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Thermal Syphoning


Ray
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As some will know, we continue to have problems with our twin coil calorifier failing to retain hot water overnight despite the water being piping hot when we go to bed and often after cruising continuously for at least twelve hours during the day.

 

The engine thermostat is set to 71 degrees.

 

As far as I know we have done all we can to stop what I understand is called thermal syphoning. We have a non return valve on the calorifier feed from the engine and the other (upper) coil is connected to the Alde boiler via an electrically operated valve that shuts that circuit down when the Alde isn't required to heat the calorifier. We've added a thermostat to the calorifier to remote control the Alde and the associated valve.

 

There's a pressure reducing valve on the cold water input to the calorifier in case this has any bearing on why the water won't stay hot i.e. if the water pressure in the supply is too great and is forcing the (cold) water to over-mix with the hot in the cylinder and thus cool it down.

 

I'm about to purchase a cylinder jacket in the hope that the heat loss is through the calorifier's normal insulation but before I do I wonder if someone could enlighten me how thermal syphoning works/happens?

 

My understanding is that the hot water in the calorifier will cause water to flow around the coils once the main heat source, external to the cylinder is withdrawn. If we didn't have an NRV the water movement in the coil circuit would effectively dump the heat from the calorifier to either the engine or the skin tank. However, as hot air/water rises, once the temperature of the water in the lower part of the calorifier - that around and below the lower coil - has been cooled by the syphoning, doesn't the coil water movement cease as there is nothing to cause the water to circulate? Surely this would leave the heated water in the top half of the cylinder untouched and available in the morning? Or have I missed something?

 

Does this prove that, if the NRV has stuck open, we must therefore be losing heat through the cylinder's insulation rather than through syphoning as we still ought to have about half a cylinder of hot water in the morning?

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Will do, although by morning the water is normally so cool that its temperature is not much above the ambient air temperature so checking pipework as you suggest is difficult but we'll have another go next time we're out cruising.

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none return valve on the cold supply in curred my problems

 

Thanks. That could be worth trying - I have a spare. But have you any idea why it solved the problem?

Edited by Ray
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If it's only lukewarm by the morning, then do what WotEver suggests, but sooner - after your engine should have cooled, but before all your hot water has gone walkies, (say a couple hours after you stop engine ?).

 

If either the feed or return from the engine circuit to the calorifier feels warm any significant distance from thee calorifier, then there is a good chance your calorifier is still trying to heat your engine at the expense of cooling itself.

 

I would have thought that most commercial calorifiers have probably enough insulation that adding more would not make a massive difference to heat loss, (but I could be wrong).

 

I still think an engine stat of only 71 degrees is a lot of your problem.

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If it's only lukewarm by the morning, then do what WotEver suggests, but sooner - after your engine should have cooled, but before all your hot water has gone walkies, (say a couple hours after you stop engine ?).

 

If either the feed or return from the engine circuit to the calorifier feels warm any significant distance from thee calorifier, then there is a good chance your calorifier is still trying to heat your engine at the expense of cooling itself.

 

I would have thought that most commercial calorifiers have probably enough insulation that adding more would not make a massive difference to heat loss, (but I could be wrong).

 

I still think an engine stat of only 71 degrees is a lot of your problem.

Our cylinder gets to 80 deg and we both shower in the morning some 14-15 hours after the engine is stopped.

It is possible that the NRV is in the system that it opens with the flow of water thermosyphoning as well as the direction of the pumped water. The only answer is as posted earlier,check all pipe work after say 4 hours,

The NRV in the cold supply will stop hot water entering the cold system every time you open a cold water tap prior to the pump starting, at which point you pump cool water back into the cylinder.

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If it's only lukewarm by the morning, then do what WotEver suggests, but sooner - after your engine should have cooled, but before all your hot water has gone walkies, (say a couple hours after you stop engine ?).

 

Never really thought of that and will give it a try. I presume that I'll be able to tell whether the engine/pipework heat is the residue from the day's cruising - I don't know how long it takes for the engine etc. to cool down naturally.

 

If either the feed or return from the engine circuit to the calorifier feels warm any significant distance from the calorifier, then there is a good chance your calorifier is still trying to heat your engine at the expense of cooling itself.

 

I would have thought that most commercial calorifiers have probably enough insulation that adding more would not make a massive difference to heat loss, (but I could be wrong).

 

I don't doubt what you're saying. However, a colleague reported that he'd added a jacket to his home (insulated) cylinder and noticed a distinct difference in the heat retention period. Surely, the abundance of cylinder jackets for sale at retail outlets would tend to support this thinking.

 

I still think an engine stat of only 71 degrees is a lot of your problem.

 

You wouldn't believe the trouble that we've had trying to locate a different rated thermostat. If we had a (vehicle) registration number it would be easy but because we don't we've not been able to locate one of the right size. I have to admit after three or four weeks trying, we put the original back.

 

Snip

 

It is possible that the NRV is in the system that it opens with the flow of water thermosyphoning as well as the direction of the pumped water. The only answer is as posted earlier,check all pipe work after say 4 hours.

 

Someone suggested that before but I wouldn't know how to stop it doing so if it is. Does anyone have any ideas?

 

The NRV in the cold supply will stop hot water entering the cold system every time you open a cold water tap prior to the pump starting, at which point you pump cool water back into the cylinder.

 

I haven't thought of that although, at present, I don't have an expansion vessel on the cylinder's outlet so would need to fit one as well as the NRV or have the PRV dripping.

 

As mentioned originally, the Alde can also heat the top half of the cylinder from the top coil (and as we tend to stay on the boat on its mooring quite a lot, is the way we quite often heat the water). The (domestic) cylinder thermostat (that fires the Alde) is set to a little over 50 degrees, which we find acceptable. I haven't monitored it too closely but I wonder how often the thermostat should fire the Alde as a result of heat loss if there's no water usage in the interim.

 

I'm sure that I've seen it said that heat loss is around 1 degree per hour. If that is so, then we should expect the (engine heated) hot water to stay hotter than the thermostat trip temperature for somewhere around twenty hours. Even at a three degree loss per hour that would give us about seven to eight hours worth of hot water.

 

In the meantime, back to my original post. Can anyone explain why thermal syphoning doesn't stop once the temperature of the water in and around the bottom coil of the calorifier is less than the temperature of the water above the coil. I thought hot water rises (clear of the coil) and there's nothing above the coil to cool that water down except minimal loss through the cylinder walls.

Edited by Ray
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Have you checked the pressure relief valve is not dribbling water? PRVs when worn will act as an expansion vessel and open slightly and dribble, this loss is made up with cold water. Does your pump cut in for no apparent reason? if so check or replace the PRV.

 

Colin

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As far as I know we have done all we can to stop what I understand is called thermal syphoning.

AIUI the correct expression for this mechanism/effect is 'thermo-syphon'. Many people in the plumbing and heating industry call it a thermal syphon but for me, that grates!

 

We have a non return valve on the calorifier feed from the engine and the other (upper) coil is connected to the Alde boiler via an electrically operated valve that shuts that circuit down when the Alde isn't required to heat the calorifier. We've added a thermostat to the calorifier to remote control the Alde and the associated valve.

 

Provided both valves are closing fully I'm reasonably confident your hot water loss will not be the result of thermo-syphoning. The motive force for a thermo-syphon is very tiny and even if the flow direction is the same as your non-return valve it is unlikely to be strong enough to overcome the hairspring which holds the NRV closed.

 

There's a pressure reducing valve on the cold water input to the calorifier in case this has any bearing on why the water won't stay hot i.e. if the water pressure in the supply is too great and is forcing the (cold) water to over-mix with the hot in the cylinder and thus cool it down.

 

No. The PRV is there only to protect the calorifier from bursting from being over-pressured in the event of, say, water pump pressure switch failure.

 

I'm about to purchase a cylinder jacket in the hope that the heat loss is through the calorifier's normal insulation but before I do I wonder if someone could enlighten me how thermal syphoning works/happens?

 

I'm inclined to think this will prove a waste of money. The amount of heat energy being lost to cool the tank from arounf 70C to 30C is enormous. I think a toally uninsulated tank might achieve this rate of heat loss but not one with factory insulation already on it.

 

 

My understanding is that the hot water in the calorifier will cause water to flow around the coils once the main heat source, external to the cylinder is withdrawn. If we didn't have an NRV the water movement in the coil circuit would effectively dump the heat from the calorifier to either the engine or the skin tank.

 

Sort of. It all depends on the relative heights of the calorifier, engine and skin tank. The thermo-syphon effect causes to all parts of the system to tend towards the same temperature at a given height. The hottest water accumulates at the top and the coolest at the bottom. So if the top of your calorifier is lower than the top of skin tank then yes I'd expect the skin tank to be capable of draining the heat energy from your calorifier and dumping it out through the swim wall. Same for the engine.

 

However, as hot air/water rises, once the temperature of the water in the lower part of the calorifier - that around and below the lower coil - has been cooled by the syphoning, doesn't the coil water movement cease as there is nothing to cause the water to circulate? Surely this would leave the heated water in the top half of the cylinder untouched and available in the morning? Or have I missed something?

 

Does this prove that, if the NRV has stuck open, we must therefore be losing heat through the cylinder's insulation rather than through syphoning as we still ought to have about half a cylinder of hot water in the morning?

Your thinking is correct but only if the top of the calorifier is higher than the top of the swim. The water in the tank will stay hot only at levels higher than the swim.

 

I suggest fitting a pair of manual (ballofix) isolating valves in the pipes connecting your calorifier to the engine. Easy to do and useful for all sorts of reasons. Then you can close them and definitively rule out thermo-syphoning through the swim. I doubt any thermo-syphoning around the Alde would be capable of dumping that amount of heat energy.

 

Another possibilty as Big COL suggests is that your PRV is letting by. Even a tiny dribble will drain the tank of hot water in a few hours. Does your water pump run briefly every twenty mins or so even if no-one has used a tap?

 

MTBM

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Vertical:

boat80.jpg

 

Tony

 

Somebody's been looking at my web site!

 

The PRV that I'm referring to is a Pressure Reduction Valve in the cold water supply, turned as low as I can to minimise the pressure of the cold water supply entering the cylinder.

 

The Pressure Release Valve on top of the cylinder doesn't drip at all. There's no Expansion Vessel so me thinks that the expansion is taking place back along the cold water inlet side of things (using the accumulator as an effective expansion vessel). The pump seldom cuts in when we're not running water - it has once or twice, but we've eventually put that down to the water in the cylinder cooling and contracting and dragging cold water back in when the accumulator is almost empty, causing the pump to cut in briefly to recharge to accumulator.

 

The cabin bilge is dry and there's no evidence of any leaks.

 

I'm planning to fit a non return valve on the cold water supply over the next few days and add an expansion vessel to the hot water output at the same time.

 

As can be seen from the picture above, the top of the calorifier is higher than the top of the engine and the top of the swim.

 

The calorifier is connected directly to the engine, not via the skin tank, the top of which is probably about the same height as the engine connections to the calorifier.

 

Is there anyone else on here with a Beta 38/43 and a SureCal twin coil vertical 55 litre vertical calorifier and do they have similar problems?

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I had some work done on my engine cooling system and then the domestic system started losing heat overnight. I've discovered that they accidentally reversed the flow and return to the calorifier coil. Next step is to swap them over and see if that helps. Could that be your problem too?

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As can be seen from the picture above, the top of the calorifier is higher than the top of the engine and the top of the swim.

 

Its not the height of the top of the calorifier that matters, its the height of the cooling watercoil at the calorifier end compared with the engine.

 

Looking at your pic the height of the engine and the height of the calorifier coil look to be much the same, and though I can't see it in the pic, presumably the skin tank is also at something like the same level. So if the engine and/or skin tank is cold (and the skin tank will cool down very quickly after the engine is stopped) and there is hot water in the bottom half of the calorifier, then if the pipework layout (and engine water pump, thermostat, any valves etc) allows it, you can get warm water in the calorifier coil rising, moving across to the top of the engine/skin tank, down through the engine/skin tank, giving up heat on the way, then back across to the bottom connection on the calorifier. That would chill the bottom half of the calorifier tank pretty quickly, giving much less chance of keeping the rest hot until morning.

 

David

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Its not the height of the top of the calorifier that matters, its the height of the cooling watercoil at the calorifier end compared with the engine.

 

Looking at your pic the height of the engine and the height of the calorifier coil look to be much the same, and though I can't see it in the pic, presumably the skin tank is also at something like the same level. So if the engine and/or skin tank is cold (and the skin tank will cool down very quickly after the engine is stopped) and there is hot water in the bottom half of the calorifier, then if the pipework layout (and engine water pump, thermostat, any valves etc) allows it, you can get warm water in the calorifier coil rising, moving across to the top of the engine/skin tank, down through the engine/skin tank, giving up heat on the way, then back across to the bottom connection on the calorifier. That would chill the bottom half of the calorifier tank pretty quickly, giving much less chance of keeping the rest hot until morning.

 

David

David

 

Thank you for that explanation. I hadn't cottoned on to the fact that it was the height of the coil, rather than the top of the tank that made the difference although thinking about it now it is pretty obvious.

 

The question now is, can I do anything to stop this happening as the NRV doesn't appear to?

 

I think that I've seen something about routeing the pipework to the floor and back up again fairly close to the cylinder. Is that a solution? Is it the feed or the return that needs to be modified?

 

Can you also confirm whether the feed from the engine goes to the upper or lower coil connection on the calorifier - the connections are one above the other?

 

Thanks.

 

Ray

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The question now is, can I do anything to stop this happening as the NRV doesn't appear to?

A mechanical solution could be to put a solenoid valve, powered from the engine electrics, in the engine's calorifier circuit.

 

However if there's a simple fix with re-routing pipes then you won't need my sledgehammer for this particular nut.

 

Tony

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It may be early days yet, but we managed to get out on the boat yesterday.

 

We got back around 4pm and had the engine off within thirty minutes - a long story about a failing pump out point!

 

Admittedly there was only one of us on board last night, so we probably drew off a little less hot water than normal but when we

checked around 7.30am this morning the hot water was still warm enough for the Alde boiler Idriven by the cylinder thermostat) not to fire up when I turned it on.

 

All we've done is to:

 

1. Add a non return valve in the cold water feed to the calorifier as suggested by cloggy above.

2. Fit an expansion vessel to the hot water output from the calorifier

3. Adjust the mixer on top of the calorifier so that we draw a litttle less hot (and some cold) when we open taps, rather than draw neat hot water all the time and add the cold at the taps.

 

In fairness we have also adjusted the thermostat on the cylinder (which might affect when the Alde fires) but the water still seemed comfortably warm when I ran some off.

 

I suspect that we may still need to tweak everything but we may be winning.

 

Thanks for all your suggestions.

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the flow should go to the top of the coil in your cylinder and return from the bottom this stops thermo syphonic action being caused by the water in the cylinder

a non return valve should be fitted as close as possible to the cylinder on the flow pipe and a zone valve (solenoid) should be immediately after the cylinder. this in effect keeps the small amount of water in the coil stationary. link the zone valve to the ignition circuit that way it closes automatically when you stop the engine

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