Design Help - Plumber not helping

[DeeJunFan]

Hi Guys,

 

I have been busy busy with the build and have been trying to arrange a million and one things.

 

So as it stands we are planning all our bathroom and other plumbing arrangements.  We have been talking to a plumber, who was recommended but it seems the idea of an informed client isn't something he is ever really used to dealing with.   Even at that i dont know much i'm just asking some questions he obviously has never been asked.

 

So at the moment we are looking at the following setup.

 

4kw PV array with immersion diverter

12kw stove with 2kw to room the rest to back boiler

9kw ASHP

 

I was humming and haa-ing over the ASHP but i think i'll just go for it and try to make the best use out of it.

 

Told the plumber i was in 2 minds about the ASHP but could he do all the work so that i could hook it up down the line if required.  He said fine but would need to do some "checks" to make sure we could heat the house with just PV/Immersion/Stove.  

 

He phoned back a few days later, said he had spoken to "Plumbing Merchant"  (who also sells ASHPs) and they said i would definately need an ASHP as "People only use PV for washing machines and tumble dryers" and "stoves are not compatible with UFH"  He told me this was the opinion of the Plumbing merchant.  I asked why was a stove not compatible, he said because of the temp diff between stove and UFH, i asked was there not a mixer etc to bring the water temp down he said yes we could reduce the temp.  I then asked another few questions and we got to, "if you want to go with a TS you need to make sure you keep the temp of it at 60 degrees"  i asked why, he said to ensure you could have a good amount of showers.  I mentioned in the in-line water heater, he asked me to send him some details so he could google it.  He couldn't tell me which was the best setup.  Buffer/UVC/TS etc.

 

So i pretty much got off the phone thinking i would need to design the system i want myself and get the plumber to just to the work.

 

Anyway, what i was thinking was i could give it a stab and hopefully the kind plumbing brains on here could tell me its shite and how to really do it!

 

Basic design

 

We shower and bath kids in the evenings. I would have the odd morning shower after gym but its only one a fairly short.

 

I was thinking 500L TS as i will have heat coming from stove and ASHP

 

Have heat coming off the PV during the day, say at 4pm if the tank temp is low fire up the ASHP to bring it up to temp

Have the in-line heater to boost if the temp drops 

 

On the really cold winter evenings we would likely be lighting the stove which could bump up the tank temp and help to run the UFH over-night etc.

 

Not 100% as to when to turn on the ASHP in the shoulder months for UFH to make sure we are warm in the mornings.  Likely have it coming on at say 6ish to get the house warmed up a bit.

 

Looking to have the TS in the attic right at the chimney breast to take the input from the stove, and be just above the ensuite and close to the main bathroom.  Guest en-suite will be out of the way a bit but not frequently used to have to accept it will be a bit of a dead leg.

 

Was thinking of running the downstairs shower/wc with under counter instant water heaters and same for kitchen/utility?  Not sure if thats a good idea or just a waste of money, but wouldn't like the after dinner clean up to take all the shower hot water.

 

Do you think i should be using a buffer tank in this setup?  Something a bit closer to the ASHP?  say if the TS is in the attic, will i need some kind of pump to get water from the ASHP up to the TS?

 

Anyway this is really all i can think off for now.  Hopefully enough to get started.

 

Thanks

Damian

 

 

 

 

 

 

 

 

[Nickfromwales]

Ok 

Having the TS in the attic could be a problem. With a solid fuel heat source you'll need an open pipe 'vented' primary heating circuit servicing that arrangement. That means you'll need a large galvanised steel feed and expansion ( F&E ) tank at a minimum of 1m above the top of the TS. 

Next problem I see is running heating and hot water via the TS for when the ashp is being used. The space heating should ideally be able to run direct ( via a small buffer ) from the ashp to the Ufh. Otherwise the ashp will need to constantly run at high temp so as not to cool the TS and render it useless for dhw. 

Lets go bite size on this . Thoughts so far?

[le-cerveau]

Whilst I am not having a stove (I have gas) I am also seriously considering an ASHP (so I can cool as well as heat) and future proof, likely, gas price increases.

As you are having a stove a TS is a must (as I understand it) and you can quite easily mix down off the TS to whatever your UFH requires.

How well insulated and leaky will your house be? As if it is near PassiveHaus standards then you are not going to require huge amounts of heat, as you are specifying 12kW stove, 9kW ASHP, either it is less efficient or you are looking at the DHW requirement also.

As for running the ASHP for morning UFH, if the TS is up to heat, you won’t need it, you can just draw off the TS, but if you want hot water for a shower then that is different.

I can’t see you needing a Buffer with a TS, it is the buffer, and the ASHP just feed the TS directly, provided your TS can cope with low grade heat as well as high grade, ie the top and bottom are separated (Akvaterm Geo style), ready to be shot down Nick!

The plumbing of it all together shouldn’t be a major issue, I wait for Nick to say otherwise, the big issue will be control:  What heat source is supplying what and when, and can you automate it?

The question is, and I think others have said this, is UFH or DHW your biggest driver (mine is DHW)?

[Temp]

DeeFunFan...

 

My system is much less complicated (oil boiler -> Thermal Store -> DHW and UFH) yet I also had nightmare of a time finding a plumber that understood such a system. My builder recommended someone that claimed to be a retired heating engineer that acted as an expert witness in heating related legal cases. Even he didn't understand how this was meant to work. In the end I drove him 200 miles to visit a company that designed and sold thermal stores. They basically taught him/us how it should all work.

 

The thermal store is the key to the system so I would go visit a few and have them make a proposal.

 

37 minutes ago, Nickfromwales said:

The space heating should ideally be able to run direct ( via a small buffer ) from the ashp to the Ufh. Otherwise the ashp will need to constantly run at high temp

 

+1   ASHP work best (highest COP) the lower the flow temperature. So they are ideal for UFH, less ideal for DHW.

 

The approach Nick suggests is good but I'm not sure how/if it could be modified to allow the stove to heat the UFH. One possibility might be to have a very well stratified TS that is much hotter at the top than at the bottom (as le-cerveau mentioned)  . The stove would heat the top and the ASHP would heat the middle/bottom. Both DHW and UFH would be drawn from around the middle. Little or no mixing down required although I would fit mixers anyway. However I think this would be a difficult system to set up and get working reliably. If the store wasn't perfectly stratified (eg it becomes stirred) then the ASHP flow temperature would have to be higher. You need to be really sure the TS company know what they are talking about and have them take you to see such a system in a customers house.

 

We have two DHW outputs on our TS feeding different parts of the house. So two sets of plate heat exchangers, two pumps, two mixers, one secondary loop pump in addition to what's needed for the UFH. This works very well in that we get two mains pressure showers with good flow rates. However the heat loss from the TS and all that the additional plumbing is significant despite being insulated. The TS is in a room the size of a small bathroom and that room gets quite toasty. I would not want to waste that heat by having it leaking into the loft space. Plan an airing cupboard?

 

PS: In case it's not obvious... I would NOT get your plumber to design this system. Get the TS company to do it and perhaps get your plumber to install it if you are happy with his work.

 

PPS: There is an issue of who is responsible if it doesn't work. Is it down to the TS company to solve or the plumber or you?

 

 

 

 

   

 

 

Edited October 11, 2016 by Temp

[le-cerveau]

3 minutes ago, Temp said:

We have two DHW outputs on our TS feeding different parts of the house. So two sets of plate heat exchangers, two pumps, two mixers, one secondary loop pump in addition to what's needed for the UFH. This works very well in that we get two mains pressure showers with good flow rates. However the heat loss from the TS and all that the additional plumbing is significant despite being insulated. The TS is in a room the size of a small bathroom and that room gets quite toasty. I would not want to waste that heat by having it leaking into the loft space. Plan an airing cupboard?

 

+1, in our design I have gone for a dedicated plant room (1st floor) next to the airing cupboard, so it can share the excess heat (minimized hopefully) but keep the clothes and equipment separate.

[Nickfromwales]

17 minutes ago, Temp said:

PS: In case it's not obvious... I would NOT get your plumber to design this system. Get the TS company to do it and perhaps get your plumber to install it if you are happy with his work.

I didn't want to say it, but I'll certainly second that tbh. . 

Hes going to the merchant for advice, which is bad on a good day. 

[Temp]

I grew up in a house that used a wood/coal burner with back boiler. My parents were more than happy to see the back of it when gas CH was installed. It's not the feeding with wood that gets to you its the emptying ash, cleaning of the glass, relighting etc.

 

On balance I would install a stove (I like a real fire in winter) but I would ditch the idea of using the stove to heat the TS and install ASHP from the outset. The cost saving on the stove and the associated back boiler, plumbing and installation would help pay for the ASHP.

 

There are other reasons for ditching the back boiler.. A new house will be well insulated that it will only be possible to run a small low output stove anyway. Are there many stoves that output little to the room and a lot to the back boiler? I've heard not. I would also want my stove room sealed so it burns outside air not room air. Again that reduces the number of options.  Is there a low output stove that can hold enough wood to still be burning next day? 

 

How big will the PV system be? Is it worth using it to heat the house or would most of the benefit come from using excess to heat the DHW only? The heating won't be on in summer and excess PV output will be down in winter (more used for lighting). I think I'd be happy with just using the excess for DHW.

 

Taken together (no back boiler and PV for DHW only) the result would be a much simpler system to install and operate......... .

 

ASHP -> small buffer tank -> UFH

 

ASHP -> Mains pressure DHW tank -> mixer -> DHW

PV -> Element in Mains pressure DHW tank

 

An off the shelf mains pressure DHW tank is also likely to be cheaper than a custom TS even though there isn't a huge difference in the parts count.

 

 

 

 

 

 

 

 

 

Edited October 11, 2016 by Temp

[Declan52]

Think you might need a new plumber as if he can't get his head round it now it's not going to get any better. 

I heat my 350l ts for half the year roughly using the pellet stove. The other half the PV does it via excess electric. PV just for washing machines my God that's some statement. Works the very best. Use the wbs just to heat the living room but if I open all the doors the heat goes round the house. In order to get the rhi payments I wasnt allowed a back boiler in my wbs. When my 7 years are up I will look into changing the wbs to one that has a back boiler. I know that I will have to add a heat dump in the attic and a few extra bits but I can live with that.

Would your truss hold the weight of a 500l ts as that would be a heavy. 

Is there any way to split the system so the ashp looks after the heating so is on low heating the water to 40 degrees and the stove and excess PV looks after the dhw side. Or is this too complicated a setup???

[Temp]

Yes that works. Think it's fairly

2 minutes ago, Declan52 said:

Is there any way to split the system so the ashp looks after the heating so is on low heating the water to 40 degrees and the stove and excess PV looks after the dhw side. Or is this too complicated a setup???

 

I think that what I was suggesting just above.

 

 

[DeeJunFan]

Hi guys,

 

Maybe the split system is what the plumber was angling at but he isn't the best communicator in the world.  And as Nick suggests i was a bit wary when he went to the Merchant to ask for advice.

 

The stove is Room sealed and all that jazz with 2kw to the room and 10 to the boiler so i was happy enough with that.  It doesnt add that much in terms of cost so no real issue with that.

 

Our attic is room in roof so i wouldn't mind the heat dump as it would be nice to get some heat up there from time to time.

 

We have a treble Truss which is 200mm deep so weight bearing shouldn't be that much of an issue but if we were going with small buffer in the utility to run the UFH and then smaller UVC in the upstairs id say we could go 350L using the split system.

 

We have an airing cupboard on the 1st floor but he wasn't sure a tank would fit.  I think it would fit ok, as its 800mm wide from memory.

 

Would say 100L buffer tank be enough to run the UFH? with a 9kw ASHP?

 

Then just do Stove/Immersion/PV to handle DHW?

[Nickfromwales]

Sounds better. 

12 minutes ago, DeeJunFan said:

 It doesnt add that much in terms of cost so no real issue with that.

It adds a lot. More plumbing, more components, you'd still need a generic rad in the top room as well as the dump rad as it'll only dump when the system overheats so you can't use it to heat 'when you like' . 

Open pipe needs F&E, without it you'd be all sealed so a much less complex setup and much easier to integrate things with /  to. 

I'd go for a TS not a buffer ( whilst an uvc is dedicated to dhw ) so you can get cold water uplift and pre heat the water going into the uvc. That'll aid downsizing, and help out massively when Pv isn't generating. 

[DeeJunFan]

4 minutes ago, Nickfromwales said:

Sounds better. 

It adds a lot. More plumbing, more components, you'd still need a generic rad in the top room as well as the dump rad as it'll only dump when the system overheats so you can't use it to heat 'when you like' . 

Open pipe needs F&E, without it you'd be all sealed so a much less complex setup and much easier to integrate things with /  to. 

I'd go for a TS not a buffer ( whilst an uvc is dedicated to dhw ) so you can get cold water uplift and pre heat the water going into the uvc. That'll aid downsizing, and help out massively when Pv isn't generating. 

 

 

I see what you mean.  I wasn't thinking of the plumbing cost just the cost difference between stoves.

 

Rodger on the overheat dump.  I wasn't planning on heating the space really so was just a nice fringe benefit when it happened.

 

You've lost me on the rest

 

Open pipe needs F&E, without it you'd be all sealed so a much less complex setup and much easier to integrate things with /  to. 

I'd go for a TS not a buffer ( whilst an uvc is dedicated to dhw ) so you can get cold water uplift and pre heat the water going into the uvc. That'll aid downsizing, and help out massively when Pv isn't generating. 

 

So i would need a TS fed by the ASHP?  What size would i need?   (Not sure i have space for 2 large tanks)

No idea what cold water uplift is.  and when you say pre-heat do you mean the TS would fed into the UVC?  Ala Jeremys buffer - thermo syphon - TS setup?  I don't know what a thermo syphon is just that its a plumbing component of some sort.

 

Cheers

D 

 

[Temp]

2 hours ago, DeeJunFan said:

I'd go for a TS not a buffer ( whilst an uvc is dedicated to dhw ) so you can get cold water uplift and pre heat the water going into the uvc. That'll aid downsizing, and help out massively when Pv isn't generating. 

 

That's interesting. So it would look something like this if I've understood correctly....

 

DHW

Primary (Presurised):

ASHP -> TS -> UVC

Stove -> UVC (Dump rad and PRV)

Secondary(Mains pressure):

Cold incoming -> Coil in TS -> Coil in UVC -> DHW (Mains pressure)

 

UFH/CH

ASHP -> TS -> UFH (Presurised)

 

PV

PV -> UVC 

 

This would mean the TS pre-warms the water on it's way through to the UVC when a tap/shower  is opened).

TS runs at UFH temperatures so no issues with the temperature uplift (Good ASHP COP). 

The TS and UVC and UFH are all unvented/pressurised and can be dosed with Fernox and something to stop legionaries if deemed necessary. .

The stove heats the DHW but not the UFH.

 

The UVC would/could be 300L-500L and the TS perhaps only 100L ?

 

Have I understood all that Nick? 

 

 

 

Edited October 11, 2016 by Temp

[PeterW]

You can now get some wood stove boilers that will fit into a sealed system just to confuse things .... this is one example 

 

 

http://www.fireplacestoreonline.com/aarrow-stratford-ebw12-woodburning-boiler-stove/?gclid=CjwKEAjwm_K_BRDx5o-sxq6ouXASJAC7TsFLp3F-ZoexXs8MmaMhLk9unC7BoPxEsav-X4dssj-3pBoCLo_w_wcB

 

[DeeJunFan]

This is the stove is was looking at.

 

There is an air supply control unit that can have a TS max set temp and dampen down the fire.  It also says it suitable for a pressurised system?  Is that the same thing?

 

http://www.stovesonline.co.uk/wood_burning_stoves/Woodfire-CX12-boiler-stove.html#entity_7739

Edited October 11, 2016 by DeeJunFan
link

[DeeJunFan]

On 10/11/2016 at 18:36, Temp said:

 

That's interesting. So it would look something like this if I've understood correctly....

 

DHW

Primary (Presurised):

ASHP -> TS -> UVC

Stove -> UVC (Dump rad and PRV)

Secondary(Mains pressure):

Cold incoming -> Coil in TS -> Coil in UVC -> DHW (Mains pressure)

 

UFH/CH

ASHP -> TS -> UFH (Presurised)

 

PV

PV -> UVC 

 

This would mean the TS pre-warms the water on it's way through to the UVC when a tap/shower  is opened).

TS runs at UFH temperatures so no issues with the temperature uplift (Good ASHP COP). 

The TS and UVC and UFH are all unvented/pressurised and can be dosed with Fernox and something to stop legionaries if deemed necessary. .

The stove heats the DHW but not the UFH.

 

The UVC would/could be 300L-500L and the TS perhaps only 100L ?

 

Have I understood all that Nick? 

 

 

 

 

Thank you for the break down?  This sounds like something that could work.  So buffer heated to say 40 and then keep the uvc around the 60 Mark or higher with stove if we go with boiler stove. I'll price both options?

 

@Nickfromwales  how does that sound?

 

Should I do away with the instant water heaters or do they still make sense?

[bassanclan]

Just one point, double check if you are in a smoke controlled area.

If you are and you want a WBS with back buner you have a choice of... the Dunsley Yorkshire!

It is the only stove that meets the DEFRA regulations (if burning logs).

[Nickfromwales]

I rang for some clarification. Here was the ( bloody quick ) reply...

 

 

Nick,

My names Jamie, I'm a heating engineer working for Stovesonline.  My colleague Susi has asked me to contact you regarding a Woodfire CX series boiler stoves.

They are designed to be installed onto pressurised systems.  The boiler is tested to work at a max operating pressure of 3 bar.

The boiler stove is also fitted with a quench coil, this is basically an indirect pipe which has cold water passed through when overheating.  The cold water which is passed through the quench coil is controlled by a overheat valve.  The valve has a probe which is fitted into the boiler stove, if the temperature in the boiler reaches 97degc then the cold water flushes through the boiler stove which in turn cools the stove down.

I've attached the installation and operating manual for you.  Please let me know if you have any questions.

Kind regards,

-- 

Jamie Bennett 
Heating Systems Manager

 

 

[Nickfromwales]

PM me and I'll fwd the pdf of the manual they sent. Alternatively you can request it yourself. . 

The unit needs a waste pipe to drain for the cold water cooling system so factor that in when considering a design. 

[Temp]

 

On ‎10‎/‎11‎/‎2016 at 11:17, DeeJunFan said:

Was thinking of running the downstairs shower/wc with under counter instant water heaters and same for kitchen/utility?  Not sure if thats a good idea or just a waste of money, but wouldn't like the after dinner clean up to take all the shower hot water.

17 hours ago, DeeJunFan said:

Should I do away with the instant water heaters or do they still make sense?

 

The kitchen/utility is going to be used regularly so I would feed it from the UVC to take advantage of the ASHP COP. If it's a long way from the UVC I would install a well insulated secondary loop so hot water comes through instantly.

 

If the downstairs shower is going to get used regularly I would also feed it DHW from the UVC (also with secondary loop for same reason). 

 

I'm not sure about using an instant water heater for the downstairs shower and basin. What sort of flow rate do you want? Personally I like a high flow rate shower and for that you need quite a powerful unit - perhaps 10-12kW although some people are happy with 7-8kW showers. How much PV are you installing? If it was only 4KW you would be importing the rest when using this shower.  

 

Would be interested in Nicks views on this? 

 

PS Size the ASHP and UVC correctly you shouldn't run out of DHW. We just have a 300L TS and haven't run out although it is recharged fairly quickly by our boiler.  

 

 

Edited October 13, 2016 by Temp

[ProDave]

6 hours ago, Nickfromwales said:

I rang for some clarification. Here was the ( bloody quick ) reply...

 

 

Nick,

My names Jamie, I'm a heating engineer working for Stovesonline.  My colleague Susi has asked me to contact you regarding a Woodfire CX series boiler stoves.

They are designed to be installed onto pressurised systems.  The boiler is tested to work at a max operating pressure of 3 bar.

The boiler stove is also fitted with a quench coil, this is basically an indirect pipe which has cold water passed through when overheating.  The cold water which is passed through the quench coil is controlled by a overheat valve.  The valve has a probe which is fitted into the boiler stove, if the temperature in the boiler reaches 97degc then the cold water flushes through the boiler stove which in turn cools the stove down.

I've attached the installation and operating manual for you.  Please let me know if you have any questions.

Kind regards,

--

Jamie Bennett 
Heating Systems Manager

 

 

And what happens to the water heated by the quench coil? does it just go down the drain? How would that work with a ST or treatment plant? What if there's a water cut (rare but possible) so shouldn't it come from a tank?
 

[PeterW]

1 hour ago, ProDave said:

And what happens to the water heated by the quench coil? does it just go down the drain? How would that work with a ST or treatment plant? What if there's a water cut (rare but possible) so shouldn't it come from a tank?
 

 

Yes - mains through the coil and straight down the drain but as its clean water it can go to rainwater or soakaway. I've also seen a thermal store for connection to a stove with a similar arrangement.

[Nickfromwales]

On 11 October 2016 at 18:36, Temp said:

Have I understood all that Nick? 

Almost   

On 11 October 2016 at 18:36, Temp said:

Cold incoming -> Coil in TS -> Coil in UVC -> DHW (Mains pressure)

Would read :

Cold incoming -> Instantaneous DHW Coil in TS -> Cold inlet in UVC -> DHW.

 

The cold uplift is basically just reference to raising the incoming cold mains temperature before it gets to the UVC, so if the buffer was at 40oC and you ran a shower at 38oC you'd expect not to deplete the UVC of energy at all. Instead the ashp would bear the brunt, which would be the better option vs reinforcing via grid electric ( if Pv is unavailable / insufficient ). Lighting the stove purely to get hot water would be a pita, so the the ashp would be connected via a dedicated coil to always make dhw production just a matter of flicking a switch. 

I've run off a few designs ( and revisions of ) for member @readiescards which originated around a similar remit. After a very brief look at the UVC + TS setup I quickly realised there was an Achilles heel to that combination. ( at the time there was a wbs with back boiler in the remit also ). The issue I kept getting stuck on was the fact that the UVC may well be sat above the pre heated water delivery temp, so if Pv had given you a very hot tank full of DHW and you drew water through the pre heat it would cool the UVC and have an adverse affect. It would only be useful heat if the UVC was sat at ~45oC where the differential then wouldn't be quite so problematic. 

With a stove, and with Pv, I don't think the dhw medium should be an UVC tbh, it's just not suitable IMHO. I'd go for a dual TS setup with the upper one being at least 500ltr. This would have a dhw coil which serves the house dhw directly. It would also have a coil for the ashp to heat it. I'd then have a third point ( flow and return tappings not a coil ) to remove heat, designed with flow to be high and return low in the TS. The plot thickens .

The ashp would feed a ( typically ) low temp second TS ( ~200 ltrs ) with a coil for cold mains uplift, but the primary purpose of that TS would be for providing space heating. It would also have another coil to accept excess heat  from the upper TS ( from the third coil ). Basically this setup would see little or no wasted heat as the secondary TS would then double up as a heat battery.

As the stove mentioned is suitable for a sealed system, you can be more creative with it under 'overheat' condition.

So, you light the stove and the primary TS gets up to target temp. The circuit starts to peak out at 90+oC and then you soon have two choices, water cooling kicks in or heat gets dumped. Obviously heat dump is a waste in typical situations, but here it becomes advantageous as you have the means to shift excess heat from the primary TS to the secondary TS and max that one out too. A cylinder stat 3/4 of the way up the primary TS, set to around 65-70oC, would bring a pump on to shift the excess heat to the secondary TS which should keep the stove away from the realms of assisted cooling ( waste dump ). 

As the stove would typically only be used for space heating and water production when space heating is required, why not make the most use of the wood burning events? With both TS's complimenting each other ( primary charging secondary and secondary providing uplift for the cold mains ( instant ) dhw coil ), you'll have enough stored heat to give Ufh through the night and have a tank of hot water for showering in the early morning. 

I don't see the point in having a wbs of lighting it is going to quickly produce heat in excess of the immediate demand, and then you end up dumping the excess. If I was going to the lengths of fitting a system to accept wood burning, and I was going to fit a buffer for the Ufh, then I'd just upsize the buffer, make it a TS, and make some meaningful use out of it  

Another upshot of this setup is that when excess Pv roofs out the primary TS it'll still pump down to the second TS, so this excess storage solution works with both the wbs and the Pv.  

Another consideration will be expansion for the entire sealed system. Both TS's would require significant expansion volume for max temp tolerance so that's another thing to factor in accordingly.  

With a bit more thought maybe there is a better way to control the relationship between the tanks, but I'll see what critique this gets before I 'fine tune' it   

 

Waste water from the boiler as per peters comment, so no issues there. 

@ProDave

If there was a water outage, burst main getting repaired I suppose is the only way for that to happen, then the boiler would over heat the TS and eventually the T&PRV on the TS would open ( at ~90oC ) and discharge the water to the tundish and D2 pipe to outside. Wouldn't be pretty, but it would deal with it. I think I'd want the D1 in 22mm ( upsize from 15mm to 22mm immediately after the T&PRV which are typically only ever in 15mm ) and then on to a 22x28mm tundish with 28mm or 35mm D2 to drain. 

Adding a cold mains accumulator would give you a reserve of pressurised cold water, so cold mains failure is addressable should it ever be a concern. 

[DeeJunFan]

@Nickfromwales

 

 

[Nickfromwales]

Tit.

pmsl.