expansion tank?

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expansion tank?

Postby duetto » Tue Jan 10, 2017 8:58 pm

hi all,

we have a model year 2003. to the best of my knowledge we don't have an expansion tank in our fw system. it appears that when we run the hot water heater we get a dripping from the pressure relief valve on the hot water heater. when the temp drops the dripping stops. we have an aftermarket isotemp water heater. the consensus on the dock is that the water heater is raising the pressure above 110 psi (the pressure relief valve open setting).

i know the question everyone will ask is has it always been this way. i have to say i can't honestly say. we've had ongoing propshaft leaking problems. we had these fixed 2 years ago but the isotemp we have had bad pressure relief valves and so more dripped water. we just upgraded to the newer pressure relief valves and the dripping question.

on a higher level, how does the pdq avoid the water expansion/dripping issue without an expansion tank?

all ideas welcome.
john & diane cummings
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Re: expansion tank?

Postby thinwater » Wed Jan 11, 2017 1:41 am

Can't say about your boat, but the PDQ 32 and 36 sail versions that I have been on have expansion tanks. When water gets hot it expands, and if it can't go back through the pump (check valve) you can get dangerous over pressure. I would never engineer a hot water system of this type with out one, and that is one of the things I used to do in my day job (refinery engineer).

Adding one should be quite simple. Do it. Don't wait.

I wonder if it failed and someone didn't replace it?
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Re: expansion tank?

Postby duetto » Wed Jan 11, 2017 7:59 am

thanks for the thoughts. it'll be interesting to see if ours is the only one without it.
john & diane cummings

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Re: expansion tank?

Postby James Power » Wed Jan 11, 2017 9:01 am

At the factory we did not install expansion tanks on the boats. The early ones had a simple pressure pump ( that could rapidly cycle depending on flow rate) and the later boats had a variable speed pump that in theory negated an expansion tank. As mentioned the the pressure relief valve will vent as the water heats, as there is a check valve in the cold suppy line at the heater. Installing an accumulator tank on the hot line seems to solve triggering the valve. I would recommend installing a tank on the cold side also.
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Re: expansion tank?

Postby thinwater » Fri Jan 13, 2017 2:26 pm

Why would you need them on both sides (you do not)? There is no check valve between the two sides.

I have a single tank and serves both sides equally. I always assumed it was factory, based on the installation and location. Apparently not.

----

For the record, installing a hot water heater in a system with a check valve on the feed side and no expansion tanks is a serious code violation. Always has been on land (100 years?), though I'm not sure about boats. I've designed many thermal systems.
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Re: expansion tank?

Postby Gusto » Fri Jan 13, 2017 9:01 pm

We inserted the hose extending from the water heater pressure relief valve into a gallon jug which differentiates the source of bilge water from the heater or somewhere else. Now I know exactly how much comes from the water heater. My latest fresh water leak came from the transom shower hose thru transom step fitting. Tasting the water is a must if your boat is sitting in salt water.

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Re: expansion tank?

Postby duetto » Sat Jan 14, 2017 9:11 am

hi,

i have setup the exact same hose/jug catcher but haven't seen any accumulation of water yet. since the discharge is a function of water heater / engine run cycles, do you have any idea how many cycles will fill the jug?

my plan is to fit an expansion tank once we return to florida.

heh, dick t, this maybe a tech topic for the owner's meeting.
john & diane cummings

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Re: expansion tank?

Postby duetto » Wed Feb 01, 2017 6:10 pm

ok, here is what i've learned. with a 6 gal water heater the water will expand approx 2-3% depending on the rise in water temperature. this is working out to approx 16 oz of engine water per heat cycle.

i have found that whale makes a ss expansion tank which has an internal membrane and a capacity of 68 oz. is my problem as simple as fitting an expansion tank which has a capacity > the heat cycle volume increase?
john & diane cummings

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Re: expansion tank?

Postby James Power » Sat Feb 04, 2017 9:14 am

I've installed a few expansion tanks of that size on the hot side and found that the pressure valve can occasionally weep. A 2 gallon unit would work better.
On a side note, the factory installed check valves to alleviate heating the water in the cold side, and also to protect the heater element from running dry in case of a water leak in the system.

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Re: expansion tank?

Postby duetto » Sat Feb 04, 2017 5:41 pm

james,

thanks for the input. the thing that attracted me to the whale is the ss tank but i'll hunt around for options.
john & diane cummings

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Re: expansion tank?

Postby Flyer » Sun Feb 05, 2017 8:02 am

Our 2003 PDQ has an on-demand water pump with no expansion tank. I have had hot water excaping from the relief valve and thought it was either a defective valve or perhaps the wrong thermostat. Perhaps an expansion tank is the solution. Where should I put it to allow for the expansion of the hot water?

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Re: expansion tank?

Postby duetto » Sun Feb 05, 2017 8:42 am

james made a point in an earlier response in this thread that pdq fitted check valves on the cold side going into the tank. if this is the case on your boat, it needs to be on the hot side. if there is no check valve it can go anywhere beyond the water pump. i'm thinking of putting it behind the medicine cabinet since we have space there. i will do in spring when we get back to fl or fall before next season. i will still keep the hose on the relief valve and overflow jug.
john & diane cummings

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Re: expansion tank?

Postby thinwater » Sun Feb 05, 2017 12:18 pm

As an 35-year engineer that has designed installed many industrial hot water systems, I can't help but feel you are making excuses for half-fixes on nice boats. Install an expansion tank. It will be safer, stress on the piping will be reduced (you are going over pressure--that is what is causing the valve to open!), equipment will last longer, and the pump will cycle less. It will be better for the water heater.

Just stop dodging the the problem with half solutions and fix it properly. My PDQ has an expansion tank and it works very well. Yes, you can get away without one, but it is not good for it.
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Re: expansion tank?

Postby duetto » Sun Feb 05, 2017 4:13 pm

thinwater,

i'm convinced! what size/capacity tank do you have on your boat?
john & diane cummings

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Re: expansion tank?

Postby AlanH » Sun Feb 05, 2017 5:09 pm

I have a 2006 with no expansion tank. We also do not have a check valve on the cold water line feeding the Hot Water Heater. We have not had any problems with the relief valve releasing water. Unless you have had the problem since the boat was new I would look at the most likely cause of the problem.

I spent seven years as a plumber about 40 years ago. The first thing I would suggest with a weeping relief valve is replacing the valve. They do eventually wear out which can cause them to dribble. Keep in mind these are temperature and pressure relief valves. The temperature is generally set to go off at 210 degrees. If your port engine is running warm, it may be a part of the problem. The pressure part of the relief valve is generally set for a pressure of 100-150 psi in marine applications. There may be a tag on your valve that tells the settings. Ours is set at 150 psi. The hot water heater is a Force 10 and it’s specs say max rating for a relief valve on it is 150 psi. Again, I would start with replacing the valve. Try to replace it with a similarly rated valve. If yours is a 150 psi you can probably just get one at Home Depot.

If you are still having the problem, an expansion tank is worth considering.

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