Vokera compact 24 heat exchanger gone - again

kevkdg · 24 Dec 2021

Thanks @Gasguru and @DP.

Both the boiler and water meter were already at the property when I moved in back in 2007. Boiler was installed in 2005, and maybe at that time there was no water meter installed hence no mini expansion vessel etc. Even so, the first heat exchanger lasted well considering as it didn't fail until 2018.

So, do I fit BOTH a Pressure Reducing Valve and a Mini Expansion Vessel? I assume if I need both the mini expansion vessel sits between the boiler and the Pressure Reducing Valve right. Or do I just need a Mini Expansion Vessel? The boiler is rated to take 6 Bar max on the DHW inlet.

I hope that damage is not instant as it will be next week now before I can either do it myself or get a plumber in.

vulcancontinental · 24 Dec 2021

The advice above is correct and parts of Leeds can be 'blessed' with water pressure that exceeds 12 bar I am told when industry shuts down overnight.

As mentioned the maximum pressure DHW side is 6 bar so personally I'd fit both pressure limiting and expansion vessel. That would also be the case if you replaced the whole boiler as most are warrantied to 8 bar now which would still not be enough. Brass components are often recommended by installers in the area as they can take higher pressures but the O rings can't so even some brass containing boilers are only warrantied to 6 bar.

kevkdg · 27 Dec 2021

Thanks @vulcancontinental

I find pushfit easy for DIY installing in confined spaces and have used JG SPeedfit in the past. Only concerns might be around earth continuity of pipework.

I was thinking of the following:
Reliance Valves Predator Compact 312 PRV 15mm x 15mm | Pressure Reducing Valves | Screwfix.com

Couple with a "shock Arrestor"/"Mini Expansion Vessel" - There seem to be two types of these, ones that you inflate and ones that are more like tubes e.g. Sioux Chief DW660-H Water Hammer Arrestor ¾" BSP Connection | Hammer Arresters | Screwfix.com.

I was looking at the below, BUT it doesn't specify it is suitable for potable water?

Ideal Heating Expansion Vessel Kit 0.16Ltr | Flues & Boiler Accessories | Screwfix.com

I can't find a JG speedfit tee for 2 x 15mm and 1 x 1/2" female connector as you can buy just the mini expansion vessels on their own such as:

Mini Expansion Vessel Shock Arrester - 11355 (bes.co.uk)

Any tips would be great.

I was planning on fitting in the boiler cupboard, but it is quite tight. Looking at cutting a section of the piping out for the cold mains feed to the DHW input on the boiler. Cleaning it up and trying to get BOTH the Pressure Reducing Valve and Mini Expansion vessel in there. Might have to put the PRV on the vertical section and the Mini Expansion Vessel on the horizontal section just below the black flow control knob.

Gasguru · 27 Dec 2021

The 312 pressure reducing valve is also available with speedfit connections. You should connect it downstream of your incomming stopcock to ensure the cold tap pressure is also reduced. You want balanced pressures at the taps/mixers.
Go for the proper mini expansion vessel.
AFAIR the plastic blanking plugs on the PRV are a 1/8 BSP female thread...get a rear connection gauge to screw in so you can set an appropriate pressure.

kevkdg · 27 Dec 2021

OK, thanks. So the Pressure Reducing Valve can/should be installed just after my main stop cock? I do wonder though what the effect might be on showering etc? Would it be beneficial to take a pressure reading first (overnight when it's likely at its highest) before going to all the trouble as I assume if my mains pressure is OK and not too high it won't be needed. The problem I've had with heat exchangers might actually be down to the fact I have no mini expansion vessel fitted as I do have a water meter. I do sometimes recall the upstairs shower would let off some water randomly for a few seconds when turned off, I now understand the potential reason why and that it is probably not imply siphoning but more than likely expanded pressure in the pipes.

I was thinking of fitting this near the boiler:

Altecnic Shock arrester c/w tee and gauge - PVRSK (macdonaldplumbingsupplies.com)

Gasguru · 27 Dec 2021

You could plumb that in first and then see what pressures you're getting...but turn the stopcock back on slowly so as not to damage the gauge in case you have more than 6 bar.
No shower should need more than a few bar pressure.

kevkdg · 28 Dec 2021

Thanks. I must admit I get confused between flow and pressure. The boiler has a black knob which acts as a stop cock, but apparently turning this down reduces the flow but not the pressure.... can't get my head round that, as if you turn it down to a dribble then surely you have affected the pressure and flow?

DP · 28 Dec 2021

kevkdg said:
Thanks. I must admit I get confused between flow and pressure. The boiler has a black knob which acts as a stop cock, but apparently turning this down reduces the flow but not the pressure.... can't get my head round that, as if you turn it down to a dribble then surely you have affected the pressure and flow?

That knob is adjusted at commissioning stage to deliver, when fully opened, a set flow at the tap. An adjustment collar within becomes a stop when preset position is reached to arrest the valve opening for the flowrate specific for that model

kevkdg · 30 Dec 2021

Tested the mains pressure today at both mine and my neighbours outside taps. They were both the same at 5 bar. The boiler DHW side is ok to a max of 6 bar. I assume then I only really need the mini expansion vessel.

Gasguru · 30 Dec 2021

Although the pressure is 5 bar it could exceed 6 bar when taps are shut (especially 1/4 turn) and washing machine/dishwasher solenoid valves slam shut due to inertia.
The pressure will also increase due to heating expansion despite the mini vessel constraining it somewhat.
I would fit the pressure reducing valve and set it to say 3 bar.

kevkdg · 31 Dec 2021

Ok, thanks. I can't access the area around the mains stop cock to fit a pressure reducing valve there as suggested by @Gasguru , I am also concerned that if I fitted it just after the stop cock it would affect for example my garden hose etc. The piping for which is all behind a kitchen cabinet.

I could however fit it just before the cold DHW feed to the boiler along with the mini expansion vessel (I assume in the following order on the cold feed: Pressure Reducing Valve > Mini Expansion Vessel > Boiler DHW Cold Input).

Another concern would be dropping the pressure to say 3 bar (as suggested) and how that might impact the hot water supply to the shower which is in an en-suite in the loft conversion, so 2 floors higher than the boiler as I assume pressure reduces with height/distance?

Gasguru · 31 Dec 2021

Each floor up is a 0.25 bar drop in pressure.
Ideally you want similar pressures at the hot and cold for each tap/mixer, just regulating the cold feed to the boiler gives an unbalance. You also need to consider whether the unregulated cold could presurise the hot (defeating the whole point of the PRV) and that can happen through a mixer.
Cut out the backpanel and re-jig the hose if necessary.

vulcancontinental · 31 Dec 2021

If you live in an area with a lot of industry drawing water during the day you need to check when industry is shut down.

kevkdg · 2 Jan 2022

Thanks everyone. Just tested the pressure at just after midnight and still 5 bar. Got someone to run a tap and shut it off quickly and the pressure quickly spiked at 7 bars before the gauge settled back to 5 bar a few seconds later. I assume it is these spikes that a mini expansion vessel absorbs.

kevkdg · 2 Jan 2022

I am thinking the simplest thing would be to remove the compression fitting used for the central heating filling loop on the mains cold supply to the boilers DHW inlet as circled in red in the photo. Replace this with a 15mm to 15mm Tee that also has a 1/2 female connector for the mini expansion vessel. Then a small length of 15mm copper pipe coming out the tee and re-instate the filling loop connector. I think this is easiest and from what I can ascertain there the mini expansion vessel doesn't have to be in the line of water flow, it just needs to absorb the temporary expansion (spike in pressure). From what I read you inflate the mini expansion vessel to your water pressure plus half a bar so that it only comes into effect when the pressure rises over half a bar more than your designated pressure.

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Vokera compact 24 heat exchanger gone - again

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