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Old 07-08-2018, 09:15 PM   #1
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Freshwater pump pulses only on hot?

The Jabsco pump in my Eastbay died, so I replaced it with the same model. Just unplugged the old hoses and wires and put in a new one. No other changes to the freshwater system at all.

When running cold water taps the flow is steady and the pump makes a consistent noise. When running hot (or any portion of warm utilizing hot) the pump seems to pulse instead of pumping steady. The flow itself is only slightly interrupted, it doesn't chatter at the spigot. But I can definitely hear the pump going on/off at about a half-second interval. There's also some dip in the 12v system, revealed by the lights dipping in time with the pump pulsations. This happens at any of the galley and head taps or shower heads. I did not try the hot/cold washdown at the transom.

I couldn't diagnose what failed in the old pump as the screws attaching the wet end (with the impeller/diaphragm in it) were stripped. Trying to drill them out fails as the housing plastic starts melting.

Since the problem only occurs when using hot water, I'm wondering if there's potentially a restriction or something that might be causing a pressure problem?

Anyone else run into a similar kind of problem? And found a fix?
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Old 07-08-2018, 09:59 PM   #2
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I had a similar problem on an old boat of mine. No matter what there are always more losses/restrictions in the hot water loop by essence of having the hot water heater. Check the hoses leading to/from for possible kinks.

On mine it was not kinks, and the water heater was just a normal domestic unit, but I solved the problem by adding an accumulator tank which cut down on the pump cycling tremendously. I don't remember the size exactly, but it was just a normal Home Depot-type unit, with air bladder, but it was big. Like 15 gallons or so.

Start with tracing the lines and hoses first though.
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Old 07-09-2018, 07:05 AM   #3
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Do you have a tempering valve (anti scalding) on the output of the water heater?

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Old 07-09-2018, 07:18 AM   #4
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Factory plumbing diagrams showed an accumulator, but none was present. Previous owners made a change, I'd guess. There were some other mounting holes adjacent to the pump.

Odd thing is nothing at all has been moved to changed with relation to the rest of the plumbing. I only moved the 1' or so lengths of the hoses leading to the pump.
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Old 07-09-2018, 07:27 AM   #5
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try opening all hot taps and let them run to vent any air in the tank and lines
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Old 07-09-2018, 08:31 AM   #6
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Quote:
Originally Posted by wkearney99 View Post
Factory plumbing diagrams showed an accumulator, but none was present. Previous owners made a change, I'd guess. There were some other mounting holes adjacent to the pump.

Odd thing is nothing at all has been moved to changed with relation to the rest of the plumbing. I only moved the 1' or so lengths of the hoses leading to the pump.
If there isn't an accumulator tank, is the pump one of the variable speed type? I'm not well versed on variable speed pumps, but would assume that air pockets (maybe in the water heater) might cause pulsing. If there is a check valve on the inlet to the water heater, that would probably prevent pulsing when only using the cold water if the air pocket was in the water heater.

Ted
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Old 07-09-2018, 10:26 AM   #7
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I had a variable speed pump in a previous boat. I never got it to work to my satisfaction, it made a lot of noise and never really got a good even flow so I returned it and put in a new pump and a tank. Worked well and the noise from the variable speed pump was gone.
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Old 07-09-2018, 06:56 PM   #8
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Do you have a tempering valve (anti scalding) on the output of the water heater?
I don't know. I'll have to check. I have noticed that the hot water comes out pretty darned hot. It's heating only via electric, no engine loop. The schematic makes no mention of a tempering valve.
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Old 07-09-2018, 07:01 PM   #9
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If there isn't an accumulator tank, is the pump one of the variable speed type? I'm not well versed on variable speed pumps, but would assume that air pockets (maybe in the water heater) might cause pulsing. If there is a check valve on the inlet to the water heater, that would probably prevent pulsing when only using the cold water if the air pocket was in the water heater.
It's a Jabsco Par Max Plus 82500-0092. It may or may not be the correct pump for the situation. I was faced with loss of water and two days time before a week long trip. So I replaced what was in there. Hopefully not a $200 mistake, but I'm prepared to eat that if I should use something else instead. Likewise, I'll add an accumulator if that'd help.

How would I eliminate air pockets? We ran the water a fair bit for meals and showers. The pulsing didn't change. What's the most direct route to purging a pocket from the water heater tank?
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Old 07-09-2018, 07:32 PM   #10
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Quote:
Originally Posted by wkearney99 View Post
It's a Jabsco Par Max Plus 82500-0092. It may or may not be the correct pump for the situation. I was faced with loss of water and two days time before a week long trip. So I replaced what was in there. Hopefully not a $200 mistake, but I'm prepared to eat that if I should use something else instead. Likewise, I'll add an accumulator if that'd help.

How would I eliminate air pockets? We ran the water a fair bit for meals and showers. The pulsing didn't change. What's the most direct route to purging a pocket from the water heater tank?
A picture of the water heater would help. Is the water heater outlet on the side or top? If it's on the side, is the safety value above it? If so, the safety valve would likely have a toggle lever that you can lift to vent air with. While you're looking, see if there are any plugs on the top of the water heater.

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Old 07-09-2018, 08:33 PM   #11
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Yes, a picture would help. I have hundreds taken on the boat thus far and have not found the ones I took of the water heater! I'll be back to the boat in a few days. With luck the safety valve will be toward the top.
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Old 07-10-2018, 05:12 AM   #12
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"I have noticed that the hot water comes out pretty darned hot. It's heating only via electric, no engine loop. The schematic makes no mention of a tempering valve."


They expect you to select an output temperature with the thermostat.
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Old 07-10-2018, 05:30 AM   #13
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Quote:
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"I have noticed that the hot water comes out pretty darned hot. It's heating only via electric, no engine loop. The schematic makes no mention of a tempering valve."


They expect you to select an output temperature with the thermostat.
Agree w FF...
Pull the cover on the WH
Usually there is an adjustable Tstat
It may have temp markings or not but try turning it down a little.
On previous boat I added an accumulator... actually a home water heater expansion tank... only about 1 gal cap'y but it smoothed out pump a lot.
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