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The Norfolk Broads Forum / Broads Boat Owners Q & A / New calorifier fitted, your help needed.
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Posted By Discussion Topic: New calorifier fitted, your help needed.

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Regulo
Jun-07-2012 @ 9:42 PM                           Permalink
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Hi, all,

I have fitted a new calorifier tank (Surecal 55 litre) on the boat, which comes pre-fitted with a temperature mixing valve. Lashings of lovely hot water. But that's the problem I now have. When I first open a hot tap, my water system pump doesn't run for a good 15-20 seconds, so there's no cold water being supplied to the mixer valve. Result: scalding hot water from the tap! Once the pump cuts in the temperature is controlled as expected. What is the cause of this? When I had my old tank with a separate temp mixer valve, this didn't happen, so what's different now? Surecal mention an expansion bottle to relieve pressure should be fitted to the hot water take-off, but I never had one before, and I can't see how relieving the pressure on the hot would affect my pump not cutting in, as I've got a non-return valve fitted on the cold inlet to the tank. I don't know what my pump operating pressure is set to, should it be higher than now to cut in quicker? I'm a bit baffled on this, so over to you, panel!

Regards, Ray.

If it's neither here, nor there, then where is it?

ditchcrawler
Jun-08-2012 @ 8:19 PM                           Permalink
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When the water heats in your calorifier the pressure also rises, it can't feed back to the cold water system due to the NR valve. So when you turn on the hot tap the hot water is fed by the built up pressur in the calorifier and the pump doesn't cut in until the cold water presuer and the hot drops low enough for the pump to cut in.

londonrascal
Jun-09-2012 @ 1:26 AM                           Permalink
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Ray, this is about as non-technical as one can get - but as a stop gap before a solution can be found - does turning on the cold tap first to get the pump going mean the hot is not soo hot when it comes out the tap (I don't mean a mixer tap) but the fact that the water pump would be running to begin and not have to cut in after a few seconds?



| Robin |


Antares_9
Jun-09-2012 @ 10:07 AM                           Permalink
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Ditchcrawler has it right, in order for a mixer valve to work efficiently (at least the inexpensive ones) the hot and cold pressure must be fairly closely balanced, the higher pressure present in the calorifier needs to be reduced to equal the pump pressure, or the pump pressure increased. As the manufacturer suggests, a pressure vessel mounted after the calorifier should make this possible assuming a decent pressure from the delivery pump, in fact the pressure vessel could be fitted before the calorifier provided it was downstream of any NRVs. They are easy to adjust with a decent bike pump to allow the required balance. Water pressure gauges are very cheap and easily fitted with a tee if you want to get really technical, I use them to balance wet heating systems quite a lot. I do find it a bit odd however that it is taking so long to equalise and would expect only about five seconds of lag unless the cold pressure is really low. Its also worth a mention that the calorifier pressure can't be that high or it would be emptying into the bilge via the PRV.

East Coast Marine Heating & Electronics

This message was edited by Antares_9 on Jun-9-12 @ 10:12 AM

Siddy
Jun-09-2012 @ 9:59 PM                           Permalink
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Ray which way is the cold flow to mixer valve. If it's above the hot and your make hot at too a temp it can produce air and backing up the cold pipe which could hold your pump off. Check if air is coming out of the tap. Even try crack a joint (pipe joint lol) on the cold to see if there's air.


Siddy

Regulo
Jun-11-2012 @ 11:03 AM                           Permalink
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Hi, all,

Sorry for the delay in replying, I've been away over the weekend.

Thanks for all your thoughts and advice, I've been in deep thought over this, and you are all correct (as expected!) in your answers. The problem lies in the combination of the mixer valve being factory fitted directly on the tank take off, and the so-called cold feed (also factory fitted) being taken from the bottom cold inlet, which is combined with the PRV. Further back towards the pump lies the non-return valve. As the water heats up and expands it pressurises the tank and all associated pipework "downstream" of the non-return valve. It also heats up that pipework through convection such that even the cold feed to the mixer valve is at tank temperature. Thus, when a hot tap is opened, scalding hot water issues until the pressure falls until the pump cuts in, and even then the pump has to clear all the hot water "backed up" in the cold feed beyond the non-return valve before the mixer valve can do it's job. It takes about half a basin of water before I get water at the temperature I've set on the mixer valve.

Now, having said all that, if I fit an expansion tank, I'll still have a delay in temperature stabilisation, due to the "cold" feed to the valve being taken from the tank inlet. I have decided to set the tank mixer at it's maximum, and return to my old arrangement of having a remote mixer valve, fed with cold from the pump side of the non-return valve, so it is truly cold.

Does all that make any sense? If it does, I must have failed in my attempts to bore and baffle you all to unconciousness! Smile

Any further thoughts welcome.

Regards, Ray.

If it's neither here, nor there, then where is it?

Antares_9
Jun-11-2012 @ 11:29 AM                           Permalink
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Roy, if, and only if  Wink  I have understood you correctly, the pressure present on the hot side will still need to be balanced for the remote mixer to operate correctly.

East Coast Marine Heating & Electronics

Regulo
Jun-11-2012 @ 1:21 PM                           Permalink
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Hi, Antares_9,

Agreed, an expansion bottle will be fitted. On my previous set-up, I didn't have an expansion bottle, I fitted an adjustable PRV in the hot water take-off pipe, set to just over the pump cut-off pressure. I suppose I could refit that as an alternative to an expansion bottle. Although this did result in a loss of vented hot water on expansion, I was happy knowing the pressure in the hot tank and pipework couldn't rise to anywhere near a dangerous level. And the mixer worked fine, with only an initial burst of very hot water before stabilising.

I'm at a bit of a loss as to exactly how an expansion tank actually works. If it's an enclosed vessel, surely it simply pressurises the air inside? How does that reduce the water pressure?

Regards, Ray.

If it's neither here, nor there, then where is it?

Antares_9
Jun-11-2012 @ 2:16 PM                           Permalink
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It has a membrane with air on the other side of the water. Usually adjustable via a schrader valve and bike pump. A lot more useful to stop pump cycling and smooth out flow but as the space for the hot water is not fixed it has a little room for expansion without a pressure increase provided the size is large enough. I use them in unvented wet heating systems for just that reason.

East Coast Marine Heating & Electronics

This message was edited by Antares_9 on Jun-11-12 @ 2:17 PM


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