Groundwater Well, Diaphragmed Hydro-Tank, and Thermal Expansion Tank need?

[Ruralist]

Stay with me--its a somewhat hard question to setup correctly. The question is given both before and after additional information.

Question: Will the installation of a flow check valve in the mechanical room between the cold water supply from a 100 gallon hydro-tank and the addition of a properly sized thermal expansion tank reduce heat driven pressure fluctuations better or faster than relying on the heat-pressure adsorption back at the hydro-tank some 150 feet away? Will the thermal expansion tank sitting directly off the hot water heater's cold supply just work better and prevent pressure flexing of system components either up or down stream of the hot water heater?

This forum seems to be the place to get answers not found readily from other resources. I have read many of the threads here concerning the "need" for a thermal expansion tank(ET) in closed systems and/or whether an open system would benefit from an ET. I also understand the situations where backflow prevention devices of several types may be required from a municipal system. I am not connected to a municipal system. I have also found several somewhat different but mostly similar definitions of "Open" and Closed" system. I pose a few questions here to gain more insight and decide if I should install a thermal expansion tank in the configuration I have for added safety and/or reducing stress-flexing the pipes and fittings down and/or upstream of the hot water heater.

Configuration:
Potable Supply (cold): Groundwater well with submersible pump delivering 3 to 5 gallons per minute into a 100 gallon bladder hydro-tank with a static operating pressure around 40 psi. Flow check valve on supply side of hydro-tank prevents water from returning to submersible pump or its supply line.

Hot Water Heater: Gas fired condensing high-efficiency hot water heater which will be set higher than 120 degrees F with thermostatic mixing valve for domestic hot water. A flow check valve on the cold supply to mixing valve is proposed to prevent migration of hot water there. The hot water heater will also be used for some space heating through a heat exchanger and a closed-loop hydronic system. No questions regarding the hydronic system, a thermal expansion tank is required in this typical closed-loop system. The groundwater source and hydro-tank configuration seems to be an "open" system as long as there is no backflow prevention between the hydro-tank and the hot water heater. The pressure and temperature ratings of the hot water heater and hydro-tank differ to some degree and could play a role in what is best.

Question: Will the installation of a flow check valve in the mechanical room between the cold water supply from a 100 gallon hydro-tank and the addition of a properly sized thermal expansion tank reduce heat driven pressure fluctuations better or faster than relying on the heat-pressure adsorption back at the hydro-tank some 150 feet away? Will the thermal expansion tank sitting directly off the hot water heater's cold supply just work better and prevent pressure flexing of system components either up or down stream of the hot water heater?

 

[Valveman]

As long as there is no check valve between the well pressure tank and the water heater it will double as a thermal expansion tank. It doesn't matter how far away the tank is. Are you seeing pressure fluctuations from thermal or just guessing it will happen?

 

[Sponsor]

 

[Ruralist]

Thanks for the reply.
System not installed yet, so I am "guessing" it will happen less or not at all if the thermal expansion tank was adjacent to the heat source. Trying to install system in such a way as to minimize flexure of system components due to thermal expansion. I am understanding that the water transfers the pressure change throughout the system instantaneously or nearly so. I have seen examples of installing the thermal expansion tank and relying on the well pressure tank. I guess I am trying to decide if a few more dollars buy more system longevity/safety or not. Or if the pressure increases at the well tank due to thermal expansion play any detrimental role in the operational aspects of the well motor pressure control (on/off) set points. Likely, just too much thinking (guessing) on my part.

Thanks for the dialogue.

 

[LLigetfa]

Proximity of an expansion tank or pressure tank is not a factor. Why do you think it would be?

 

[Reach4]

A flow check valve on the cold supply to mixing valve is proposed to prevent migration of hot water there.

You bring up an interesting point. If you were to add an effective check valve between the pressure tank and the WH, you would need a thermal expansion tank. There are things called heat traps, and some of these have a ball.

I would presume these are designed to not totally seal, or else we would have had a lot of discussions of problems with built-in heat traps. Actually there are noise, and perhaps other problems, that cause many plumbers to remove the heat traps. But I don't think that isolating the WH from the thermal expansion tank is one of the cited reasons.

 

[Ruralist]

Thanks for the discussions in all directions.

" Why do you think it would be?" was asked above.

I am currently understanding more about the way in which the thermal expansion tanks are sized, pre-charged with air, and placed into service in closed systems. Since these are dedicated system components, they are operated and maintained (O&M) as such. This O&M is straight forward. Relying on the well pressure tank to accommodate the volume of a few more gallons of water due to thermal expansion as if this volume was delivered by the well pump is straightforward and reasonable.

I also think in terms of failure modes. Although, sometimes my failure modes are not very likely; and I need feedback.

Here is a failure mode:

If any of valves between the well pressure tank and the hot water heater are closed, there is no place for thermal expansion to go. Not a likely failure mode I "guess"? This is were my guessing comes into play.

The hot water is used in the house, the hot water tank is full of nearly cold water and is on, and the cold-water supply to the house has been turned off right after the hot water tank filled. In this mode, the Temperature & Pressure safety valve pops but not before all the fixtures in the house are subjected to the same pressure of around 150 psi. Is this right?

 

[Reach4]

The hot water is used in the house, the hot water tank is full of nearly cold water and is on, and the cold-water supply to the house has been turned off right after the hot water tank filled. In this mode, the Temperature & Pressure safety valve pops but not before all the fixtures in the house are subjected to the same pressure of around 150 psi. Is this right?

Pretty much right.

Note the classic heat traps are just a rise and fall in the pipe. http://waterheatertimer.org/images/Heat-trap-install-900.jpg Nothing that could cause a worry about pressure isolation.

 

[LLigetfa]

The hot water is used in the house, the hot water tank is full of nearly cold water and is on, and the cold-water supply to the house has been turned off right after the hot water tank filled. In this mode, the Temperature & Pressure safety valve pops but not before all the fixtures in the house are subjected to the same pressure of around 150 psi. Is this right?

Not a common scenario IMHO but within the realm of possibility. There are often other water use devices that in theory may relieve some of the pressure besides the TPR. Toilets, ice makers, humidifiers, and RO filters will often release some excess pressure.

Also, the volume of water from expansion is not gallons you allude to unless that was a typo. Normal expansion is less than a gallon but of course since water cannot be compressed, it has to go somewhere. On most piping systems, a small percentage of it actually stretches the pipes. I have often turned off the main valve to service my plumbing and am surprised at the volume I get out under pressure when I open a tap. Granted, I have both an iron filter and a softener that must stretch a little.

 

[Valveman]

The pressure tank will handle the once or two of water you get from thermal expansion. But if you close a valve to the house, you should also turn off the water heater. That valve would isolate the pressure tank from the water heater.

 

[Ruralist]

Thanks for the contributions to all on this subject. I am planning at this time not to install a separate thermal expansion tank on the cold water feed line adjacent to the hot water heater unless there is a compelling reason to place a backflow preventer between the well pressure tank and the hot water heater--I see no reason for such a backflow device at the present time.

And, yes--Valveman --turning of the hot water heater off when the water supply to the house is shut off is the right O&M in this case.
Thanks again