Hot water/calorifer expansion

[grama]

I have a fresh water cooled marine engine with a domestic hot water system (electric pump and accumulator)............ When the engine heats up the domestic hot water and it
thereby expands......if you don't relieve the pressure (by using the taps)
where does the extra volume/pressure go? I know where mine goes because the pressure
relief valve operates and I have some water in the bilges by the calorifier. But
this can't be right. I don't want leaking water every time the engine runs. I did in fact fit a short
length of pipe from the relief valve to a collector bottle last year......as a tell-tale, but as
a temporary measure to save the water collecting in unwanted places. What sensible solution is there to this?

 

[Giblets]

Not too familiar with marine hot water calorifiers but should there not be an expansion vessel somewhere to take up the water expansion? If there is then it sound like it's kaput as the pressure relief valve is a safety device for over-pressure situations only not for coping with the expansion on a regular basis.

 

[Fr J Hackett]

I have a fresh water cooled marine engine with a domestic hot water system (electric pump and accumulator)............ When the engine heats up the domestic hot water and it
thereby expands......if you don't relieve the pressure (by using the taps)
where does the extra volume/pressure go? I know where mine goes because the pressure
relief valve operates and I have some water in the bilges by the calorifier. But
this can't be right. I don't want leaking water every time the engine runs. I did in fact fit a short
length of pipe from the relief valve to a collector bottle last year......as a tell-tale, but as
a temporary measure to save the water collecting in unwanted places. What sensible solution is there to this?

What you are doing is common practice

 

[Skysail]

Several possibilities:

replace the Pressure Relief Valve, they do sometimes fail. First try turning the knurled knob to see if this displaces any debris in the valve.

Fit an expansion tank - see Cleghorn Waring's website for advice.

Take an exhaust pipe overboard through the hull.

Continue with the exhaust bottle!

All these have been successfully applied by many Moody owners where the exhaust water problem seems to be a 'feature' of the marque.

 

[grama]

common Problem...

There is , I feel , no problem with the pressure relief valve. Its doing exactly what its meant to do. Its just there has to be somewhere for the heated water to expand to........being a pumped hot water system (with accumulator) it'll already be up to "pressure" before the engine has warmed up. I've spoken to Pat Manley (PBO Tech) and he has a collector bottle which he empties from time to time. Do everybody do this ????

 

[Strathglass]

I have a fresh water cooled marine engine with a domestic hot water system (electric pump and accumulator)............ When the engine heats up the domestic hot water and it
thereby expands......if you don't relieve the pressure (by using the taps)
where does the extra volume/pressure go? I know where mine goes because the pressure
relief valve operates and I have some water in the bilges by the calorifier. But
this can't be right. I don't want leaking water every time the engine runs. I did in fact fit a short
length of pipe from the relief valve to a collector bottle last year......as a tell-tale, but as
a temporary measure to save the water collecting in unwanted places. What sensible solution is there to this?

I have two expansion chambers in my freshwater system one is near the pump in the cold water side. This gives some expansion capability and helps limit the pump from hunting by adding some hysterisis to the system.

The cold water feed to my calorifier is fed through a non return valve and there is a further expansion chamber after this valve. Without this each time the water heats up there would be some water expelled by the safety pressure release valve on the calorifier due to the expansion of the water. This is what you are experiencing.

If you add one or two expansion chambers you will get rid of the problem.

Iain

 

[PetiteFleur]

I just take the pipe into an old plastic milk bottle jammed in the bilges. I used to empty it regularly but now don't bother until layup time. Never gets more than a quarter full.

 

[sarabande]

you might try fiddling with the pressure relief valve 'tap' when the system is cool. Sometimes debris can get trapped between the seal and the seat, and under increased (hot water) pressure there's a leak.

At least it shows that the valve is working. I had a calorifier where it jammed, and when the pressure became too much, it used to blow the hot water inlet pipe off the spigot, and dump all the lovely hot water in the bilge. A new valve cured the problem.

 

[fireball]

and dump all the lovely hot water in the bilge.

The thermocouples went on the last boat immersion heater ... pressure relief valve opened and the pump emptied the tank (and the cold water tank) into the bilge .... loverly clean bilges!!

 

[Cliveshep]

I have a fresh water cooled marine engine with a domestic hot water system (electric pump and accumulator)............ When the engine heats up the domestic hot water and it
thereby expands......if you don't relieve the pressure (by using the taps)
where does the extra volume/pressure go? I know where mine goes because the pressure
relief valve operates and I have some water in the bilges by the calorifier. But
this can't be right. I don't want leaking water every time the engine runs. I did in fact fit a short
length of pipe from the relief valve to a collector bottle last year......as a tell-tale, but as
a temporary measure to save the water collecting in unwanted places. What sensible solution is there to this?

Take yourself off to a plumbers/heating merchant (they are far cheaper than chandleries) and purchase a small expansion bottle such as would be used on a combi boiler. As a guide, about 200mm long x 125mm diameter will serve most small boat systems. It should ideally be stainless. Ensure that the cold feed to the calorifier has a loop in in to prevent a thermal syphon back from the calorifier but do not fit a non-return valve as the loop takes care of that issue. Then tee into the outlet side of the freshwater pump, which I assume has a pressure switch, and install the pot with one of those flexi pipes sold in plumber's mercahnts, homebase etc, you want the one with the swivel and fibre washer at one end, the other end connecting to the tee. The swivel connects the pot (expansion vessel) which you clamp to any convenient structure but with the bicycle valve at the top.

They are usually over-pressured, so let some air out with the pump on but at rest, (i.e. the pressure switch is satisfied and the pump is not actually running) until the pump kicks in. Once the pump stops let out a little more air until it starts again. What you are trying to achieve is the internal diaphragm squeezed up about 2/3rds of its way inside the pot although you won't see that of course. If you over-do letting the air out pump it up with a foot pump or a quick jab at the local garage. This will then accomodate expansion pressure created by the heating calorifier.

As a bonus, you'll find the pump won't cycle half as much with an expansion vessel in the line.

 

[Anthony]

Ensure that the cold feed to the calorifier has a loop in in to prevent a thermal syphon back from the calorifier

Hi, sorry I didnt quite understand your comment about the loop; surley the feed to the calorifier is always under pressure from the cold water pressure system so how can you have a syphone break? I am guessing I have misunderstood somthing...

Ants

 

[MoodySabre]

Run a hose from the PRV to a skin fitting above the water line. I used a plastic fitting on the transom because my calorifier is at the aft end of the cockpit locker.

 

[grama]

Latest....

Right, I've changed the pump pressure switch from 40psi to 25psi.......and kept the Jabsco 1litre accumulator........and added a Cleghorn 2 litre expansion tank. The accumulator precharge is 10psi, and the expansion tank set to 25psi.........all complies with Cleghorns recommendations.........and the calorifier still shoves out a cup of water when the engine warms up !!! Today I vented the water pressure (opened the taps) with the waterpump off and set off with zero pressure.........and lo and behold no evidence of water from the overpressure valve...........I'm thinking now that either the valve is faulty, or there's some debris making it release prematurely..........that I must check.

 

[Bilgediver]

I have a fresh water cooled marine engine with a domestic hot water system (electric pump and accumulator)............ When the engine heats up the domestic hot water and it
thereby expands......if you don't relieve the pressure (by using the taps)
where does the extra volume/pressure go? I know where mine goes because the pressure
relief valve operates and I have some water in the bilges by the calorifier. But
this can't be right. I don't want leaking water every time the engine runs. I did in fact fit a short
length of pipe from the relief valve to a collector bottle last year......as a tell-tale, but as
a temporary measure to save the water collecting in unwanted places. What sensible solution is there to this?

This might be a common experience however fitting an accumulator in the calorifier will stop this being necessary...I cannot understand why peeps are reluctant to fit them and instead wear out PT valves and fill bilges losing water in the process.

 

[Cantata]

Piping it overboard as per MoodySabre's post is the simplest, cheapest and foolproof solution, and keeps your bilges dry. If you observe you are losing a lot of water, you need a new PRV, about 12 quid from a heating suppliers. Take the old one with you as there are different connection regimes on different valves.