Too answer some questions:
The expansion tank goes on the cold water intake. The pressure in the bladder should be set at the cutoff pressure of the pump so that water only goes in the tank during expansion.
Groco makes good stuff for a princely sum. The same product can be had for a lot less. My preference is the Watts PLT line.
https://www.watts.com/products/plum.../water-heater-accessories/expansion-tanks/plt
The PLT-5 should be more than enough for a marine water heater. Under $40 plus shipping.
Home Depot is showing a similar size different brand for around $25.
https://www.homedepot.com/p/Everbil...VCTKGCh18lQEaEAQYBSABEgJvZ_D_BwE&gclsrc=aw.ds
Ted
The exact item you are looking for is a water heater expansion tank. Required on residential water heater installs in most locations now. Home Depot, Lowes etc have them.
This is assuming that there is nothing wrong with your T&P valve. Which there may be. Also if you use engine heat to heat the water you could be going over temperature for the valve.
Ted, putting the expansion tank on the cold water side being the correct and only method is most confusing to me. The tank is installed in-line. I just don't see that installing it on the output side makes a bit of difference in terms to limiting the internal pressure of the water heater especially if a directional (in only) cold water supply valve is used as some installations recommend. What am I missing? My relief valve weeps ever so slightly, not enough to justify adding an expansion tank, but I do want to understand just in case.
Snip
. But, as others have noted, and I learned this myself on some websites yesterday after my initial post, it does not appear these accumulators are designed for hot water and should be plumbed in to the cold side. Installing an accumulator helps with other things like constant pump cycling for small faucet draws, but it would not solve my problem I now know, which is also confirmed by the replies on this thread.
I was told (second hand from the hot water heater manufacturer) one method to prevent the RV from constantly lifting is by plumbing in an accumulator on the hot side. But, as others have noted, and I learned this myself on some websites yesterday after my initial post, it does not appear these accumulators are designed for hot water and should be plumbed in to the cold side. Installing an accumulator helps with other things like constant pump cycling for small faucet draws, but it would not solve my problem I now know, which is also confirmed by the replies on this thread.
One more thing – it appears code criteria for boats concerning hot water heater relief valves has changed. These heaters are now required to have 100 psi relief valves, where on shoreside (similar rated T/P heater units) they are 150 psi. The manufacturer doesn’t note the MAWP on my heater plate, by design I assume because less is more. I think I just figured out the solution.
The T&P valve on a water heater is not something to be trifled with.
In our marine plumbing, a check valve is typically located in the cold water inlet to the water heater. This check valve serves a different purpose than the dual check in a domestic application, it's intended to prevent backfeed of hot water into the cold system. It's function is more comfort than safety. Still, it can create conditions where the thermal expansion overpressures the relief valve setting. Installation of a thermal expansion tank in the cold water piping is ineffective because the check valve prevents flow back into the tank's bladder. The solution is not to plumb the tank into the hot water piping since the expansion tank isn't rated for hot water.
The solution lies in the water heater inlet check. Since that check valve is NOT a safety device, the simplest solution is to allow minimal flow back through the check valve. Scoring the seat of the valve or drilling a tiny hole in the flapper will allow minimal backflow while retaining the function. If there's an accumulator tank on the plumbing system, it will likely have the capacity to absorb the thermal expansion, or if not, a larger or additional tank can be installed- in the cold water piping where it's intended.
One option is to just remove the check valve. You will probably get some thermal mixing, but it's better than running your plumbing system and the explosion release pressure.
The solution lies in the water heater inlet check. Since that check valve is NOT a safety device, the simplest solution is to allow minimal flow back through the check valve. Scoring the seat of the valve or drilling a tiny hole in the flapper will allow minimal backflow while retaining the function. If there's an accumulator tank on the plumbing system, it will likely have the capacity to absorb the thermal expansion, or if not, a larger or additional tank can be installed- in the cold water piping where it's intended.
. I think the point Ted is making that some are missing is if you place the expansion tank on the water heater inlet side between the check valve and the WH the heated & expanding water can back up into the expansion tank but NOT into the cold water system (which is the reason for the check valve. It may mean moving the check valve a short ways if it is currently immediately at the WH inlet. Move it to the inlet side of a "T" for the expansion tank.
Creating a "leaky" check valve can certainly solve the problem. I think the point Ted is making that some are missing is if you place the expansion tank on the water heater inlet side between the check valve and the WH the heated & expanding water can back up into the expansion tank but NOT into the cold water system (which is the reason for the check valve. It may mean moving the check valve a short ways if it is currently immediately at the WH inlet. Move it to the inlet side of a "T" for the expansion tank.
A check valve on the incoming water to your water heater serves no purpose and should be removed. Water cannot go 'backwards' through your water heater. A small expansion tank installed in the cold water line near the water heater is appropriate to prevent your temperature and pressure relief valve from weeping to account for .
I'd disagree. That check valve does serve a purpose, that's to prevent backflow of hot water into the cold piping. It may not always occur, but it can happen more easily in a marine water heater because of the piping. The check valve prevents that. So "should be removed" doesn't always apply.
How can it backflow? What in marine plumbing can make water flow backwards? Does it flow back through the pump and refill your tank as well?
How can it backflow? What in marine plumbing can make water flow backwards? Does it flow back through the pump and refill your tank as well?
I installed a watts as described by Ted and very much happy with it, cheap, well made and you can easily pressurize the tank to your need.Too answer some questions:
The expansion tank goes on the cold water intake. The pressure in the bladder should be set at the cutoff pressure of the pump so that water only goes in the tank during expansion.
Groco makes good stuff for a princely sum. The same product can be had for a lot less. My preference is the Watts PLT line.
https://www.watts.com/products/plum.../water-heater-accessories/expansion-tanks/plt
The PLT-5 should be more than enough for a marine water heater. Under $40 plus shipping.
Home Depot is showing a similar size different brand for around $25.
https://www.homedepot.com/p/Everbil...VCTKGCh18lQEaEAQYBSABEgJvZ_D_BwE&gclsrc=aw.ds
Ted
How can it backflow? What in marine plumbing can make water flow backwards? Does it flow back through the pump and refill your tank as well?
How can it backflow? What in marine plumbing can make water flow backwards? Does it flow back through the pump and refill your tank as well?