I am in troubleshooting mode and am worried I am missing something obvious to try.

After I run my sprinklers, my hot water is diminished for 2-3 hours but overall pressure is fine. It is not cold but I have to turn things like the shower valve more than usual to achieve the same temp I get at all other times.

House ties into the water line about 6 ft from the meter, sprinkler valves are about 6 ft past that in a straight line. I haven't dug anything up but I assume each is just a split off the same line from the meter. I don't think its possible the sprinklers are somehow tied into hot water lines - would have taken extra effort to achieve this. Supply is City of Houston water via an HOA.

House is a 2 yo new construction with two 40 GAL hot water heaters in paralell(2 cold lines in, 2 hot lines out) in the attic(4th floor equivalent).

Sprinkler system is three zone. First zone is just a few sprinkler heads, second is drip lines for foundation, and third is a 600sf backyard with ~9 sprinkler heads. I run them to cycle and soak so the 1st and 3rd zone get 30 minutes each over the course of 2 hours once a week. I am 99% sure there is no pump so it can't be over drawing(I don't see a pump sooo?). As far as I know, backflow preventer is functional but I'm not sure that is a factor. No obvious leaks.

My current hypothesis is that water is flowing by the the house split quickly on the way to sprinklers and creating a siphon effect drawing stored water out of the hot water heater and then my cooler water is a result of the WH catching back up after the sprinklers stop.

What else could it be? Anything I should check? Contractors were sloppy, anything I should look for that is supposed to prevent this design or material wise?

Long post but I have no idea what is relevant information. I am stumped. Thanks anyone with an idea!

Are you positive the two things are related?

Our water heaters sometimes go into a low energy mode where they stop keeping the hot water at the normal temp. So then when you go to shower or shave you get not as hot water and it takes a while to catch back up. Read the manual for your water heaters and see if this applies to you as well.

Timber08 said:

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My current hypothesis is that water is flowing by the the house split quickly on the way to sprinklers and creating a siphon effect drawing stored water out of the hot water heater and then my cooler water is a result of the WH catching back up after the sprinklers stop.

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If this was the case, your sprinkler system would be creating a vacuum inside your WH, and you wouldn't have reduced hot water while it catches up, you'd have none. A traditional tank WH operates by cold water coming in forcing heated water back out the top. If there was an air void, the influx of cold water would only push out air, causing sputtering in the line, and you'd likely be hearing what's known as "water hammer" in the pipes.

Does this happen at a particular time of day, or is it more prominent in one part of the house over another?

Picard said:

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Are you positive the two things are related?

Our water heaters sometimes go into a low energy mode where they stop keeping the hot water at the normal temp. So then when you go to shower or shave you get not as hot water and it takes a while to catch back up. Read the manual for your water heaters and see if this applies to you as well.

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I guess I can't say with certainty they are related yet but it is 100% correlation. Run the sprinklers and I will have weak temperature. This is true whether I run them Monday or Wednesday etc. as I do move it depending on rain so it can't be an unrelated hot water use that happens to occur the day I water.

If I don't run the sprinklers, first shower of the day is always capable of scalding hot.

There are instances where we have a little cooler water without running the sprinklers but I would chalk that up to normal increased demand(back to back showers and running the dishwasher at the same time etc).

Try an inline back flow preventer/check valve?

hurricanejake02 said:

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Timber08 said:

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My current hypothesis is that water is flowing by the the house split quickly on the way to sprinklers and creating a siphon effect drawing stored water out of the hot water heater and then my cooler water is a result of the WH catching back up after the sprinklers stop.

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If this was the case, your sprinkler system would be creating a vacuum inside your WH, and you wouldn't have reduced hot water while it catches up, you'd have none. A traditional tank WH operates by cold water coming in forcing heated water back out the top. If there was an air void, the influx of cold water would only push out air, causing sputtering in the line, and you'd likely be hearing what's known as "water hammer" in the pipes.

Does this happen at a particular time of day, or is it more prominent in one part of the house over another?

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I don't mean catches up in the sense that the water refills, I would think that doesn't take long at all or I would have the sputtering. I mean coming back to temp(not water level) just as if I had emptied it by turning on all the hot water taps, waited just a minute, then hopped in a tepid shower. So 4am the sprinklers create the vaccum(maybe???), 5am it refills, 6am I take a shower with full pressure but my 80 gallons isn't to temp yet. Maybe still no merit to this idea?

I have only watered in the mornings but it has only happened on mornings I watered. So for instance I watered Mon, Wed, Mon in the last two weeks and only had the issue those days. All other days its 110-120 degrees at max hot like I have it set at the exact same time.

I do feel/think its more pronounced the hotter it gets outside but that doesn't explain 90F outside Monday has an issue and 90F outside Tuesday no issue.

Mr. AGSPRT04 said:

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Try an inline back flow preventer/check valve?

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That is what I have been thinking but was hoping I might be missing something. Just didn't want to treat a symptom instead of an underlying cause.

I'm also not thrilled about introducing a complexity that can leave my whole house without water if it fails. Especially if nothing is actually harmed by whatever is happening.

Do you even have a Backflow Prevent Value (BPV) on the sprinkler system?

This was my first thought when I read your initial post...

Gary79Ag said:

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Do you even have a Backflow Prevent Value (BPV) on the sprinkler system?

This was my first thought when I read your initial post...

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I do have one, similar to this BACKFLOW PREVENTER PRESSURE VACUUM BREAKER PVB.

Okay, that good news...

Nest question is...is it working properly?

I had a sprinkler system installed and the city required that a licensed irrigation company representative inspect and test it 2 years later. Sure enough, it failed and it had to be replaced.

I'm no expert here regarding this, so maybe the experts on here can clarify if my concern is a real issue or if I'm barking up the wrong tree!

Perhaps you could try shutting the water inlet valves to the water heaters, then run your sprinklers, then re-open the valves and try the shower for hot water.

If the sprinklers are somehow pulling hot water from the water heaters, closing that valve will prevent it.

Also - is the water coming from your sprinklers hot?

GigEmAgs08 said:

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Also - is the water coming from your sprinklers hot?

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this is what I was wondering.

GigEmAgs08 said:

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Also - is the water coming from your sprinklers hot?

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I'll add that to my list to check this weekend.

I'm also thinking to hang out in the attic while they run to see if I can hear/feel any movement around the WH with everything else shut off.

Couple of things:

1. Sprinklers cannot suck water out of your water heater. If the hot water was being sucked out of your water heater, something would have to replace that water, either more water or air. You have already said that the water isn't sputtering air after the sprinklers, so it isn't air replacing the water, and it can't be cold water as that is what you are claiming is sucking the hot water out.

2. Have you actually measured the hot water only temperature at both these times? Do this at a sink that you can turn the cold water off at the wall to make sure that the blend valve isn't malfunctioning somehow. Also measure the cold water only both before and after the sprinklers run

My guess is you will find that the hot water is the same temperature both times. I will guess that your cold water is actually much cooler after the sprinklers run as they are completely draining the reservoir of warm water in the distribution to your house, and bringing in cooler water from the water main.

Additionally, your shower blend valve may be set to include too much cold water as a safety measure to prevent scalding. There are adjustments for that.

CalAG said:

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My guess is you will find that the hot water is the same temperature both times. I will guess that your cold water is actually much cooler after the sprinklers run as they are completely draining the reservoir of warm water in the distribution to your house, and bringing in cooler water from the water main.

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This was actually my guess as well. That the hot water isn't less hot, just that cold water is colder.

You can't suck water out of the hot water heater. The whole system is pressurized, so as your water flows past the house split, water would be forced into the house in the same manner that it is forced to the sprinkler system, not sucked out. It isn't forced into the house because everything in the house is closed and the only opening is the sprinkler heads. Basic hydraulics says this is not the case.

CalAG said:

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My guess is you will find that the hot water is the same temperature both times. I will guess that your cold water is actually much cooler after the sprinklers run as they are completely draining the reservoir of warm water in the distribution to your house, and bringing in cooler water from the water main.

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CalAG for the win

That is much much more logical. I'll test with a thermapen to satisfy my curiosity but I think you're right.

Thanks everyone!