Getting hot water from cold lines (after draining heater?)

[wmb]

I have a weird problem which has resurfaced after going away for several months.

When we first got the trailer, with the water heater on and full of hot water, we would get a stream of hot water for a few seconds after turning on a cold outlet (including flushing the toilet).

This went on for a few weeks and then the problem went away and we haven't seen it happen at all until this week, when it started happening again after I drained the water heater and replaced the drain plug.

Now it's doing the same thing. If the water heater has been on overnight, when opening the cold side we immediately get hot water for a few seconds, then it goes cold, or the first flush of the toilet flushes hot water.

It almost seems like the water heater is heating the water in the pipes. Could the heater tank inlet valve be allowing hot water back into the supply pipes?

[DryFly]

"It almost seems like the water heater is heating the water in the pipes. Could the heater tank inlet valve be allowing hot water back into the supply pipes?

Look around your hot water heater to see if a cold water line is resting against the hot water line that comes immediately out of the tank. Your water heater is a cold water in, hot water out device; I don't see how allowing hot water back into a supply pipe would be causing your issue.

[perryg114]

Did you leave the bypass valve on that is used so you can drain, blow out, or fill with lines antifreeze without having to deal with the extra volume of the water heater? This would be a valve between the cold water inlet and the hot water outlet going to the water heater.



Perry

[wmb]

Originally Posted by perryg114

Did you leave the bypass valve on that is used so you can drain, blow out, or fill with lines antifreeze without having to deal with the extra volume of the water heater? This would be a valve between the cold water inlet and the hot water outlet going to the water heater.



Perry

I've never touched the bypass valve. We've never had to winterize, but the previous owners did.

[Zil]

I had to install a hot water tank in a shop garage. The tank was tapped into the cold water line before a eye wash fountain. The fountain had hot water. The cold water in the heater when heated, expanded into the cold water line. I tried a check valve, that resulted in the pressure release valve pulsing water. I ended with a new cold water line back to the entry point of the system. Not the same as an RV but could lead you to the problem.

[perryg114]

Check the valve and see if it is closed. It should be closed. The valves to and from the water heater should be open.



Perry

Originally Posted by wmb

I've never touched the bypass valve. We've never had to winterize, but the previous owners did.

[james.mileur]

Just reading your air cushion - drain plug thread.

IF you have restored your air cushion, and IF your water pump doesn't start immediately when you open a cold faucet, and IF the physical arrangement of cold & hot tubing is such that the hot water pushed backwards out by the expanding air cushion until the pump starts is into the main cold supply to the other fixtures, I can postulate a shot of hot water out of the cold fixtures, IF you follow.

[AlinCal]

If thermal expansion is the culprit, before you run any water and you can get access to your cold water feed line into the w/h you can feel the temperature with your hand.
This is common in residential situations where the thermal expansion causes pressure to increase and push the cold supply back to the meter, an expansion tank at the w/h solves this problem.
on your trailer though assuming you are on your tank and pump system this cant really happen because of the check valve in the pump, so then the pressure relief will dump the excess. In your rv this is the job of the air cushion.

[james.mileur]

Originally Posted by AlinCal

on your trailer though assuming you are on your tank and pump system this cant really happen because of the check valve in the pump, so then the pressure relief will dump the excess. In your rv this is the job of the air cushion.

I believe this could happen on the tank and pump, since the water heater and cold main to fixtures are all downstream of the check valve. My air cushion sometimes supplies water for a couple seconds before the pump kicks on. I'm only assuming it pushes hot water out during that time. I just haven't noticed the hot slug of water, perhaps due to the actual arrangement or length of tubing to/from the intersection.

[AlinCal]

I suppose it could to a limited amount.
My basic thinking is, if you have the hot migrating into the cold supply something has to allow it to travel. You will get some of that as pressure increases though.

If the system has an accumulator tank at the pump this could happen.

[KK4YZ]

It’s possible that the H/C valves are open in the shower. If the valve at the shower head is closed you’d never know it. I believe this could cause cross flow between the hot & cold systems. Check the outside shower as well.

[GCinSC2]

OK what the heck.

Is your water hose in direct sunlight and heating up the water in the hose and as soon as you run water thru it, problem disappears?

[1970 Safari]

Originally Posted by KK4YZ

It’s possible that the H/C valves are open in the shower. If the valve at the shower head is closed you’d never know it. I believe this could cause cross flow between the hot & cold systems. Check the outside shower as well.

I am leaning towards this. I have a shut off installed on my home shower head and one afternoon I forgot to turn off the water in the shower and only used the shut off.

When my wife went to do dishes she complained the hot water was cold (opposite of your issue but same cause, maybe). To make a long story short- checked the water heater, cranked it up, shook my head- next day she took a shower and said I had left the shut off open....problem solved http://www.airforums.com/forums/imag...s/innocent.gif

[Wazbro]

If you are running the furnace 1 of the ducts could be against the cold water line and heating it.

[wmb]

Thanks for all the replies.

It's stopped doing it again now.

From a couple of comments here it seems it's possible that it could happen when on tank + pump... Well, even though we had city water we did run the pump to improve the low pressure from the city supply. We've since turned off the pump and that seems to have changed things.

As to GCinSC2's comment about sunlight on the hose, I do remember wondering if that was the cause last time this happened, last summer when we were in 105 temperatures in Utah. But the last week or so here in Texas has been overcast and chilly, so I don't think it's that this time.

[SSquared]

I have had this happen too. I think it is the air cushion in the water heater. That holds pressure, and can push out a fair bit of water when a valve is open. I don't think there is any backflow preventer on the water supply into the heater, so it seems hot water would push out into the cold line, until the pump kicks in and overpowers it.

We seem to have been able to reduce/eliminate it by flushing water through all the fixtures to make sure there are no bubbles elsewhere (such as the outside shower)