or faucet cartage …. Maybe the pressure drop over comes the heat trap, the piping between the coil and storage tank…