relief / water expansion from calorifier

[clyst]

At present the relief valve discharges into the GRP tray under the engine sump and hence requires frequent emptying .

My thoughts are to discharge the relief water overboard via a skin fitting. The problem I can see is a slight possibility of sea water cross contamination. To over come this a small "header tank" for the relief valve hose to discharge into and then drain overboard could be fitted. My problem is since the calorifier is fitted approx on the waterline the header tank would have to be sited a couple of ft above WL but would the relief be strong enough to reach the tank ?? I know its a bit like "how long is a bit of string" but any ideas ?? I quess I could just rig a poly hose to the expected height and see what happens .

Does anyone's relief discharge overboard ?

 

[nigelmercier]

Does anyone's relief discharge overboard ?

Only after a few beers!

As far as I know, mine has no such outlet.

 

[theoldsalt]

The back pressure on the PRV due to a header tank being a couple of feet above it is very small in comparison with the pressure inside the calorifier that is being relieved. (2 feet of head produces approximately 1psi - ie a half of 1psi per foot of head)

The domestic water pressure is usually at about 20psi so the PRV will be set above this, otherwise it will constantly blow off each time the water pump cuts in. So maybe the PRV should be set to 25 or 30psi and should rarely blow off.

However why go to all the bother of a header tank and an overflow through the hull? if the PRV is leaking it either needs replacing or is set to too low a pressure. I suggest you buy a new PRV for a pressure above your domestic water presssure (see on the pump). They are readily available for a few pounds. To cater for the then rare blow off add a length of pipe into a drinks bottle to keep your bilge dry. My bottle gets emptied about twice a year.

 

[CharlesSwallow]

The back pressure on the PRV due to a header tank being a couple of feet above it is very small in comparison with the pressure inside the calorifier that is being relieved. (2 feet of head produces approximately 1psi - ie a half of 1psi per foot of head)

The domestic water pressure is usually at about 20psi so the PRV will be set above this, otherwise it will constantly blow off each time the water pump cuts in. So maybe the PRV is set to 25 or 30psi.

However why go to all the bother of a header tank and an overflow through the hull? if the PRV is leaking it either needs replacing or is set to too low a pressure. I suggest you buy a new PRV for a pressure above your domestic water presssure (see on the pump). They are readily available for a few pounds. To cater for the then rare blow off add a length of pipe into a drinks bottle to keep your bilge dry. My bottle gets emptied about twice a year.

I entirely agree with this but would offer a further suggestion. It is my experience that the valve can get furred up in hard water situations and the seat becomes compromised to the extent that it is practically leaking all the time. Does your pressure pump run for short periods when no water is being drawn off? Either remove it and clear it in a pot of limescale remover or as has already been suggested, replace it.

Chas

 

[Robin]

Some pressure systems are set at much lower pressures I think. Our last boat the relief valve would drop water into the bilge when we first bought it so I went the tonic bottle route initially as a solution. When that became a PITA to remember to empty I put the hose into the top of the adjacent gas locker from which it drained overboard, intending later to install a skin fitting. Ten years on I sold the boat and it still drained into the gas locker.

 

[clyst]

Correct me if I'm wrong but since the water system is a closed pressurised set up isnt the relief valve there to relieve the pressure caused by water expanding when heated ??

 

[theoldsalt]

CharlesSwallow,

Yes thank you for adding that suggeston.

It does sometimes work. Most PRVs can be rotated by hand. That causes them to open momentarily and the blow-off clear any build-up on the seating. However that build-up can have already have damaged the seating of the valve. Also just the fact that the valve is dribbling can erode the seat much like a dripping tap at home that won't shut off even after a new seal is fitted due to a damaged seat.


Clyst,

It is still worth a try and if not succesful, replace the PRV.

 

[johnphilip]

Sheer relief

Correct me if I'm wrong but since the water system is a closed pressurised set up isnt the relief valve there to relieve the pressure caused by water expanding when heated ??

I am sure you are right, As water cannot be compressed like air its expansion in a fixed volume can exert huge pressure, hence the PRV. Why not rig a discharge pipe through the cockpit seat near the cockpit drain?

 

[theoldsalt]

Clyst,

Correct me if I'm wrong but since the water system is a closed pressurised set up isnt the relief valve there to relieve the pressure caused by water expanding when heated ??

That is true but the system is not perfectly rigid. There is usually a small amount of air trapped in the calorifier and the hoses can expand with increases in pressure. This combined with the PRV relief pressure being several psi higher than the pump pressure means that the PRV only blows off very occasionally.

If system has very little build it flexibility and the PRV still blows off too frequently then this can be overcome by fitting an accumulator to allow for some expansion. This has the added benefit of smoothing out the flow of water from the pump.

I do not have an accumulator fitted and as previously stated the PRV rarely blows off

 

[CelebrityScandel]

On all our boats over the years we have always had the calorifier relief valve discharge side just piped back to a freshwater tank; after all, that is where the water came from in the first place. Also, if the water is heated from the engine or a generator so the calorifier is in use while cruising away from power supplies then the potential loss of any potable water is a concern .

A tank vent is a convenient place to Tee into and the pipe can be clear unreinforced plastic so you can see if it is leaking (although there will always be at least the sign of the fresh water pump cycling too when no other demand on it) and it will stand the small quantity of heat energy in the dribble of any leak.

 

[theoldsalt]

CelebrityScandel,

What an interesting alternative solution and worthy of noting.

(On a similar vain - I have always wondered why toilet cistern overflows are not fed into the bowl instead of outside the house where they discolour the wall and freeze in winter)

 

[GrahamM376]

Does anyone's relief discharge overboard ?

Yes, our does. Piped it to a skin fitting as it discharged a small amount into the bilges every time the water heated up. Put a swan neck in the pipe and, because the calorifier is pressurised, no back contamination. I the PRV were to fail, we would hear the pump running.

 

[westhinder]

Does anyone's relief discharge overboard ?

On my new ( to me ) boat the discharge is piped to a skin fitting. I think there is no swan neck or one way valve in the pipe but I will have to check next time I'm aboard.

 

[ghostlymoron]

CelebrityScandel,

What an interesting alternative solution and worthy of noting.

(On a similar vain - I have always wondered why toilet cistern overflows are not fed into the bowl instead of outside the house where they discolour the wall and freeze in winter)

I've installed 2 new toilets recently and the overflow is via the internal siphon so no external overflow required.

 

[nedmin]

Why not fasten a small bore pipe to the PRV and let it drain out the engine room breather vents.

 

[Phil.C]

At present the relief valve discharges into the GRP tray under the engine sump and hence requires frequent emptying .

My thoughts are to discharge the relief water overboard via a skin fitting. The problem I can see is a slight possibility of sea water cross contamination. To over come this a small "header tank" for the relief valve hose to discharge into and then drain overboard could be fitted. My problem is since the calorifier is fitted approx on the waterline the header tank would have to be sited a couple of ft above WL but would the relief be strong enough to reach the tank ?? I know its a bit like "how long is a bit of string" but any ideas ?? I quess I could just rig a poly hose to the expected height and see what happens .

Does anyone's relief discharge overboard ?

Had the same problem with mine, removed PRV from calorifier and piped to potable expansion vessel with PRV teed off between. expansion vessel needs to be at least 1/20th of the volume of the calorifier. Works fine without any loss of water!

 

[BlueChip]

At present the relief valve discharges into the GRP tray under the engine sump and hence requires frequent emptying .

My thoughts are to discharge the relief water overboard via a skin fitting. The problem I can see is a slight possibility of sea water cross contamination. To over come this a small "header tank" for the relief valve hose to discharge into and then drain overboard could be fitted. My problem is since the calorifier is fitted approx on the waterline the header tank would have to be sited a couple of ft above WL but would the relief be strong enough to reach the tank ?? I know its a bit like "how long is a bit of string" but any ideas ?? I quess I could just rig a poly hose to the expected height and see what happens .

Does anyone's relief discharge overboard ?

Mine just vents into a gallon poly bottle in the engine bay.
It only needs emptying every month or so, so for me it wouldn't be worth the extra plumbing to fit skin fitting.
If you are losing a lot of water it's probable that your PRV is rated too low for your system and is venting too early or it's rated only slightly above the pressure switch on your water pump.

 

[MoodySabre]

Moodys discharge water into the bilges. I have put a hose on my PRV and taken it to a plastic skinfitting on the transom.

 

[KellysEye]

>Mine just vents into a gallon poly bottle in the engine bay.


We use a hose from the vent to an old conditoner container.

 

[nigelmercier]

CelebrityScandel,

(On a similar vain - I have always wondered why toilet cistern overflows are not fed into the bowl instead of outside the house where they discolour the wall and freeze in winter)

There was (perhaps still is) a regulation in the water bye-laws that states the overflow must be clearly visible. I think some modern cisterns can be made to overflow into the bowl, one of mine is coupled this way.