Plumbers STUMPED!! Irrigation causing Airlock in Hot Water tank

Hello, I've had a couple of plumbers out here, and they left in defeat, stumped as to what is going on:

Every morning after the sprinkler runs, we have no hot water in the house (no drips, no noise, just nothing coming out of faucet). Cold water is fine. After much research in the forums, I narrowed it down to an airlock being created in hot water line (or electric HW tank) after the sprinklers turn on or off. Reason being, after I open up all the hot water outlets in the house for a while (sometimes hours), the hot water will suddenly 'pop' out and start flowing again.

I've got a 6 zone system, but only use 4 zones. Automatic controller. Run them for only about 5 minutes now everyday around 3:30am, one zone per day (2 zones on 2 days a week). Yesterday I tried closing the sprinkler's shut off valve about 50% to reduce the pressure, but no noticeable change. There's no hot water coming out of sprinklers.

City water. No softeners, filters, or anything else of the kind on premises. The water here is pretty hard (Tampa, FL), so sediment may be a concern, but why is the irrigation system the catalyst? And why does it just 'pop' back to normal. No loud sound, no sediment comes out, and the tank does not suddenly start filling up when it does either, it just flows normally again.

All white PVC around the house, and looks like CPVC coming in/out of hw heater.

House is only about 10 y/o, water heater the same, but even at it lowest setting, it heats up extremely hot, so it's working great. We've had low hot water pressure issues on and off since we moved in last year, but after sprinklers turned off for winter, problem resolved. Now, sprinklers back on, problem back.

Any ideas what could be going on here? Thanks for your help!

An "air lock" is a funny thing to diagnose in a pressurized system, but we may need to ask some questions since the plumbers at your home had the benefit of being there to see everything.
Do you have a regulator? Do you have a backflow device on the house and on the irrigation system? Do you have a thermal expansion tank somewhere else in the house, since one is not seen in the picture?

More pictures would also be nice, for the reason explained above, of the water service, the tee for the irrigation system.
Also, do you have a recirculation pump on the water heater?

To me it seems that if there are ball checks in the water heater supply nipples on the hot and cold sides, they may be getting hung up and causing the problem. More opinions are sure to follow to help you.

install a vacuum breaker above the water heatr on the cold side

http://www.watts.com/pages/_products_details.asp?pid=815

Thanks for the replies. Here is a picture of the irrigation T with the backflow preventer on it. Right behind it is where I believe the main splits off to the house. I don't see any tanks or attachments of any kind anywhere (ie recirculation or expansion tanks, etc...).

I don't see any other valves anywhere else. Could it be not an 'airlock' but something else that is sticking when the irrigation systems kicks on/off?

I'm just saying that an "air lock" is usually associated with gravity type drainage and not in pressurized systems.
Frodo did suggest a method of relieving a vacuum in the plumbing system, but one would have to ask how the vacuum was created by the irrigation system to be sure it would be the right fix...he could be right that it would solve the problem but we need to remember that there were low pressure problems without the irrigation system in use.
Any others want a stab at it?

Thanks! You're absolutely right, how to be sure if it is air/pressure causing this at all or something else? The low hot pressure in the house was there before but it all went away as soon as the day the hot water 'popped' and I was not using the sprinklers. After that, the hot water flowed great until the day I started the sprinklers back up, and it returned. Cold water pressure has always been fine.

I just don't see any other valves anywhere that could be sticking, just the 3 shutoff valves that are open. What else should I check?

I could be way off base here but I think your irrigation system causing this problem is incorrect Irrigation is tied into you main water supply out in yard and has a back flow preventer on it to prevent problems like this . there should never be a reason an air lock would travel and lock up only your hot side. Issue is probably at water heater. Depending on age of water heater there are little check valves in the galvanized nipples in some that I have seen on several occasions cause this type of issue your experiencing. What would need to happen is either wh be replaced ( highly recommended it over 7 years old) or check valves removed. Again this is not a supper common problem which is why u might be getting a Mis diagnoses. At the very least having someone come and and remove them would make u no worse off then you are. If the check valves are malfunctioning they will block flow going either in or out of wh if I was your plumber that's where I would start

I'm going to agree with djmayhill.

They used to make nipple that had a small ball in them.
They were designed to prevent heat from conducting out of the supply lines when there was no water flowing through it.
Cold side floated up and would get pushed down with water flow.
Hot side was heavy and sank to bottom of nipple and would rise up with water flow.

Problem with those, they would rattle.
But it just might be possible that one is getting stuck. Very old but I have seen weirder things happen.

djmayhill said:

Depending on age of water heater there are little check valves in the galvanized nipples in some that I have seen on several occasions cause this type of issue your experiencing. What would need to happen is either wh be replaced ( highly recommended it over 7 years old) or check valves removed.

Click to expand...

Maybe take Model No of WH and GOOGLE for manual. It should have info if it came with the heat loss traps installed, if they were not removed @ install.

I could be wrong, but

look at the OP's picture of he W/H you will not see any nipples in the top of the W/H

looks to be cpvc male adapters connected to the unit

frodo said:

I could be wrong, but

look at the OP's picture of he W/H you will not see any nipples in the top of the W/H

looks to be cpvc male adapters connected to the unit

Click to expand...

Takes care of the dielectric problem. I would worry about blow-out though...

i suggest you dig up the incoming line to see if your house is hooked up to the

sprinkler vacuum breaker , it should not be

Yea I did not know there was a picture. I would agree that's the case based off what I can see. Still the irrigation system being the source of the problem is very strange being that it is a pressurized system and air locks don't usually occur. Although I'm not there to see the scope of it. I would still do more investigation around wh because if only hot side is affected than that would be the starting point. Being the wh is 10 yrs old i would highly recommend replacement. That way one could rule out it being the source of the issue. It doesn't have much time left anyway. IF the irrigation is the source and that a big if in would make sure the backflow preventer is still functioning

Very Odd indeed.

I'm guessing that the plastic ball valve near the ground is on line coming from the house to the back flow and not the main service to the house.
If so do you a photo of the main service valve to the house.
This no hot water flow is through out the home at all fixtures?

On the hot supply behind the 90 is there a valve?
Disconnect the water heater from the system and see it your problem is still occurring.
Cut the supplies off from the heater and tie them back together with 2 90's and cycle the irrigation. does the no flow issue continue.
If not then just replace the WH like djmayhill suggested.

frodo said:

I could be wrong, but

look at the OP's picture of he W/H you will not see any nipples in the top of the W/H

looks to be cpvc male adapters connected to the unit

Click to expand...

Good eye, frodo. unless the OP can confirm or refute that the mentioned fixes have been looked into by any of the plumbers, I would say trying your recommendation would be the way to start. It would be easy and could do no harm.

Mighty thanks to all for all the help! Everything you guys said made sense, so I'm going to spend some time working on it. The great thing is when I posted initial question, I had nowhere to start, but now I have a few options to look into. Worst case is replacement, but as pointed out, it's due anyway, but at least I can send it off knowing I tried...

Much appreciated, now I see why chicks dig y'all and the rest of us want to be you...