Expansion vessel or accumulator (leaky PRV)

[Tessy]

In the process of trying to figure out weather or not I have a faulty PRV on the top of my calorifier, I have realised that my expansion vessel is actually installed on the cold side before water enters the calorifier. Am I right in thinking that this makes the tank an accumulator, rather than an expansion tank for hot water? Would setting the correct pressure on this vessel make it a hot water expansion vessel, even though its on the cold side? Basically I'm trying to see if I can stop the PRV from kicking out perfectly good hot water, which it has been doing too much of lately. I have a new PRV ready to fit, but I'm not actually sure it is broken. I find my installation odd, is it not common to have a vessel fitted on the hot side? 

[Onewheeler]

If you have a non return valve between the expansion/accumulator tank and the calorifier the PRV will pass water and you may be in danger of bursting the calorifier. That's the first thing to check. Otherwise it might be a knackered PRV. Twiddling the red knob (oo-err) may help the valve reseat if there is scale there.

[Keeping Up]

19 minutes ago, Tessy said:

I find my installation odd, is it not common to have a vessel fitted on the hot side? 

Common but not universal. My installation, for example, has the non-return valve on the outlet of the calorifier, which is just as effective at preventing back-syphoning but which eliminates the need for an expansion vessel on the hor side (there is the usual accumulator in the cold supply, which effectively absorbs the expansion).

 

I agree, the first step is to play with the knob for a bit.

[nicknorman]

28 minutes ago, Tessy said:

In the process of trying to figure out weather or not I have a faulty PRV on the top of my calorifier, I have realised that my expansion vessel is actually installed on the cold side before water enters the calorifier. Am I right in thinking that this makes the tank an accumulator, rather than an expansion tank for hot water? Would setting the correct pressure on this vessel make it a hot water expansion vessel, even though its on the cold side? Basically I'm trying to see if I can stop the PRV from kicking out perfectly good hot water, which it has been doing too much of lately. I have a new PRV ready to fit, but I'm not actually sure it is broken. I find my installation odd, is it not common to have a vessel fitted on the hot side? 

It is normal to have an accumulator on the cold side, usually near the pump. It is optional to have an expansion vessel on the hot side. Without an expansion vessel on the hot side, on might expect the occasional squirt out of the PRV when the water heats up, but not a significant loss of water.

 

If you are losing significant hot water out of the PRV, either it is faulty or the pump pressure is too high.

 

Since you already have a new PRV a you might as well fit it.

 

If you want to check the pump pressure, proceed as follows.

 

First, ensure there is reasonable air pressure in the accumulator. To do this, turn off the pump and open the cold tap. Then check the air pressure in the accumulator using a car tyre type pressure gauge. It should be around the pump cut-in pressure which is typically around 15psi. Ideally it should be a couple of psi below pump cut-in pressure but It is not too critical, something around that is OK. If it is badly wrong, correct with a foot pump or by letting air out.

 

Now close the cold tap and turn on the pump. When the pump cuts out, the air pressure in the accumulator will be the same as the water pressure, so you can measure it. It should be well below the PRV and calorifier rated pressures.

Edited November 12, 2020 by nicknorman

[ditchcrawler]

You will unfortunately read loads of posts on social media saying you don't need an expansion vessel with people who are quite happy that the PRV lifts when the tank heats up. The PRV should be a safety device and not a control device. I would advise having an expansion tank and some calorifier manufactures not only recommend it but make it part of their approved installation method and without it they wouldn't need to honor their warranty as the equipment was not installed correctly. 

[Onewheeler]

24 minutes ago, ditchcrawler said:

You will unfortunately read loads of posts on social media saying you don't need an expansion vessel with people who are quite happy that the PRV lifts when the tank heats up.

A separate expansion vessel is only needed (in addition to an adequately sized accumulator) if there is a NRV on the cold feed to the calorifier. The NRV in my opinion only solves a hypothetical problem in most installations and in my opinion. I don't have one on one boat and took it out on the other. No issues with warm water from the cold tap and even if there was it would only need to be run for a few seconds.

[Tony Brooks]

6 minutes ago, Onewheeler said:

A separate expansion vessel is only needed (in addition to an adequately sized accumulator) if there is a NRV on the cold feed to the calorifier. The NRV in my opinion only solves a hypothetical problem in most installations and in my opinion. I don't have one on one boat and took it out on the other. No issues with warm water from the cold tap and even if there was it would only need to be run for a few seconds.

Pretty much the same for me except I move the NRV to the pump inlet side so when pump valves start leaking the NRV takes over and holds pressure in the system. If you have a reverse layout boat with the gally at the stern then i suspect you stand more chnace of getting hot/warm water out of the cold taps.

[Jen-in-Wellies]

1 hour ago, nicknorman said:

t is normal to have an accumulator on the cold side, usually near the pump. It is optional to have an expansion vessel on the hot side.

It is also normal for many (not all) of those cauliflowers with no expansion vessel to subsequently split. Mine did. The replacement, with expansion vessel has so far not and been in a lot longer than the original. There should be somewhere for expanding water to go as it heats, other than spiking the pressure and relying on the PRV working to prevent failure. There are several ways of arranging this as this thread shows.

Jen

[Tessy]

Can confirm that there is NO NRV between tank and calorifier. The tank is set to 15psi (pump cut in). Does this mean that it is being used as an expansion vessel when the water is hot? If so I'll need to change this PRV. New one hasn't come with any gasket of any kind, how does one typically ensure a good deal to the calorifier? As you can see from the pic, old one might need insulation dug away first. Never done this before.

[Jen-in-Wellies]

Should be able to do it without digging any more insulation away. Turn off water pump and open a tap to depressurise system. Undo the nut on the end of the copper pipe going in to the PRV and pull away the pipe. rotate the PRV anti-clockwise to remove it from the threaded stud that goes in to the cylinder. To fit a new one, wrap about a half dozen turns of PTFE tape around the threaded stud, then screw the new PRV on. This should seal it, but if it doesn't, remove and put more turns on. The copper pipe going in should seal again on the compression olive with just being nipped up with a spanner. Don't put lots of force on this. Buy the type of PRV that goes 1/2" BSP inlet to 15mm compression outlet for easiest plumbing. Close the tap, turn on the pump and check for leaks. Done!

 

Oh yes and with no non return valve, your accumulator is also acting as an expansion vessel.

Jen

Edited November 12, 2020 by Jen-in-Wellies

[nicknorman]

6 hours ago, Jen-in-Wellies said:

It is also normal for many (not all) of those cauliflowers with no expansion vessel to subsequently split. Mine did. The replacement, with expansion vessel has so far not and been in a lot longer than the original. There should be somewhere for expanding water to go as it heats, other than spiking the pressure and relying on the PRV working to prevent failure. There are several ways of arranging this as this thread shows.

Jen

I don’t disagree, but I think there are plenty of boats around with no EV. Ours included! Yes it is on my list of jobs ...

[Jen-in-Wellies]

15 minutes ago, nicknorman said:

I don’t disagree, but I think there are plenty of boats around with no EV. Ours included! Yes it is on my list of jobs ...

It was with me too. Really must get round to fitting an expansion vess- Oh dear, the cauliflower is bust. ?

It's the reason why I can get a bit preachy on the subject. ?

The end of the calorifier is nigh! Repent! Repent!

Edited November 12, 2020 by Jen-in-Wellies

[Ex Brummie]

For around £7-£8, you can get a PRV from a plumbers merchant with a pressure gauge. This will identify any re-occurrence in the future. If the valve has failed is leaking only when you heat up the water, then your problem is with the system. All the knob twirling in the world isn't worth it. Change the PRV for one with a gauge and follow on from there.

[nicknorman]

1 hour ago, Jen-in-Wellies said:

It was with me too. Really must get round to fitting an expansion vess- Oh dear, the cauliflower is bust. ?

It's the reason why I can get a bit preachy on the subject. ?

The end of the calorifier is nigh! Repent! Repent!

OK hint taken, I’ll move it up the list!

[cuthound]

On 12/11/2020 at 11:34, Jen-in-Wellies said:

It is also normal for many (not all) of those cauliflowers with no expansion vessel to subsequently split. Mine did. The replacement, with expansion vessel has so far not and been in a lot longer than the original. There should be somewhere for expanding water to go as it heats, other than spiking the pressure and relying on the PRV working to prevent failure. There are several ways of arranging this as this thread shows.

Jen

 

Luckily for me, my calorifier split just before I bought the boat, at only 6 and a half years old, 580 engine hours. The lack of expansion vessel cost the previous owner nearly £800.

 

 

On 12/11/2020 at 20:01, nicknorman said:

OK hint taken, I’ll move it up the list!

 

I would do it ASAP, maybe my boat was unlucky, but the calorifier really didn't last long without an expansion vessel before it split.

[nicknorman]

1 hour ago, cuthound said:

 

Luckily for me, my calorifier split just before I bought the boat, at only 6 and a half years old, 580 engine hours. The lack of expansion vessel cost the previous owner nearly £800.

 

 

 

I would do it ASAP, maybe my boat was unlucky, but the calorifier really didn't last long without an expansion vessel before it split.

Ours is 10 years old and nearly 4000 engine hours so the end is probably nigh...

[blackrose]

Without an EV in the system my PRV used to weep constantly. I installed the EV in the hot supply pipe from the calorifier, but it doesn't really matter which side of the calorifier the EV is installed, as long as it's downstream of the NRV. Effectively anything downstream of the NRV comprises the same hot water vessel.

 

If you don't have a NRV in the cold feed pipe then you don't need an EV, but then that only works because the calorifier must be heating upstream water in the cold feed pipe. I don't really know how energy efficient one setup is compared to the other.  Anyway, for anyone fitting an EV, the pressure should be set to the pump's cut out pressure or slightly above, with the pump switched off and taps open to relieve pressure in the system.