Vented Cylinder with ASHP?

[JamesPa]

I'm struggling with where (or whether) to put a UVC in my retrofit, because the  existing vented cylinder is in the 'middle' of the house (on the first floor) and there is no obvious route for the vent pipe.  I do need to replace the cylinder, the existing one has only a 0.7sq m coil and, based on a 55C flow temp,  wouldn't get above about 40C before the heat pump starts cycling due to the relatively small transfer to the tank with such a small coil area.

 

So I'm faced with various not very easy and quite disruptive options to fit a UVC, or simply replacing the exiting more or less like for like with a vented cylinder but with a larger coil.

 

Obviously it would be nice to have mains pressure hot water, but it isn't essential and its a lot of disruption.  is there any good reason to avoid combining a vented cylinder (with a sufficiently large coil) with an ASHP?

 

 

 

 

Edited February 12, 2023 by JamesPa

[ProDave]

For the vent pipe you either need access to an outside wall, probably the least disruptive route (smaller holes to drill) or a soil stack.

 

Care to do some sketches of the layout of your house, first and second floor and I am sure someone can find a route for you.

[JamesPa]

  On 12/02/2023 at 14:24, ProDave said:

For the vent pipe you either need access to an outside wall, probably the least disruptive route (smaller holes to drill) or a soil stack.

 

Care to do some sketches of the layout of your house, first and second floor and I am sure someone can find a route for you.

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Thanks for the offer.  There is a route, but the combination of plumbing through that route and the other changes to the system to go mains pressure is quite disruptive.  So Im exploring alternatives while accepting that I may end up with a UVC, hence the question about whether there is any reason to avoid a large coil vented cylinder.

 

I suppose one option might be to put in a uvc, but plumb it vented.  Then I could 'upgrade' later!

[Marvin]

We use an unvented cylinder with no problem....

[JamesPa]

I think people might have misunderstood the question.  Im asking about using a vented cylinder (ie the 'old' type with a cold water tank in the loft) with an ASHP as opposed to an unvented one (which most seem to specify).

 

My current cylinder is vented (I have a cold water tank in the loft).  To convert to unvented is a pain because the run for the pressure relief 'vent' involves much disruption.   So Im wondering about replacing my current vented cylinder with another vented cylinder (retaining all the plumbing) the only difference being a bigger coil.  Of course I wont get mains pressure hot water, but I don't at present.

 

Can anyone see a problem?  

[Nickfromwales]

  On 12/02/2023 at 21:32, JamesPa said:

I think people might have misunderstood the question.  Im asking about using a vented cylinder (ie the 'old' type with a cold water tank in the loft) with an ASHP as opposed to an unvented one (which most seem to specify).

 

My current cylinder is vented (I have a cold water tank in the loft).  To convert to unvented is a pain because the run for the pressure relief 'vent' involves much disruption.   So Im wondering about replacing my current vented cylinder with another vented cylinder (retaining all the plumbing) the only difference being a bigger coil.  Of course I wont get mains pressure hot water, but I don't at present.

 

Can anyone see a problem?  

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You need to ask the cylinder manufacturers to be honest. Unless you show us a link to a cylinder you're considering using, were unable to help ( or offer meaningful replies ). You'd then also need to check this with the ASHP manufacturer before getting any traction.

[JohnMo]

There you go

https://www.cylinders2go.co.uk/shop/uncategorised/telford-open-vented-300-litre-twin-coil-solar-and-heat-pump-stainless-steel-3-pockets/

[Iceverge]

I can't see any logical reason for a vented cylinder being a problem. 

 

Maybe there will be more costly antifreeze/inhibitor used as it'll need to fill the attic tank too. 

 

I'm aware the regs/manafactuers instructions specify that an unvented cylinder needs to have a tundish visible and waiting a specified distance of the cylinder,  so you can see if the T+P valve is discharging warning of an issue. 

 

Why can you not run large diameter copper piping from the UVC T+P valve vertically upwards (AKA a solid fuel vented cylinder) and from there to a discharge a safe location. 

 

I can't see why this wouldn't work or be dangerous?

 

It might solve @Thorfuns basement pump issue .

 

 

 

[Nickfromwales]

  On 12/02/2023 at 22:35, Iceverge said:

Maybe there will be more costly antifreeze/inhibitor used as it'll need to fill the attic tank too. 

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Nope. That would be if it were a vented thermal store   This would be heated via a coil, so the brine and DHW would never meet.

 

  On 12/02/2023 at 22:35, Iceverge said:

see if the T+P valve is discharging warning of an issue. 

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No T&PRV on an open vented cylinder.

 

  On 12/02/2023 at 22:35, Iceverge said:

Why can you not run large diameter copper piping from the UVC T+P valve vertically upwards (AKA a solid fuel vented cylinder) and from there to a discharge a safe location. 

 

I can't see why this wouldn't work or be dangerous?

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This is a bone of contention, and a question I have asked of some cylinder manufacturers. I've not seen a reason why the factory fitted T&PRV could not be unscrewed and then relocated to the highest point of the discharging pipework run. Obviously the T part of T&PRV would be ineffective at that point, but why cant we have a capillary wire, like a fire valve of an oil boiler type, to allow remote relocation? 

Answer is; too many risks associated with human error eg bad / lazy fitters I expect.

[Iceverge]

Vindication from on high😁. 

 

What would be wrong with leaving the T+PRV on the tank as normal and running say 22mm copper vertically upwards to the loft (with insulation when it's outside the thermal envelope) and then back down to a safe discharge location? 

 

Realistically I reckon you could change to barrier pipe once a suitable distance from the T+P valve as the water would surely have cooled. 

 

 

[JamesPa]

  On 12/02/2023 at 23:20, Nickfromwales said:

  On 12/02/2023 at 22:35, Iceverge said:

Why can you not run large diameter copper piping from the UVC T+P valve vertically upwards (AKA a solid fuel vented cylinder) and from there to a discharge a safe location. 

 

I can't see why this wouldn't work or be dangerous?

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This is a bone of contention, and a question I have asked of some cylinder manufacturers. I've not seen a reason why the factory fitted T&PRV could not be unscrewed and then relocated to the highest point of the discharging pipework run. Obviously the T part of T&PRV would be ineffective at that point, but why cant we have a capillary wire, like a fire valve of an oil boiler type, to allow remote relocation? 

Answer is; too many risks associated with human error eg bad / lazy fitters I expect.

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I guess the answer is that there has to be a visible indication of the discharge.

 

Having said that the regulations only specify what must be achieved not how and its clear that there are multiple ways of achieving it.  Obviously the easy way is to follow the guidance or manufacturers instructions and the problem is that if you dont then its ultimately down to building control to determine if the 'how' conforms, which becomes a risk for an installer.

 

No doubt some more innovation is due in how to vent an unvented cylinder.

 

 

[JohnMo]

  On 13/02/2023 at 17:53, JamesPa said:

ultimately down to building control to determine if the 'how' conforms

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But is it down to building control, doesn't the mandatory installers certificate, trump building control?

[Iceverge]

  On 13/02/2023 at 17:53, JamesPa said:

guess the answer is that there has to be a visible indication of the discharge

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Why not have it outside the house in a visible location. Guarded by mesh to prevent scalding and discharging into a gutter? 

 

  On 13/02/2023 at 18:18, JohnMo said:

But is it down to building control, doesn't the mandatory installers certificate, trump building control

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I believe so, MIs>BRegs

[ProDave]

  On 13/02/2023 at 18:47, Iceverge said:

Why not have it outside the house in a visible location. Guarded by mesh to prevent scalding and discharging into a gutter? 

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That is an allowable option, that is what we have.  The discharge needs to be visible, above ground, at least 100mm above the ground, drain to somewhere that drains away (BC accepted ours discharging onto a French Drain) and covered with a cage.

[Jimmyjazz]

Having a vented cylinder with a large area coil (high gain) should be no problem. There are manufacturers that do this. Try McDonald cylinders. They would be able to help.

[ReedRichards]

  On 12/02/2023 at 14:17, JamesPa said:

Obviously it would be nice to have mains pressure hot water, but it isn't essential and its a lot of disruption.  

 

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But worth it, in my opinion.  For a while I had the discharge pipe from my cylinder running along the skirting board to an external wall.  Then we revamped the bathroom and I could run it under the floor to an internal drain pipe.  The tundish incorporates a non-return valve so you don't get drain smells.