Calorifier

[chris w]

Why does it look sensible?

 

The TRV is a safety device pre-set to 3 Bar and should never open in normal use, if it blows it means the calorifier is over pressured!

Most people don't have an expansion vessel on their calorifier so the PRV will open every time that they heat up the water in the calorifier due to expansion. This is what it is meant to do - it's a waste of hot water, but that's life without an expansion vessel.

 

NRV's ................... should not be needed anyway!

 

Another oft-quoted reason for having an NRV is to prevent hot water running back up the inlet pipe, mixing with the cooler water and promoting bacterial growth.

 

A common mistake that you have made is to adjust this pressure and assume the volume when the system is pressurised

No I haven't actually. I always set the expansion vessel pressure and the accumulator vessel pressure with the system unpressurised.

[Guest TerryL]

name='chris w' date='Nov 1 2008, 07:57 PM' post='291288'

Most people don't have an expansion vessel on their calorifier so the PRV will open every time that they heat up the water in the calorifier due to expansion. This is what it is meant to do - it's a waste of hot water, but that's life without an expansion vessel.

No that's what you think it is meant to do and it's unneccessary and eventually ruins the valve, they can stick, then what? If the accumulator is sufficiently sized it will do the same job as the expansion vessel and in fact the excess pressure is only present during heating up and until a tap is opened.

 

Another oft-quoted reason for having an NRV is to prevent hot water running back up the inlet pipe, mixing with the cooler water and promoting bacterial growth.

I can't see that happening with separate pipe runs and it won't run back with an expansion vessel fitted locally.

 

No I haven't actually. I always set the expansion vessel pressure and the accumulator vessel pressure with the system unpressurised.

I'm very glad to hear that!

[chris w]

No that's what you think it is meant to do and it's unneccessary and eventually ruins the valve, they can stick, then what? If the accumulator is sufficiently sized it will do the same job as the expansion vessel and in fact the excess pressure is only present during heating up and until a tap is opened.

 

You seem to miss the point that:

 

(i) the vast majority of boaters (based on historical posts here) do NOT have an expansion vessel.

 

(ii) they do not have a pump accumulator of appropriate size to act as an expansion vessel. A typical pump accumulator will be about 2 litres, while a typical expansion vessel will be about 8 litres. These two vessels differ considerably in size and most people will not want to fit an 8 litre expansion tank under the cupboard adjacent to the pump.

 

(iii) they DO have an NRV and therefore one cannot use the accumulator as an expansion vessel for the calorifier.

 

Any or all of these will mean that the PRV will continually dribble when the calorifier is heated to accomodate expansion. There have been innumerable posts on here previously, with boaters asking why their PRV's "leak". The reason is (apart from one or two that leak when cold, due to a fault) is that the PRV is doing its job based on normal expansion of the calorifier's contents.

 

The water pressure once a tap is opened (and then closed again) will instantaneously be the pump's cut-out pressure (~30psi typically). Assuming the heating is still on, then the pressure will increase once more until the PRV or the expansion vessel (if fitted) takes over.

[Guest TerryL]

You seem to miss the point that:

 

(i) the vast majority of boaters (based on historical posts here) do NOT have an expansion vessel.

 

(ii) they do not have a pump accumulator of appropriate size to act as an expansion vessel. A typical pump accumulator will be about 2 litres, while a typical expansion vessel will be about 8 litres. These two vessels differ considerably in size and most people will not want to fit an 8 litre expansion tank under the cupboard adjacent to the pump.

 

(iii) they DO have an NRV and therefore one cannot use the accumulator as an expansion vessel for the calorifier.

 

Any or all of these will mean that the PRV will continually dribble when the calorifier is heated to accomodate expansion. There have been innumerable posts on here previously, with boaters asking why their PRV's "leak". The reason is (apart from one or two that leak when cold, due to a fault) is that the PRV is doing its job based on normal expansion of the calorifier's contents.

 

The water pressure once a tap is opened (and then closed again) will instantaneously be the pump's cut-out pressure (~30psi typically). Assuming the heating is still on, then the pressure will increase once more until the PRV or the expansion vessel (if fitted) takes over.

 

And you miss my point, it is not necessary to have a substandard and problematic system for the sake of a £25 8ltr expansion vessel that can be fitted anywhere convenient. In fact a larger expansion vessel will transform the water performance considerably.

[chris w]

And you miss my point, it is not necessary to have a substandard and problematic system for the sake of a £25 8ltr expansion vessel that can be fitted anywhere convenient. In fact a larger expansion vessel will transform the water performance considerably.

 

I'm not arguing that it CAN'T be done nor even that it shouldn't be done, just that most people WON'T do it as they are either not bothered or, more likely, they wouldn't know HOW to do it without paying an outsider.... and most of those would be baffled based on my experiences of marina "engineers".

[Guest TerryL]

I'm not arguing that it CAN'T be done nor even that it shouldn't be done, just that most people WON'T do it as they are either not bothered or, more likely, they wouldn't know HOW to do it without paying an outsider.... and most of those would be baffled based on my experiences of marina "engineers".

Why not use a plumber then?

[chris w]

Why not use a plumber then?

Plumbers and heating engineers are usually reluctant to work on a boat because they are not certain of the conditions under which the system will be used and boats are not covered by the usual domestic regs. Yet of course their work still has to be warrantied. Not worth the hassle to most guys. Also, many plumbers are not qualified to touch oil-based heating systems.

 

Further, many boaters won't pay the kind of money that good plumbers want today. Plumbers are in demand and can charge £300 per day. To understand the system, part drain the system, fit an expansion vessel, refill and commission the system plus travelling time may work out £150 labour + parts.

Edited November 1, 2008 by chris w

[Guest TerryL]

Plumbers and heating engineers are usually reluctant to work on a boat because they are not certain of the conditions under which the system will be used and boats are not covered by the usual domestic regs. Yet of course their work still has to be warrantied. Not worth the hassle to most guys. Also, many plumbers are not qualified to touch oil-based heating systems.

So for someone like me who does both marine and domestic heating, plumbing and electrics, there could be quite a bit of potential business out there, even just advising?

[chris w]

So for someone like me who does both marine and domestic heating, plumbing and electrics, there could be quite a bit of potential business out there, even just advising?

You may need the money. or was that a sneaky way of getting a commercial advert on the forum?

Edited November 1, 2008 by chris w

[Guest TerryL]

You may need the money. or was that a sneaky way of getting a commercial advert on the forum?

 

Well I might need the money, there's always some b*****d wanting to take if off me!

 

I was just wondering how people manage if they can't get these jobs done. It must be a nightmare judging by the threads on here.

[ChrisPy]

earwig go again !!

[Batavia]

Why does it look sensible?

My "sensible" comment referred to the basis of Chris P's (too many people called Chris here) calculation, not the adequacy of the size of the accumulator he had used in his calculations for the required duty.

 

In simple pressure systems like this, the PRV should not lift in normal operation - it is there as a safety device - as there is a significant probablity that it will not reseat properly.

 

The only answer is a larger vessel - unless the elasticity of the calorifier itself provides a significantly increased volume (which I would doubt for most boat calorifiers).

 

Chris

[Smelly]

Isn't this rather clouding Troll's question?

 

Isn't it inevitable, accepting that it's the tank and not just the insulation, that his PRV has stuck closed?

[chris w]

In simple pressure systems like this, the PRV should not lift in normal operation - it is there as a safety device - as there is a significant probablity that it will not reseat properly.

 

The only answer is a larger vessel - unless the elasticity of the calorifier itself provides a significantly increased volume (which I would doubt for most boat calorifiers).

 

Chris

 

If there is no expansion vessel fitted then the PRV will open everytime the water heats up. It's the only place for the water to go when it expands. Having a larger vessel won't make any difference as that will just fill up full too and the same situation will then obtain.

 

Chris

[Troll]

Isn't this rather clouding Troll's question?

 

Isn't it inevitable, accepting that it's the tank and not just the insulation, that his PRV has stuck closed?

 

 

Well I have to admit this has all gone way over my head, and I still dont really know why my calorifier has swollen a little either side of its straps.

 

I fitted it myself though, I marked everything very carefully and its working fine now, just interesting to see what the experts think

[RLWP]

If there is no expansion vessel fitted then the PRV will open everytime the water heats up. It's the only place for the water to go when it expands. Having a larger vessel won't make any difference as that will just fill up full too and the same situation will then obtain.

 

Chris

 

And, as predicted, we don't have and never have had an expansion vessel. And every time we turn the water pump on it pressurises the calorifier. And everytime it heats up the expanded water has nowhere to go because of the valves in the pump and the taps being shut. So eventually the PRV opens and spills a few CCs of water into the bilge. Everytime.

 

Fortunately the pressure is quite low so the calorifier doesn't swell up like a balloon. I wonder why the OPs did that? I wonder what pressure that would take? I wonder how much water that would take too? I wonder what would happen when he opened the hot tap the first time? I wonder if this could have been in any way due to the expansion of a few litres of water through a few degrees c? I haven't done the sums but it seems unlikely.

 

Richard

[chris w]

Water is incompressible (to a first approximation) so if it expands at all, even having been heated a small amount, it has to go some place. It can't be "squashed" into the original space. To be pedantic the commpressibility of water is actually 0.005%; pretty close to nothing

 

Chris

[RLWP]

Water is incompressible (to a first approximation) so if it expands at all, even having been heated a small amount, it has to go some place. It can't be "squashed" into the original space. To be pedantic the commpressibility of water is actually 0.005%; pretty close to nothing

 

Chris

 

True. As I'm being too idle to do the sums, what's the change in volume then for the temperature increase you might expect in a calorifier? Volume is about 30 litres, temperature from 5 to 60 degrees c?

 

Ah, from this site, about 2% So 2% on the diameter? Doesn't sound like ballooning.

 

 

Richard

 

Oh dear, then we have to do a shaky calculation about how much volume causes "ballooning" in a calorifer. How big a balloon? Zeppelin, Montgolfier, Blimp, Condom?

Edited November 3, 2008 by RLWP

[chris w]

The usual figure to use for a calorifier to be safe is 4% expansion. The total expansion of water from freezing to boiling is 4.3%

 

Chris

Edited November 3, 2008 by chris w

[ChrisPy]

The usual figure to use for a calorifier to be safe is 4% expansion. The total expansion of water from freezing to boiling is 4.3%

 

Chris

just found a wonderful reference on a website selling expansion vessels to the building trade. Expansion is quoted as 0.032%, whereas what they meant was 3.2% or an expansion factor of 0.032. .... thus endangering anyone who uses takes the reference seriously. Probably written by a marketing executive after rubbishing the engineer's draft text.

Edited November 4, 2008 by ChrisPy

[Batavia]

And, as predicted, we don't have and never have had an expansion vessel. And every time we turn the water pump on it pressurises the calorifier. And everytime it heats up the expanded water has nowhere to go because of the valves in the pump and the taps being shut. So eventually the PRV opens and spills a few CCs of water into the bilge. Everytime.

 

Richard

 

There seems to be a mismatch between the "few ccs" through the TRV described here and the litre or so which should occur according to the calculations. How is this difference accounted for? I would suggest one of the mechanisms I listed in a previous post.

 

How thick is the wall of a typical calorifier? Knowing this, we can work out the expansion of the calorifier itself (purely out of interest).

 

Also, I would re-assert that in a properly-designed system, the expansion vessel should be able to handle the expansion without the TRV lifting - from what has been said, it just means that the ones people typically fit are too small.

 

Chris G

[ChrisPy]

I suspect many systems do not have a NRV, so the cold system accumulator also acts as an expansion vessel. which helps ........ or not!

[Batavia]

I suspect many systems do not have a NRV, so the cold system accumulator also acts as an expansion vessel. which helps ........ or not!

Especially if somone opens a tap while the calorifier is heating up!

[Guest TerryL]

Especially if somone opens a tap while the calorifier is heating up!

The accumulator needs to be of a reasonable capacity to prevent the pump from firing up everytime water is dawn off and to give a good smooth flow, so without a NRV in the line it would be quite capable of absorbing the calorifier expansion while heating up. Common sense should tell people not to take the cold feed from the calorifier cold inlet anyway but to take it directly from the pump accumulator. However you are unlikely to get hot water reversing out of a vertical or horizontal calorifier in practice because the bottom part is always unheated and cold due to stratification. This is another carefully thought out myth easily overcome by common sense and good plumbing practice.

Edited November 4, 2008 by TerryL

[mikevye]

We had calorifier problems out of warranty and bought a bare shell with twin coils from surecal who recommended an 8 litre accumalator fitted on the HW outlet. I thought at the time it was well over the top for a 65 litre tank but after reading the forum comments I now realise its not. Buying the bare shell was half the price of a retail unit and allowed for the fitting of Hep2o fittings straight onto the coil pipes instead of compression fittings.