water heater temp fluctuates with pump cycling

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Ghost

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I've recently noticed that my water heater temperature has more of a tendency to fluctuate from hot to cold when taking a shower. The source of the fluctuations of course is related to the cycling of the water pump, which otherwise seems to work as it always has, at least that's what the water pressure and my well trained ear for what's "normal" tell me.

Researching the issue, I have come to the conclusion that adding a thermostatic mixing valve on the water heater could have some benefits, especially when the water tank is super hot from the diesels running (which has nothing to do with the water temp fluctuations as of late, though it might help cure it).

That said, the fact that I was not in want of a TMV previously tells me that ultimately "something" else has changed.

Since I am not aware of any check valves in my water system, it begs the question in an otherwise simple system, what is causing the problem that was not present in the past?

Any ideas? Only thing that comes remotely to mind is the pressure tank on the water pump. What say you all?
 
I think you will find a check valve in the inlet fitiing (adapter) to the water heater. You may have to unscrew it from the water heater to see it.

Check it and replace it if it has gone bad or add one in the feed to the water heater.

Then see what happens.
 
Ghost I'm wondering if you gave up drinking and began to notice? :D We had this issue on our boat and it was there from the start. So I added a shower valve that was a constant temperature valve and all is well. Perhaps as Jim mentioned your surge tank is low on air or water logged. This would be indicated by frequent pump cycling. At least it was the case for our system. The pump was cycling very quickly and all I had to do was pump up the bladder again.
But I still like the anti-scald valve for the shower and heartily recommend the installation.
 
Negative Boatbum, I do seem to recall a 30YO Port in my recent past!!!

Thanks guys. I'll inspect for a check valve on the WH inlet, see if the pressure on the surge tank is still registering and then will probably go ahead with the TMV.
 
The pressure on the surge tank will register with water or air.
 
So how do you check it (there is a gauge on it)? Turn of the water pump, depressurize the faucets and then check to see if the gauge shows pressure?
 
The accumulator should be checked for pressure with an air pressure gauge when the water pressure at the tap is zero. What I did was turn off the dockside and water pump. Then I opened the valves and drained the system to relieve the pressure.
Next a gauge when on the accumulator valve. It's like a regular tire air valve.

Note:

If you pump it up to spec, and ours is 25 psi, you may induce hammering. I had to back off the pressure quite a bit before that stopped. It was in the instructions for the accumulator to back off until the hammering stopped. Eventually I was down to 10 or 12 psi.
 
Even if the pressure tank is not set to the correct pressure, the water pressure will be the same for hot or cold water. So I question how this could make the water temperature vary.
 
pressure may be constant but flow will take the path of least resistance. does the shower get colder or hotter when thepump cycles on?
 
No when cold water goes into the hot water tank it expands making the hot water all the time to be at higher pressure, if there is a one-way or check valve. The cold water pressure would go down due to the pump, bad surge tank, the hot would still be at a higher pressure and the shower temp. would change.
My surge tank does not have a bladder so if the water pressure is off the pressure is 0.
 
Heating the water will expand it but will not increase the pressure while someone is using hot water. It's been awhile since I worked with an accumaltor tank but if memory serves me, they are pressurized at the pressure that the water pump turns on. When the pump is on it will allow water into the tank. After the pressure builds up and the pump shuts off, the internal bladder will then push out the water. This allows for a steady flow of water and reduces the number of times the pump from cycling on and off.
 
The point is the shower has 2 sorces of water one from the hot side and one from the cold with a check valve they are really 2 seperate systems as far as pressure goes. If the cold side pressure drops the hot side won't therefore the shower water will get hotter.
I had that problem with the dock side water, if my neighbor would use his hose the shower would get real hot for a second. I replace my check valve with 10 extra feet of extra hose and the problem went away. The reason the check valve is there is to stop hot water from backing up into the main line, if it does you can get a burst of hot out of the cold side. The extra hose provided enough expansion volumn to stop that.
 
the hot usually has a check valve. That means that you have two different pressure levels in the hot and cold especially with engine heating. If there is any air in the hot side that can cause uneven flow as can any restriction in the cold side such as a partially plugged filter screen. I would check all filters and bleed the hot side . Shutting off the water flow at a shower wand as with a submarine shower will probably cause hot to back flow into cold because of the pressure difference and you will get a burst of hot.
 
This is why most people who have served aboard a Navy ship will jump out of the shower when a nearby toilet flushes. You could always count on a short blast of superheated steam if you did not get out in time.

Mike
 
The hot water may have higher static pressure because of heating but a soon as a faucet is opened, that higher pressure is relieved and the pressure is the same for hot and cold water.

The change in shower temperature when a toilet is flushed is because of pressure drop caused by long and small water lines. You won't have this in a typical boat.

The OP's problem is apparently a new problem so it's not caused by the design of the plumbing system. It has to be a failed component.
 
quote:
Originally posted by rawidman

The hot water may have higher static pressure because of heating but a soon as a faucet is opened, that higher pressure is relieved and the pressure is the same for hot and cold water.

The change in shower temperature when a toilet is flushed is because of pressure drop caused by long and small water lines. You won't have this in a typical boat.

The OP's problem is apparently a new problem so it's not caused by the design of the plumbing system. It has to be a failed component.

Click to expand...

I agree with the above. However, a major reason why a shower will become hotter when a toilet is flushed is due to the pressure drop in the cold water. Maximum water pressure will be obtained when there is no flow; the more the flow the lower the pressure. The longer the hot/cold water runs are from the input source the greater the effect.
Since the OP's is getting a fluctuation in temperature, the first thing is to try and hone in on whether it's related to the hot water or cold water side. Closing one eye and just assuming it's on the hot side may not resolve the issue and waste quite a bit of time.
 
Thanks for all the thoughts everyone!

I'm working remote from the boat today (and thinking about going somewhere and dropping the hook tonight/tomorrow), so was able to take a quick peek. There IS in fact a check valve on the cold water inlet. I think I'll swap that out if I can find one at the local hardware (Anacortes itself does not have a large store, but a decent chance I can find something. I also switched to the secondary water pump (there is a backup pump on my boat, very convenient for long cruises to who knows where, if the primary fails, swap two valves and throw an electrical switch and your water is back).

To clarify, my temp fluctuations happen with ONLY the shower running, not connected to dock water, so toilets/sinks, etc.. are not contributory to THIS particular issue. Honestly, it really makes that old check valve start to look guilty if its not opening/releasing easily.

On the way up yesterday I stopped by Lowes and looked for a thermostatic mixing valve. Mistake. The plumbing "manager" (about 19) didn't know what that would be used for but at least knew some of the faucets had one integrated. I SHOULD have just ordered one from Amazon earlier this week and so now, I'd have to drive 30 minutes one way just to find a plumbing store to get one. Since I am actually working today, I think I'll just ignore it for another week and go boating instead.

One last comment. While I have no intention of building out the differential equation to describe the effects of increased water pressure due to thermal expansion against other factors, I will go so far as to suggest that in my mind that effect will be a distant contributor ONCE the faucet is open and the water pump starts cycling. The water pump pressure cycling and components in that group of mechanisms surely have to take precedent before all others.

Again, massive thanks for the VERY helpful mindshare from EVERYONE. THANKS!!!!!
 
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