UVC safety question

[Gone West]

I’m looking at how to connect up the UVC which is part of the Genvex Combi I’ve just bought. The UVC is heated with an EASHP which will heat the water to a maximum of 55C. It has a 1kW immersion to increase the temperature if required. Genvex say the maximum tank temperature should not exceed 65C. Reading up on safety devices on the internet it says there should be three independent ones. It looks like the Combi has two manual reset thermal cut outs with 80C stamped on them. Does that mean I would need just a pressure relief valve to have a safe system?

 

 

The top part is an anode, under that two reset buttons and at the bottom the immersion.

 

[Gone West]

@Nickfromwales whoops, sorry should I have put this under 'Boilers and hot water tanks'?

[Nickfromwales]

@PeterStarck

If it's unvented then does it not already have a temperature & pressure relief valve factory fitted ?

[Gone West]

1 hour ago, Nickfromwales said:

@PeterStarck

If it's unvented then does it not already have a temperature & pressure relief valve factory fitted ?

There's no mention of a temperature & pressure relief valve in the Genvex documentation and it's not marked on their diagrams. The UVC is in the bottom part of the unit and is completely boxed in except for the front. The only other thing on the front of the tank is a brass plug near the top of the tank.

[Nickfromwales]

28 minutes ago, PeterStarck said:

There's no mention of a temperature & pressure relief valve in the Genvex documentation and it's not marked on their diagrams. The UVC is in the bottom part of the unit and is completely boxed in except for the front. The only other thing on the front of the tank is a brass plug near the top of the tank.

Do you have a link to the installation / service guides?

 

edit to add : 

Does the unit have any discharge / overflow pipework / connections?

[Onoff]

Page 5 of the install manual says the supply pipe must be fitted with a pressure reduction valve:

 

https://www.google.co.uk/url?sa=t&source=web&rct=j&url=http://www.genvex.dk/en/products/combinated/combi-185-bp/&ved=0ahUKEwiHtaag9PXWAhWEORoKHf9kB5IQFghGMAo&usg=AOvVaw15g1a-4AjOv14v6hhfd60d

 

And page 7 shows a 6 Bar safety valve on the diagram...

Edited October 16, 2017 by Onoff

[Gone West]

12 hours ago, Nickfromwales said:

Do you have a link to the installation / service guides?

 

edit to add : 

Does the unit have any discharge / overflow pipework / connections?

Clive has linked to the installation guide that I've got. The tank doesn't have any overflow pipework. I've installed a pressure reducing valve set to 3 bar on the cold supply inlet. I think the 6 bar safety valve referred to is the expansion vessel.

[dpmiller]

An expansion vessel is NOT equivalent to a safety valve. What happens when the bladder fails?

 

Me, I'd be removing that brass bung and getting  at least a safety valve but better a T&PRV in there.

[Gone West]

Just now, dpmiller said:

An expansion vessel is NOT equivalent to a safety valve. What happens when the bladder fails?

 

Me, I'd be removing that brass bung and getting  at least a safety valve but better a T&PRV in there.

I'm not thinking an expansion vessel is the same as a safety valve. When you say remove the brass bung and replace with a T&PRV how could you run the discharge pipework. When the front cover is on there is no room and the only exit for pipework is under the unit towards the rear.

[Nickfromwales]

@PeterStarck

The 6 bar reference is your need for a backup should the PRedV fail . 

Basically on any 'control group' aka 'multiblock' you have your primary PRedV and then directly after it you have your failsafe PRV set at 5-6 bar dependant on the UVC's maker plate / MI's where it'll state max operating pressure before failure eg "3 bar working / 10 bar max" etc.

6 bar is a good average so go with that.

Clearly you have no space inside the unit so we're into remote mounted controls which need to be directly adjacent to the 'UVC'. 

Do you have any additional gubbings which were manufacturer supplied, eg to construct said control group?

 

Also, your EV will connect to the control group  

[Nickfromwales]

1 hour ago, dpmiller said:

An expansion vessel is NOT equivalent to a safety valve. What happens when the bladder fails?

 

Me, I'd be removing that brass bung and getting  at least a safety valve but better a T&PRV in there.

We need to be 100% sure that there isn't already one on it / in it, which needs terminating.

@PeterStarck, best to make a quick phone call to technical to confirm / deny the above . 

[Gone West]

14 minutes ago, Nickfromwales said:

We need to be 100% sure that there isn't already one on it / in it, which needs terminating.

@PeterStarck, best to make a quick phone call to technical to confirm / deny the above . 

It is manufactured in Denmark and I bought it direct from Denmark. Before buying it I spoke to the UK distributors but as I didn't buy it from them I can't see them being helpful. I can try Genvex direct in Denmark. I was thinking that as it has two manual reset thermal cut out switches all I would need possibly is a remote pressure relief valve connected to the DHW outlet? It didn't come with any extra fittings. Maximum pressure is 12 bar, operating pressure is 3 bar.

Edited October 17, 2017 by PeterStarck

[Nickfromwales]

It's an unvented water heater fitted into a uk home so it's G3 all the way I'm afraid.

This doesn't allow for interpretation, you need to follow it to the letter. BCO will want in on this for your completion cert, even though they probably don't know what it is you do, and you need to make them aware of it so your compliant and it's recognised in your paper trail for insurance purposes.

The ovetheat stat is one, not two means of achieving 'safety' as the regular temp control stat isn't a safety device, just a regulator   

The 6bar PRV is another, so the 3 / 3.5 bar PRV will be your third. To be honest you need to seek out a G3 fitter and get them on board now otherwise you may do this wrong and have to pay them to correct it. 

Criteria for D1&2 discharge need to be spot on, length of vertically falling pipe before / after the PRV & tundish etc and so on.

 

[Gone West]

2 minutes ago, Nickfromwales said:

It's an unvented water heater fitted into a uk home so it's G3 all the way I'm afraid.

This doesn't allow for interpretation, you need to follow it to the letter. BCO will want in on this for your completion cert, even though they probably don't know what it is you do, and you need to make them aware of it so your compliant and it's recognised in your paper trail for insurance purposes.

The ovetheat stat is one, not two means of achieving 'safety' as the regular temp control stat isn't a safety device, just a regulator   

The 6bar PRV is another, so the 3 / 3.5 bar PRV will be your third. To be honest you need to seek out a G3 fitter and get them on board now otherwise you may do this wrong and have to pay them to correct it. 

Criteria for D1&2 discharge need to be spot on, length of vertically falling pipe before / after the PRV & tundish etc and so on.

 

OK thanks. It has a regular temperature control stat and two manual reset thermal cut outs. There are around a dozen of these fitted in the UK that I know of so it's been sorted, if only I knew how. I'll try to find a friendly G3 fitter. Thanks for your help.

[Jeremy Harris]

Page 14 seems to have the control group, and it's all external.   The snag is that there is no expansion vessel shown, and it doesn't look like there is an internal one, either.  It may be that, given the high max working pressure (10 bar), plus the high PRV pressure (6 bar) that this unit is designed to work in the same way as our boiling water tap boiler, where the PRV blows off every time the tank heats up, as a means of controlling expansion.  That was made in Belgium, so it it could be that this is just the way things are done in other countries, and that would tally with the check valve arrangement, where they seem to have gone overboard to make sure that there can be no expansion flow back to the supply. 

 

Here we tend to use an expansion vessel and not let the PRV blow off.  So, it looks to me that if a suitably sized expansion vessel was fitted next to the point where the PRV (Item 7 on the diagram of the control group) connects then it should be compliant, subject to the points that @Nickfromwales has made about G3, tundishes etc.

Edited October 17, 2017 by JSHarris

[Gone West]

4 hours ago, JSHarris said:

Page 14 seems to have the control group, and it's all external.   The snag is that there is no expansion vessel shown, and it doesn't look like there is an internal one, either.  It may be that, given the high max working pressure (10 bar), plus the high PRV pressure (6 bar) that this unit is designed to work in the same way as our boiling water tap boiler, where the PRV blows off every time the tank heats up, as a means of controlling expansion.  That was made in Belgium, so it it could be that this is just the way things are done in other countries, and that would tally with the check valve arrangement, where they seem to have gone overboard to make sure that there can be no expansion flow back to the supply. 

 

Here we tend to use an expansion vessel and not let the PRV blow off.  So, it looks to me that if a suitably sized expansion vessel was fitted next to the point where the PRV (Item 7 on the diagram of the control group) connects then it should be compliant, subject to the points that @Nickfromwales has made about G3, tundishes etc.

I did wonder if it was a European thing not having an expansion vessel. There is another 3/4" BSP take off from the bottom of the tank that they call a 'hot water circulation pipe'. I'm not sure what it's supposed to do but it may be possible to connect a pressure relief valve to it.

[Jeremy Harris]

30 minutes ago, PeterStarck said:

I did wonder if it was a European thing not having an expansion vessel. There is another 3/4" BSP take off from the bottom of the tank that they call a 'hot water circulation pipe'. I'm not sure what it's supposed to do but it may be possible to connect a pressure relief valve to it.

 

I think it may be.  I've seen a sealed mains water pressurised electric water heater in France that had no expansion vessel, just a PRV that dribbled a bit every time the heater came on, the same system that our boiling water tap heater tank uses.  They seem to certify the tanks to a high max pressure, then just allow a small amount of water to blow past the PRV to control the pressure. 

 

In terms of safety, it's arguably better than our system, as at least the PRV gets exercised regularly, so you know it's working OK.  With our system, the only time you know if the PRV is working is if the expansion vessel loses pre-charge or fails.  The snag then seems to be that a PRV that's not operated for years, may well then leak all the time a bit when first operated from an over-pressure event.  I've heard of this happening with combi boilers, when a loss of pre-charge in the expansion vessel has caused the PRV to operate and then not seal properly again afterwards, so needing replacement.

 

 

 

 

[Gone West]

13 minutes ago, JSHarris said:

 

I think it may be.  I've seen a sealed mains water pressurised electric water heater in France that had no expansion vessel, just a PRV that dribbled a bit every time the heater came on, the same system that our boiling water tap heater tank uses.  They seem to certify the tanks to a high max pressure, then just allow a small amount of water to blow past the PRV to control the pressure. 

 

In terms of safety, it's arguably better than our system, as at least the PRV gets exercised regularly, so you know it's working OK.  With our system, the only time you know if the PRV is working is if the expansion vessel loses pre-charge or fails.  The snag then seems to be that a PRV that's not operated for years, may well then leak all the time a bit when first operated from an over-pressure event.  I've heard of this happening with combi boilers, when a loss of pre-charge in the expansion vessel has caused the PRV to operate and then not seal properly again afterwards, so needing replacement.

 

 

 

 

Thanks, that's interesting. Does their system meet our regulations?

[Jeremy Harris]

1 hour ago, PeterStarck said:

Thanks, that's interesting. Does their system meet our regulations?

 

I'm not sure.  I would guess that you could argue that the manufacturer's instructions trump the method of compliance in the approved documents (in this case Part G3).  It seems perfectly safe to me to allow expansion via the manufacturer's rated PRV, as long as that PRV is fed to a drain that complies with our regs in terms of a visible means of discharge, slope and lengths of pipe, protection from scalding at any external discharge point, etc.

 

There's a small problem in that having an external discharge outlet means an air leak, but you may well be able to argue that discharging through a long enough internal drain, with a trap, is OK.  This is the argument I used with building control for the Sunamp PV PRV.  I argued that the temperature could never exceed 58 deg C, so was safe to discharge via an internal drain with an internal trap on the ground floor.  I fitted a plastic tundish right at the PRV, to provide the required visible indication of discharge.

 

In your case, the argument would be whether or not the tank could ever exceed the 65 deg C maximum temperature and what would happen if it did.  Clearly if only run from the integral heat pump it cannot, as the heat pump flow can't ever get up that high, it's physically limited by the properties of the refrigerant gas used.  That leaves a fault developing in both the small immersion heater thermostat and the over-temperature cut out at the same time as the only condition where the tank could exceed 65 deg C.  There is a good argument that, with no expansion vessel fitted, the PRV will open as normal, but stay open and clearly indicate that there is a problem.  My view is that this meets the spirit of the regulations themselves, if not the letter of the suggested method of compliance with the regulations in Approved Document Part G3. partly because the maximum heat input is only just over 1 kW, so not anything like the power available from a boiler or solid fuel appliance.  Worth noting that the Approved Documents are not the regulations, just one way that the regulations can be complied with.

Edited October 17, 2017 by JSHarris

[Nickfromwales]

1 hour ago, JSHarris said:

They seem to certify the tanks to a high max pressure, then just allow a small amount of water to blow past the PRV to control the pressure. 

I think our water regs frown on such an arrangement, certainly anywhere you are storing above a thimble full of water. The reason being is that your basically wasting clean fresh drinking water by design, and with an UVC it'll be with enough frequency and enough volume to warrant a design for the safe management and prevention of such waste. 

 

30 minutes ago, JSHarris said:

There is another 3/4" BSP take off from the bottom of the tank that they call a 'hot water circulation pipe'. I'm not sure what it's supposed to do but it may be possible to connect a pressure relief valve to it.

That's a provision for ( as we'd call it ) a hot return circuit ( HRC ) for the prevention of dead legs or delays in getting hot water to the furthest outlets from the cylinder.

Dont connect the PRV to it as itcwont comply with G3 ( distances from D1 to tundish and tundish to D2 next invert etc. 

Also, don't connect the EV here as the water will be warm / hot. Connect the EV as per my earlier post, to the cold pipe AFTER the control group, teed in accordingly. 

Remember there can be no valves of any sort between the point that the EV is fitted and the cold connection into the UVC. The reason being that you must never be able to remove or isolate the means of expansion inadvertently or by a valve failure. I do, on larger installs with multiples of EV's, make a manifold and connectcall the EV's via locksheild gate valves for ease of isolation, testing and repressurising  ( which have shrouds on the gate stem so have to be consciously opened or closed manually ) as they save the entire system being drained to carry out such maintenance, but I would not do this where there is only one. 

@PeterStarck

Can you get a bent iron into that 1/2" brass plug and take a pipe horizontally to some internal space and fit the PRV there, and still get the case on ?

[Jeremy Harris]

@Nickfromwales the second quote above was from @PeterStarck, rather than me, and I agree with your take on what that connection is for.

 

Given that there are a handful of Genvex Combi 185 installations here in the UK, it would be good to track down one or two and see what they did.  Adding an expansion vessel seems relatively easy, and could be done on the cold side with the rest of the control group, but it may be that the Danish system of not using an expansion vessel at all, and just using the PRV to release the excess from expansion may be OK.  The volume of lost water is tiny when the thing is operating normally, maybe a cupful a day, if that.  The only time the PRV is going to release a significant flow is under a multiple fault condition, where the tank sensor fails, the Optima controller selectively fails, the low power immersion control circuit fails, the immersion thermostat fails and the over-temperature cut-out fails.  That seems to me to be a pretty low risk.  Our Genvex Premium 1L has basically the same controller, the Optima 300 in our case, and it's pretty fail-safe (too fail-safe in some respects, like when it comes to things like filter change alarms).

 

So, for the PRV to let by continuously, with the tank at a temperature above 65 deg C, requires several simultaneous failures to occur, any one of which would flag an alarm on the Optima.  Firstly, the tank temperature sensor would have to fail, and my guess is that normally failure of this sensor automatically triggers a fail-safe shutdown in the Optima.  If, for some reason, that didn't happen, and the small immersion stayed on, and no water was drawn off, then the thermostat in the immersion should turn it off at 65 deg C.  If the tank sensor failed, and the Optima failed to fail safe and the immersion thermostat failed closed, then the over-temperature cut-out should operate.  If the latter fails as well (so now we have a case where four systems have failed) then the tank could boil and continuously vent via the PRV.

 

Frankly I can't see that this is a scenario that could realistically occur, because as soon as the tank gets above 65 deg C the heat pump will refuse to deliver heat (it will probably throw a refrigerant over-pressure fault), and so there will be yet another signal to the Optima to shut the unit down.  I can't see how the Optima could end up accepting two critical shut down failure signals yet continue to send the signal to keep the immersion element powered on.

 

Overall, my view is that the control group should be installed exactly as per the manufacturers instructions, with no additions or attempts to over-think things in order to try and make this unit's plumbing look like that on a UVC.  It is not a UVC in the conventional sense at all, but an integral part of a well-engineered system, that if installed in accordance with the MI's should comply with UK regs, subject to the drain outlet from the PRV being fitted to UK requirements.

Edited October 17, 2017 by JSHarris

[Nickfromwales]

I agree with your thinking, but that doesn't matter one jot as it's not open to our interpretation.

It's a mains pressurised unvented water heater. Black and white case here to chose ignorance over regulations, excusing my bluntness, which will serve you ill. In Denmark you can follow the installation guide as is, but do the MI's state that it's ok to do so outside of Denmark, or does it say words to the effect of "please check this installation complies with the local / regional water regs of the country of installation" etc etc? Best to check that imo, before advising directiy against it ?

Regarding PRV discharge......

If a million of these units got sold in the U.K. then that's a million tea cups of water unnecessarily dumped to drain, and in the ( EU driven ) U.K. that's not acceptable, particularly when just adding a £50 EV negates it. No sale, sorry. 

@JSHarris I wasn't referring to the eventuality of the PRV continuously discharging, so apologies for any bum steer there. Afaic it would be free to do so in either scenario so the matter is moot. 

Lets be honest, your fitting everything else anyway, as it's all bolted into the control group and built into the immersion, ( as per @PeterStarck's comment, correcting my assumption that the immersion was only single failsafe when it's actually dual  ) so why not add the EV and have no discharging water ? 

If this is to be recognised for exactly what it is, and signed-off and registered according to our regs, then it'll need the control group and the EV, and I wouldn't install one any other way. The fact it's so easily, and reasonably cheaply negated, adds to my argument. 

If, however, someone says it doesn't need to comply with G3 because the BCO has called it, then fill your boots. Until then, fill out your G3 benchmark book . 

 

[Jeremy Harris]

But, Part G3 is not a regulation, as I understand it, and that is key.  The regulation is the law, in this case the Building Regulations themselves, not the Approved Documents.  I know we all tend to consider the Approved Documents to be the regulations, but they aren't, they are just a bunch of guidance documents that are intended to give examples of ways in which the regulations themselves (the law, in effect) MAY be complied with.

 

For example, I can comply with a structural requirement by following a standard detail in Approved Document Part A.  I can just as easily comply with exactly the same structural requirement in maybe two or three other ways, that are not specifically listed in Approved Document Part A.

 

In essence, the core issue here is that this unit is not a standard unvented hot water cylinder at all.  It is a part of a system, that includes a collection of sensors and controls, together with heat inputs, that are not found on a standard UVC.  For example, the primary heat input comes from a direct side contact heat exchanger than cannot physically deliver more than about 60 deg C, and is limited to 55 deg C.  There is a physical limit that prevents the refrigerant gas from delivering more than about 65 deg C, if that.

 

The heat pump output dominates the heating to the integral tank, and controls how that tank is heated.  As such, the heat pump controller is the primary mechanism for determining when to turn on the auxiliary heater, so provides the first level control.  There are then a bunch of additional controls that prevent the system from malfunctioning, far more than a UVC would have.

 

Does it need, or would it even work, if the unit was modified away from the MIs in order to make it function like a conventional stand alone UVC?  TBH, I don't know.  My guess, based on my experience of programming the Optima controller is that there may be a few issues to resolve.

 

I hate to say this, but I think it's overly simplistic to consider the integrated tank in this system as if it were a stand alone UVC, just because there seems to be a risk that this safe and tested, reliable and well-proven, unit could end up not performing as it was designed to.

 

As for discharging water during the expansion stage, I'm not 100% sure that it would normally discharge.  The pressure may rise within the tank, but with the manufacturers stated 6 bar PRV, would the tank actually get to 6 bar when fed via double check valves and a 3 to 3.5 bar PRedV?  It may be that they have found that in normal use the pressure rise when operated within the normal range never actually blows the PRV?

 

I think more info is needed from other people who have installed them in the UK, and how they've done the installation.  Genvex are technically pretty switched on, and aware of the regulation and laws in all the countries they sell to, and the fact that they write a pretty clear set of instructions in good English makes me think they have thought things through, but haven't felt the need to explain why their control group doesn't seem to include an EV.

[Gone West]

13 hours ago, Nickfromwales said:

@PeterStarck

Can you get a bent iron into that 1/2" brass plug and take a pipe horizontally to some internal space and fit the PRV there, and still get the case on ?

Unfortunately there's no way of exiting the case, when the cover is on, except downwards and underneath towards the rear. Very interesting discussion between yourself and Jeremy.

[Gone West]

10 hours ago, JSHarris said:

I think more info is needed from other people who have installed them in the UK, and how they've done the installation.

I have spent quite some time trying to find pictures of installations and watching old self build programmes on TV that feature the Combi, but with no luck so far. There are small developments in the UK that have had Combis fitted but it seems they have all been installed by Total Home Environment. I can't see them being helpful as I saved money by not buying from them. It would have been simpler paying the extra £3100, but that is expensive information on how to install in the UK.