G3 and ASHPs

[JamesPa]

I know this has been raised before and I also posted a similar question on the Hot Water Tank thread (so apologies if you have read it there) but...

 

I realised the other day that the R290 Vaillants don't rely on an immersion heater for their legionella cycle, they do it natively.  Presumably others will follow suit as they introduce R290, possibly as an option, possibly as the only option (as it is with the Vaillants).

 

These being the case, is it to re-think how G3 is satisfied with a UVC connected to a heat pump? 

 

The requirement (as opposed to guidance) is 

 

(3) A hot water system that has a hot water storage vessel shall incorporate precautions to:

 (a) prevent the temperature of the water stored in the vessel at any time exceeding 100˚C; and 

 (b) ensure that any discharge from safety devices is safely conveyed to where it is visible but will not cause a danger to persons in or about the building.'

 

If we omit the Immersion altogether requirement, 3a satisfied automatically, so far as I can see without vent valve, tundish, vent pipe etc, and thus 3b does not apply

 

Now if we do want a backup, whats to stop us putting that in the heating flow pipes to the tank, rather than in the tank itself.  Answer - nothing, its called a backup heater and many HP manufacturers offer one.  Now this could boil the tank, because its in a pressurised system, but we can incorporate backup devices into the heating pipework, rather than the tank pipework to prevent that happening.  For example the usual immersion cut out valve plus a valve which diverts the heated flow away from the tank if it gets above say 95 - a bit like a car thermostat.

 

The point of all of this is to avoid having to run the vent pipes from the tank, which is often in a space inaccessible or not conveniently accessible to the outside.  Furthermore, as all of the safety features are contained in the heating system itself, the HP manufacturers can incorporate them and tank manufacturers can do what they are good at, making tanks.

 

Have I missed anything (apart from the difficulty with getting the heating industry to think outside the box (or, in this case, cylinder)?

[Beelbeebub]

i'd say the chances of a gas boiler being able to boil the UVC are pretty minimal.  As a gas appliance they have multiple safety features and sensors and are more likely to shut themselves down than run away uncontrollably. 

 

The only things that could boil a tank realistically are a solar system and an immersion.

 

Yet we have to have G3 even for an UVC just on gas.

 

In all honesty I think the UK is a little *too* careful with them, other countries seem to get away with much less installation regulation (as opposed to design and manufacture regs which do need to be tight) 

 

I kinda wonder if it has some bias in the fact it was a "foreign" and "European" system and therefore automatically inferior to the good old british vented cylinder.....

[JamesPa]

1 hour ago, Beelbeebub said:

Yet we have to have G3 even for an UVC just on gas.

I guess the question is does it really, or is it the easy solution given that (I guess) most gas installations will also have an immersion.  Also do gas boiler manufacturers tell you that their product can't get above 100, I don't recall seeing this in the specs.

 

Only a small proportion of the plumbers I have encountered think out of the box, so I wouldn't expect much innovation.

 

Re your 'kind of foreign' comment, I would say most definitely.  Also 'new' and of course 'new' things need more regulation don't they, heat pumps being the obvious example in this context.

Edited August 18, 2023 by JamesPa

[Beelbeebub]

57 minutes ago, JamesPa said:

I guess the question is does it really, or is it the easy solution given that (I guess) most gas installations will also have an immersion.  Also do gas boiler manufacturers tell you that their product can't get above 100, I don't recall seeing this in the specs.

 

Only a small proportion of the plumbers I have encountered think out of the box, so I wouldn't expect much innovation.

 

Re your 'kind of foreign' comment, I would say most definitely.  Also 'new' and of course 'new' things need more regulation don't they, heat pumps being the obvious example in this context.

The newer boilers really don't like boiling, it can damage the heat exchangers. I don't think the old cast iron jobs cared. They just kettled away.

 

But because the new heat exchangers are so thin thry could locally overheat if they boiled so the manufacturers put high limit cutouts, normally at 90/95C in the heat exchanger. 

 

Given the design philosophy of gas boilers is usually to fail safe ie if a sensor breaks stop working, i'd say there is very little chance of a gas boiler ever being able to output a flow of more than 95C and hence being able to boil the UVC. 

 

Only if an immersion heater is fitted to the UVC is there a possibility of boiling. And to be fair, immersion heaters sticking on is not that uncommon. 

[JohnMo]

Max output on my gas boiler is 85 deg, so way under the 100. Not much higher than a valiant ASHP really 

[Beelbeebub]

24 minutes ago, JohnMo said:

Max output on my gas boiler is 85 deg, so way under the 100. Not much higher than a valiant ASHP really 

And there will be an internal cutout that kills the burner if you get more than 95C, say there was a glitch with the control system, or maybe the pump failed. 

 

Point is it is highly unlikely a gas boiler will boil an UVC. 

 

If your expansion vessel was dud and the pressure relief failed (or was capped) the worst that would happen is the tank would split and leak hot water. It's unlikely to be a huge split leading to a tsunami, more of a crack and a big drip. 

 

The myth busters style steam explosion and ballistic tank (which would be very bad) is only possible if you superheat the water beyond 100C so you get a lot of water flashing to steam upon pressure release. 

[JamesPa]

1 minute ago, Beelbeebub said:

And there will be an internal cutout that kills the burner if you get more than 95C, say there was a glitch with the control system, or maybe the pump failed. 

 

Point is it is highly unlikely a gas boiler will boil an UVC. 

 

If your expansion vessel was dud and the pressure relief failed (or was capped) the worst that would happen is the tank would split and leak hot water. It's unlikely to be a huge split leading to a tsunami, more of a crack and a big drip. 

 

The myth busters style steam explosion and ballistic tank (which would be very bad) is only possible if you superheat the water beyond 100C so you get a lot of water flashing to steam upon pressure release. 

So if there is no immersion in the cylinder, doesn't that satisfy the G3 requirements on its own without all the vent faff?

[Beelbeebub]

3 minutes ago, JamesPa said:

So if there is no immersion in the cylinder, doesn't that satisfy the G3 requirements on its own without all the vent faff?

I have a feeling you might struggle to get that through. 

 

It's arse covering. 

 

A sensible approach would be all UVC have to be certified (made by registered manufacturer, clearly marked etc) but they can be installed by anyone as as long as there is no immersion (big sticker on side, immersion boss either missing or blanked off with dummy immersion covered in warning tags if one is fitted). 

 

Updating the info plate to show the coil area would be a useful thing too.