Recirculating Line Causing Excess Pressure?


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Old 09-07-08, 10:26 AM
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Recirculating Line Causing Excess Pressure?

House was built in 2000 with a recirculating hot water line driven by a Grundfos UM 15-10 B5 pump. Hot water heater installed was an AO Smith 75 gallon unit. Basically, ever since the house was built, I have been getting leakage from the TPR valve. Changed the TPR valve several times and no improvement. So over the last 8 years I have lived with the TPR valve connected to a hose that leads to the floor drain.

Last month the hot water heater started to leak from the tank and the entire unit needed to be replaced. I replaced the heater with a 50 gallon AO Smith Vertex 50 unit and I am still experiencing leaking at the TPR valve. Now I am bound and determined to figure this out once and for all!!

I also have an Amtrol ST-5 expansion tank installed just before the cold water line comes into the heater.

First thought was that the city installed backflow preventers and this was my problem. But city says we don't use backflow preventers and they came out and confirmed this. Pressure measured at the outdoor sillcock is 72PSI. I have been monitoring tank pressure and it goes as high as 155 PSI when the unit is heating water (hence tripping the TPR).

The Grundfos pump feeds the recirculated water into the cold water line just before the heater intake. There is a checkvalve between the Grundfos and the Tee into the cold supply line.

The water department guys think the Grundfos pump may be putting too much pressure into the tank. Also, they think maybe the expansion tank is either too small or not working properly. Based on what I am reading, it appears that an ST-5 may be too small for my application. But the calculations are very close and I am getting a significant amount of water from the TPR when the unit is cycling.

I have the Vertex set at 120 degrees.

There is also a checkvalve immediately before the expansion tank on the cold water supply line.

Also ... I can't find anything on a Grundfos UM series pump but my guess is that this is the same as a UP series pump. It doesn't appear that these pumps are adjustable in any sort of way.

I just measured the air pressure on the expansion tank and it is way off the scale of my tire pressure guage -- over 55PSI. And water is leaking from the air valve when I go to measure the pressure. Perhaps I am getting close to the culprit?

I am on a mission from God here. Any more thoughts?
 

Last edited by pslatin; 09-07-08 at 11:10 AM. Reason: Add information about expansion tank pressure.
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Old 09-07-08, 12:34 PM
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I do think you have found the culprit in the expansion tank. It is highly possible the bladder has ruptured and it has become waterlogged with no efficiency at all, now. I would consider lowering my incoming pressure at the PRV to about 60psi. 72 is getting borderline. Add the water heater for expansion and you 've got more pressure than the system can stand.
 
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Old 09-07-08, 01:15 PM
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First, you cannot measure the air pressure in the expansion tank when the expansion tank is connected to the system. Attempting to do so will only measure the higher pressure of either the bladder or the system pressure.

Secondly, the water coming from the air connection indicates that the bladder has failed. You need a new expansion tank and now is a good time to get the larger model.

Thirdly, that check valve on the city water supply is what is making the hot water system a closed system.

Finally, there is no way that your circulator is the cause of the water heater overpressurization. That pump, in that configuration, develops only enough pressure to cause the water to circulate.
 
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Old 09-07-08, 02:25 PM
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Thanks for the great responses.

RE: [furd]; The check valve on the cold water supply just before the expansion tank is not a "city valve", but rather it was installed by the original plumber. Is it necessary? The city tells me they don't mandate a closed system. Why would the original plumber have installed it?

So the next issue is if I go to the bigger tank, I may need to rework some of the copper to get it to fit. Question, do I have to attach the tank directly to the water line or can I drop a line down and out a bit in order to fit the tank into the space? I saw a pic on the internet of an expansion tank mounted horizontally so I wonder what I can get away with here? I have no intention of mounting horizontally but I may need to drop a line down and put in a couple of elbows and move it away from the vertical supply line.

RE: [chandler]; How do I go about "lowering the incoming water pressure at the PRV"? Someone suggested that I might need a pressure reducing valve. Is this what you were thinking?

Lastly ... the recirc line is fed back into the cold water supply. I have read and heard from others that it is also possible to feed this back into the tank at the point of the tank's drain valve. Does anyone have any argument for why this might be better? Isn't there a pipe inside the tank that brings the cold water supply to the bottom of the tank where the burner is?
 
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Old 09-07-08, 03:25 PM
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I did not write that the check valve was a "city valve". I wrote that the check valve was in the city water supply line to the heater.

You have two lines going to the cold water inlet of the water heater; one from the discharge from the circulation pump and one from the cold water supply in the house. It was this second line that I referred to as a city water supply because ultimately that is where the water is coming from. You absolutely need the check valve in the line from the pump in your present piping arrangement. You may be able to remove the check valve from the supply line although I don't recommend doing so. If you want to try it AND the check valve is of the swing type you can remove the "gate" from the inside by removing the top of the valve, removing the nut on the side of the valve where the hinge pin is located, removing the pin and then the gate, replacing the pin in the body (leave the gate out) and replacing the top of the valve. Use a piece of baling wire to affix the gate to the valve to both let others the valve function has been disabled and also so that you can find the gate to allow reinstallation if necessary. Run this way for a while and note how far back the cold water inlet gets hot. If it is less than a foot then you are home free.

You may not have a PRV (Pressure Reducing Valve) installed in your incoming water service. While 72 psi is a bit higher than "normal" water pressure I do not think it is excessive and I would not install a PRV in this case.

If your city water utility does not require an expansion tank on your water heater and you do not install a PRV AND you do disable that check valve on the water heater cold water supply then you may not need the expansion tank. If you decide to disable the check valve in question (by removing the gate), then install the water pressure gauge, use enough hot water to cause the water heater burner to light, note the water pressure and do not use any water (including flushing the toilet) until the water heater burner shuts off. If there is no pressure rise then you do not need the expansion tank.

If you do need the expansion tank please understand that it may be mounted in any orientation and it may be located some distance from the water heater AS LONG AS the connecting piping is connected to the cold water piping where it is directly in connection to the tank of the heater. In other words, there are no check valves that may isolate the expansion tank from the water heater tank. You may install an isolating valve to facilitate replacement but if you do make sure it is a ball valve and use a nylon "ty-wrap" or some baling wire to lock the valve in an open position except when actually changing the expansion tank.

Yes, you could alternately connect the circulation pump discharge to the bottom of the water heater through the brain connection. It would require a 3/4 inch brass nipple, brass tee and a ball valve along with a garden hose adapter for the ball valve in addition to an isolating (ball) valve for the pump connection. Doing it this way would almost surely preclude the necessity of the check valve in the cold water supply piping. A down side is that if you have a fairly high amount of sediment in your city water the pump will be stirring it up from the bottom of the water heater.
 
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Old 09-07-08, 06:15 PM
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Thank you Mr. Furd! This is all making a lot more sense now.

My plan this evening is to replace the ST-5 with an ST-8. The good news is based on Amtrol's website, the diameter of the ST-8 is the same as the ST-5 so it should be as easy as changing a light bulb (I hope). I was afraid that a bigger tank would have a larger diameter and this is the dimension that gives me trouble as it sits less than an inch from the vertical drop of the cold water supply line.

I have started measuring the discharge from the TPR valve when the heater is on and it is between a pint and quart per cycle.

I agree with all of your conclusions but if you would be so generous as to elaborate a bit ... to wit:

You would not recommend removing the so-called "city supply" check valve (and I agree) but what would be the benefits/reasons for keeping it? Are you worried about pushing water back to the city or pushing too much hot water into the cold line or ???

I agree with not reducing the 72PSI incoming pressure but besides my hot water tank issue, are there any other concerns with having the pressure be that high? Seems to me that "more pressure" is always preferred when taking showers or hosing off the driveway! Our sprinkler system mostly runs in the early morning hours but when that is running I need as much pressure as possible.

And lastly, as for connecting the recirc line to the tank drain valve ... I am aware of the issue with sediment and don't think that is a huge problem where we live. But I was wondering if there were truly any benefits to doing this from the standpoint of the recirc line performing better. Presently without the recirc line running, the shower furthest from the heater takes 1 min 50 sec to get hot. With recirc line, the water gets hot in 25 to 30 seconds. I would only rerun the piping if I could improve on this.

(If 25 to 30 seconds seems long, it is likely due to the bozo plumbers who installed the system when the house was being built. They did not take the recirc line up from the basement up to the second floor bathroom that is farthest from the heater. It only runs the length of the basement!)

Thanks for all your help. This is an excellent message board!
 
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Old 09-07-08, 08:45 PM
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Hello,

According to the Amtrol tank sizing chart you are on the line between an ST-5 and an ST-8.
http://www.amtrol.com/pdf/TXT%20Sizi...artsMC8520.pdf

I would change the tank to an st-8 but you should be able to get away with the st-5.Make sure to set the tank pressure appropriately. I used to leave them at factory settings until I worked for a water meter company, and learned the tank pressure should be set to about 2 lbs under the static water pressure. In your case, 72 psi.
When replacing the tank, take into consideration, the old tank is full of water and will be quite heavy compared to the new tank.

As far as your water pressure is concerned, 72 psi is fine. If you are over 80psi, you need a PRV and reduce the pressure.

As for the check valve, DO NOT remove it. It is there as a safety control. Not from you pushing water back into the lines, but to prevent a negative pressure on your line should there be a water line break or a fire, etc. You could have a hose in some chemicals when the negative pressure happens and it could cause the chemicals to get sucked up the hose contaminating not only your water system but the city water system as well.
 
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Old 10-11-08, 12:23 PM
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Problem Resolved

Sorry for taking so long to respond but after replacing the ST-5 with an ST-8 ... my TPR valve leak has stopped. Thanks to everyone for your help.

Yes the old expansion tank was heavy and the only way to drain it completely before discarding it was to remove the valve stem from the Schrader valve.

I set the bladder pressure on the new ST-8 to just over 60PSI. I was afraid to go more because I had read that you can blow the bladder if you use a compressor. I tried using a bicycle pump but was getting nowhere. Besides, my tire pressure guage only went up to about 55PSI and I had to guess the rest of the way. So question is: If the bladder pressure is UNDER the city incoming pressure of 72PSI ... what are the ramifications of this? Just curious.

The hot water heater is set at 130 and when it cycles it goes up to 99-100PSI ... a significant improvement from before when it would climb to 155PSI (and hence tripping the TPR).

New Issue: My Grundfos recirculating pump has a timer on it and I have it set to run in the morning (for adult showers) and in the evening (for kid showers). But during the afternoon (when the pump is off), if we use the shower that is farthest from the heater (65 feet horizontal, up to the second floor) ... the hot water never seems to arrive.

When I have the recirc pump totally disabled (unplugged from the wall), the hot water takes 2 minutes to arrive, but at least it DOES arrive and from that point on the shower is hot.

I don't understand why, when the pump is cycling on off during the day, I never get hot water during the "off cycle". I would expect to wait the two minutes to get hot water but then I would expect the remainder of the shower to be hot.

Any thoughts?
 

Last edited by pslatin; 10-11-08 at 12:23 PM. Reason: Add notification.
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Old 12-15-08, 12:46 PM
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Pressure Increasing

The new expansion tank continues to work well and there are still no leaks at the TPR valve.

But when I installed the new tank in October, my max pressure in the water heater dropped to about 90-95 PSI which was perfect. But slowly, over time, the max pressure is increasing, where now it is showing approximately 110 PSI.

What is happening? Do I have too much air pressure in the expansion tank? Or too little? Or is there something else going on? I can live with 110 ... hell I can live with anything under 150 (right?) ... but it seems like I am on a course back to when the TPR was leaking.

Perhaps this is what happened to my first tank (see initial note below) ... maybe there is something going on causing premature expansion tank failure.

Any thoughts?
 
 

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