Water heater pop off valve

N5IIG · 10-03-2008, 05:19 PM

Here is a good one to ponder. Seems like everytime We fire up the water heater - the hotter the water gets the valve starts to drip a small amount of water - not a real problem but annoying - I hate drips. I understand all the stuff about water expanding when heated and the air space required st the top of the tank and the lack of airspace causes the probe to flood as they call it. It only does this the first fire up after periods of no use. I can pop the valve - let excess water out - pop it shut and never another problem as long as I leave it on and pressured up by pump or city water hookup. I have changed the valve and the new ones (2) do the same thing. Can anyone explain to me why it does this after setting for a period of time (2-6 weeks) more or less. I have talked to other owners - some have the problem and some don't. My dealer who is real good says the same thing - some do and some don't. By the heater is a 6 gal gas only Atwood. I am not real concerned about it - just a pain. Any thoughts anyone.

By the way I got the front AC replaced last month with my extended warranty with no out of pocket expense. Cost was almost half of what I paid for the warranty three years ago. They even waived the deductable - works for me.

Sorry for the length but I guess this ole redneck is just wound up tonite. Take care and enjoy your travels....................Jim

N5IIG · 10-03-2008, 05:19 PM

Here is a good one to ponder. Seems like everytime We fire up the water heater - the hotter the water gets the valve starts to drip a small amount of water - not a real problem but annoying - I hate drips. I understand all the stuff about water expanding when heated and the air space required st the top of the tank and the lack of airspace causes the probe to flood as they call it. It only does this the first fire up after periods of no use. I can pop the valve - let excess water out - pop it shut and never another problem as long as I leave it on and pressured up by pump or city water hookup. I have changed the valve and the new ones (2) do the same thing. Can anyone explain to me why it does this after setting for a period of time (2-6 weeks) more or less. I have talked to other owners - some have the problem and some don't. My dealer who is real good says the same thing - some do and some don't. By the heater is a 6 gal gas only Atwood. I am not real concerned about it - just a pain. Any thoughts anyone.

By the way I got the front AC replaced last month with my extended warranty with no out of pocket expense. Cost was almost half of what I paid for the warranty three years ago. They even waived the deductable - works for me.

Sorry for the length but I guess this ole redneck is just wound up tonite. Take care and enjoy your travels....................Jim

KC1BUD · 10-03-2008, 09:36 PM

Jim,

I thought I had the only one like that. I allievate it in the same way as you do. Only mine doesn't ne 2 to 6 weeks, just from 1 weekend to the next. It has done it since I bought it. I thought abour getting a replacement but if you have had the same thing with the new ones maybe that isn't the problem.

Mine hasn't been connected to city water, and most of the places I have had it connected didn't have 40 pounds of pressure. I don't use a filter either.

John-D · 10-04-2008, 06:33 AM

The problem is not temperature (which causes a rapid flow) but pressure.. The valve is called TPR, for Temperature/Pressure relief

First, I'll bet this only happens when you are on city water right? And you do NOT have a Watts type pressure regulator in the line.

Here is what is happening.. Though there is an "Air cushion" in your water heater, if the pressure is too high to start with as the water heats the pressure increases. This happens fairly slowly as.. Well.. The heater heats fairly slowly.. Now in a home water heater water can flow back into the supply line a bit, so the over pressure is no problem, but in a motor home there is a check valve at the inlet, water can not back flow on either the city water or on the water pump.

So if you have say 45-50 PSI the pressure may go up as much a 5-10 PSI, no problem

But if the city line is pushing 100 PSI, the pressure goes up more (Due to the greater compression of the air cushion and the greater volume of water in the tank to start with) and now the pressure setting of the valve is exceeded

A new valve may help.. But a better solution is to limit inlet pressure to around 50 PSI.

NOTE there will be pressure loss going through a filter

I suggest the following chain of devices

Park Spigot-Regulator 60-70PSI--Long hose--Filter-Short hose--Regulator 45-50 PSI-Very short hose--Rig

NOTE; shield the filter and short hoses as much as you can from sun for best performance

N5IIG · 10-04-2008, 08:30 AM

Thanks for the replies. John I like your explanation but I do use a Watts regulator set at 40-45 psi and I check it periodically for accuracy. As far as pump pressure I have not checked it for accuracy but it is not excessive at the faucets. I have changed the valve twice as they are not real expensive. I don't use a filter as we only use spigot water for bathing/washing dishes etc. My water heater thermostat is not adjustable so maybe I need to check and see what the water temp is - just a thought. Anyway I appreciate your explanation and it makes a lot of sense. Thanks again and travel safe...........Jim

John-D · 10-04-2008, 04:55 PM

Well.. Then it may just be a bad valve

N5IIG · 10-11-2008, 09:10 AM

Chandler do you or anyone else have an opinion on this topic. As I said it isn't really a big deal - just a pain

..................Jim

John H... · 10-11-2008, 10:19 AM

About the only thing I would suggest is to check the valve and see if it is stiff or sticky and if so, put a small amount of light oil on the shaft. They will sometimes collect a small amount of corrosion and not seat correctly. If that doesn't help then it could be a faulty valve, in which case you have to change it if it really bothers you, (it would me)or ignore it. John H....

Chandler · 10-11-2008, 06:28 PM

Hi Jim! Well, John's explaination is basically sound. The way the tanks are designed, there should be an air bubble in the tank. That will allow the expanding water to expand :-) You know that and understand. The other function of the valve is related to temperature. If the water temp is too high, the valve can weep. So, that said, I don't think you have a pressure problem. I would think that the temp is too high initially. There should be a temp rating on the valve. Do you think you could use a digital thermometer to measure the temps when this happens? That's the only thing that is still unchanged. You've changed the valves so I'm going out on a limb and say that three bad valves is highly unlikely. Lack of an air bubble? I don't know how but maybe...That leaves the temperature. Wadda ya think?

Chandler

N5IIG · 10-12-2008, 06:16 AM

Thanks John H and Chandler. I agree with both of you and I plan on checking the water temp. I just have a feeling that the fixed thermostat is getting the water to hot and exceeding the parameters of valve or getting so close that it opens a small amount. I agree Chandler that 3 bad valves is a little unusual. John H the valves appear to operate smooth but if all else fails I will try a little lube. Thanks guys and if I find anything in my redneck scientific study (Ha) I will let you know. Thanks again and travel safe...............Jim

John-D · 10-12-2008, 12:18 PM

There are two other possibilities.. Kind of rare but they do happen.

1: Bad T-Stat allowing the heater to "over heat" this increases pressure way beyond design, Now, The odds favor this NOT being the problem because if it was I'd expect the fuse plug (TEMP) portion of the TPR to let loose and drip is not something it knows how to do..Flow like Niagra Falls, yes, Drip no.

#2 is ... Failed TPR valve. These are standard valves. Any Hardwere, plumber, Lowes, Home Depot. or you can pay Camping world prices. Take the old valve with you in any case

N5IIG · 10-13-2008, 02:42 PM

Well folks I did it all yesterday in every way you can think of. The water temp is approx 142 - so that rules out water temp being to high. I have no way of checking the pressure in tank but valve is rated at 150 psi. I realize that heating water raises pressure but I don't think it would reach 150 psi in the tank - more like water expansion by heating which semi floods the valve. I could open a faucet in coach for a second or two and it would stop dripping and never drip again during heat up process.

John H I did blow out the valve with air and lubed with light oil but did not help any.

John D as I have stated in a couple of the post I have three new valves and all three of them do the same thing. I just don't think that I got three weak valves in a row from different suppliers. Thay are all Watts valves rated at 150PSI - 210 Deg F.

My conclusion is that the water is expanding when heated (which is normal)and causing the valve to drip - loosing air bubble or just flooding a little I don't know but it is beyond me. I will just watch it and move on. Thanks to all who gave suggestions but I am out of ideas. Everyone take care and travel safe......Jim

Neil C · 10-13-2008, 07:59 PM

Your problem can be solved by installing a expansion tank in the water line. I have a 2 gallon one. (home depot) I installed one by putting a "y" hose fitting on the cold water inlet and ran a short white water hose to the tank in my area where the water comes in.
It solves two problems
1.. the blowoff valve will not leak because there will be enough air in the expansion tank to compensate from cold start to high shut off temp on water heater.
2.. this will also balance the water pressure
and your water pump will have longer on -off cycles, smoother pressure changes.. you'll like that in he shower.. sometimes it would'nt even go on for small amounts of water usage.