Plumbing system DHW design conundrum

[SimonD]

As is fairly typical my architect 'forgot' to put much thought into the aspect of plumbing in our house. I've been thinking about is for ages trying to find a good solution, but I'd like to ask for some input here, please. It's really about the DHW distribution.

 

My problem is that the DHW demand is from two opposite ends of the house. At one end I have the main family bathroom with a bath and then a guest bathroom with shower. At the other end  of the house I have an ensuite to the master bedroom, a utility room and kitchen above.

 

The current plan is to install an Alpha E-tec boiler with heat recovery unit and thermal store within, or next to the family bathroom. This is partly to do with incoming main at this very corner of the house.

 

If it's relevant, I have 7 bar standing and 3.5 bar dynamic mains pressure, 22 l/m flow rate. This is one of the reasons I'm using the Alpha heat store as it can take up to 5bar mains pressure and I can use everything I've got in terms of flow (DHW flow rate on boiler is up to 19 l/m). Its setup is also more compact than system boiler with water tank (something which the architect overlooked in his designs - and I also mistakenly overlooked at the time too!)

 

The initial sketchup of the DHW distribution was to run to a manifold which sits within the utility room and then distribute from there (at this location the manifold is more or less central in the house). The problem I've got is that this creates relatively long total runs bringing DHW back to the family and guest bathrooms. However, I'd obviously then end up with a very long DHW run from boiler to kitchen sink too. For example, in terms of meterage, I'll have 22mm or larger DHW to manifold which could be about 16m if not longer, then 15mm pipe back again to bathroom for each fitting, another approximately 16m each. The kitchen supply would only be about 4m beyond the 16m 22mm to manifold.

 

Having thought about this, I'm not convinced this is very good due to the long runs, so I'm wondering which way is best to get around it. I've got a space problem where I'm looking to install the boiler.

 

2 things I'm considering are

 

1. Whether it's better to install a manifold at the boiler to split the supplies to each side of the house and then use distribution manifolds to each of the relevant rooms once it gets there. I'm tempted to also put in a solitary 10mm plastic run directly to the kitchen sink from the boiler location, again to reduce hot water wait time and wastage.

 

2. To scrub the planned single boiler and instead install two smaller combi boilers at each end of the house - is this a crazy idea? (price wise, it's actually pretty much equal to installing the full Alpha kit and would save me money in terms of manifolds). Boiler labour cost of installation is free here for me, so not part of the equation. From a heating perspective, this could actually work quite well as I'd use each boiler for the main house zones.

 

Any thoughts and advice?

[Nickfromwales]

2 boilers is just way overkill. Scrap that idea  

Ok, does the alpha have a secondary hot return connection, or will they allow you to feed that back into the cold inlet? If you have no option for hot return then your problems can only be managed by reducing pipe size to outlet and getting rid of the huge amount of 22mm supply pipework ( as that is where all of your major dead-leg issues will stem from, not the pipe runs ). Centralise the boiler and manifolds wherever you like, and run a HRC to the furthest most outlets to suit. You can run 22mm feeds to 2x DHW manifolds either ends of house for max flow rates, that would be best case for flow, but they would need to be serviced by an HRC and be worst case for efficiency. 

[markocosic]

I'd fit the gear next to the two bathrooms.

15 mm pex to each bathroom, running past the appliances (sink first, shower last, so that the sink responds fastest), and lagged.

 

I'd run 15 mm pex to the other end of the house too. Either live with the matter ensuite pressure dropping if the kitchen/utility are used and share. Or run two pipes one to ensuite and one to kitchen.

 

In kitchen hot fill the dishwasher and tee kitchen tap before dishwasher so that the pipework up to it is warm more often than not. 

 

14 metres of 15 mm pex gives you a 1.5 litre dead leg. That'll clear in a few seconds..9 secs at 10l/min. 18 at 8l/min. Again lag it the whole way. Check pressure drops at desiredflowrates. Sounds like you have plenty available.

 

 

If you want to do better then you'll need hot water recirculation and a single 22 pex across the house. Else 22mm is dumb as it comes without recirculation. You'll be waiting an age for hot water.

 

 

Electric trace heating to kitchen tap is the other inefficient option. Depends whether you use a dishwasher out not...

 

The volumes that matter for response are between heat source and the outlet. 

 

[SimonD]

15 hours ago, Nickfromwales said:

2 boilers is just way overkill. Scrap that idea

 

Haha, yes, I thought it might be a bit over the top!

 

15 hours ago, Nickfromwales said:

Ok, does the alpha have a secondary hot return connection, or will they allow you to feed that back into the cold inlet? If you have no option for hot return then your problems can only be managed by reducing pipe size to outlet and getting rid of the huge amount of 22mm supply pipework ( as that is where all of your major dead-leg issues will stem from, not the pipe runs ). Centralise the boiler and manifolds wherever you like, and run a HRC to the furthest most outlets to suit. You can run 22mm feeds to 2x DHW manifolds either ends of house for max flow rates, that would be best case for flow, but they would need to be serviced by an HRC and be worst case for efficiency. 

 

Yes, the Alpha FlowSmart has a secondary return, or as they call it a secondary circulation coil as it simply reheats the return through the thermal store and feeds it back into the DHW flow. However, I've just had some feedback on Alpha suggesting I'd be better looking elsewhere, which, in terms of storage combi, takes me down the route of either Vaillant or Veissman (although the current thread on here about Veissman doesn't fill me with great confidence) so I'll be checking HRC capability of both these.

 

I ideally I would install the boiler centrally in the house in utility room. The problem is that the most favourable wall for the flue terminal would come out under a bridge and too close to it. It also has both door and windows too close. Another choice is to try and route it across or around the ceiling of the master bedroom and out an alternative wall - this would give a flue terminal that is under the required regs for distance to neighbours boundary and fence, but the neighbour is troublesome - my wife's reminded me that any plume is gonna cause complaints - a 45 degree diverter might work, but if it touches his fence in any way, I'm in trouble ?

 

Thanks, I'll have to look at redisigning for HRC but I do think that if I can get the boiler in the utility, that would be best..

[SimonD]

2 hours ago, markocosic said:

I'd fit the gear next to the two bathrooms.

15 mm pex to each bathroom, running past the appliances (sink first, shower last, so that the sink responds fastest), and lagged.

 

I'd run 15 mm pex to the other end of the house too. Either live with the matter ensuite pressure dropping if the kitchen/utility are used and share. Or run two pipes one to ensuite and one to kitchen.

 

In kitchen hot fill the dishwasher and tee kitchen tap before dishwasher so that the pipework up to it is warm more often than not. 

 

14 metres of 15 mm pex gives you a 1.5 litre dead leg. That'll clear in a few seconds..9 secs at 10l/min. 18 at 8l/min. Again lag it the whole way. Check pressure drops at desiredflowrates. Sounds like you have plenty available.

 

 

If you want to do better then you'll need hot water recirculation and a single 22 pex across the house. Else 22mm is dumb as it comes without recirculation. You'll be waiting an age for hot water.

 

 

Electric trace heating to kitchen tap is the other inefficient option. Depends whether you use a dishwasher out not...

 

The volumes that matter for response are between heat source and the outlet. 

 

 

Thanks for the suggestions, and makes a lot of sense. Interestingly, Emmeti make a suggestion to run HRC through their manifolds and simply t-ing off to the individual outlets from there. I'm not sure if this is the best of both worlds, or the worst. They also recommend max 12m 15mm pipe from manifold to outlet.

 

Having measured a little more, if I run the supply pipe diagonally across the house (just about possible as I have a service void under the ceiling joists), I can get a run from boiler to kitchen sink and ensuite shower down to about 17m.

[markocosic]

Who is designing (liable) and what are their objectives? (comfort / opex v capex / ethics & efficiency / future proofing)

 

 

Start with outlets. What are your flowrates (blended, hot, cold) and time to temperature requirements? (e.g. I'm fine to wash hands in anything >20C and always dishwash so don't give a monkeys about the kitchen tap hot water whereas SWMBO would freeze to death if washing hands at <35C and insists on washing kitchen oddments by hand in hot water and scattering them about their place)

 

Washing machine and dishwasher and toilets can have negligible flowrates.

 

Then the fixtures. What dynamic pressure do they require? Some are well designed and happily operate from 0.5 to 5 bar. Others are garbage and need 1.5-3 bar else they barely dribble or make a racket.

 

Then diversity. What needs to run simultaneously and (how far) can the flowrates drop when multiple outlets are in use? (e.g. kid brother runs two showers off a 35 kW combi - 10L/min each individually or 7.5L/min each if simultaneous is fine by him given how rarely this actually occurs; bathrooms in use together being rarer than fixtures in use together.

 

Then heat source. If you're in any shape or form interested in future proofing you'll have stored hot water of some form (tank or thermal store) rather than a combi. That will dictate positioning. 

 

Combis are cheap. Combis can also heat tanks in addition to supplying direct, and your don't care about the distance from combi to tank quite do much as you do combi to outlet. Tank for bathrooms so that you can utterly deluge a bath or shower at your full incoming mains flowrate. (store the water at 45-48C so that you don't need thermostatic mixer on bath, then use taps designed for a gravity pressure system that can do 30+ litres/minute at 1 bar, fed from you 22 mm off the tank) Combi then to supply ensuite and kitchen/utility directly with neverending supply. Like two boilers but not two boilers. Downside is heat losses in non heating season (from tank and pipework).

 

When gas gets banned or ethics are a consideration you switch to heat pump to the tank and a long pipe run in 15 mm pex to ensuite/kitchen. Fit a temperature controlled and timed secondary return to this so that you're not waiting too long in mornings without keeping it hot unnecessarily overnight and whilst out etc. Combi probably has a keep warm timer on it too.

 

Nothing wrong with viessmann boiler itself. Just their UK operations. The other thread was using the incorrect settings to control the boiler. Changing the setting (after also changing every part on the boiler pointlessly) addressed the issue.

 

 

 

 

 

[SimonD]

1 hour ago, markocosic said:

Who is designing (liable) and what are their objectives? (comfort / opex v capex / ethics & efficiency / future proofing)..

 

.. whereas SWMBO would freeze to death if washing hands at <35C and insists on washing kitchen oddments by hand in hot water and scattering them about their place)

 

Here you have hit the nail on the head.

 

I'm designing, and ultimately I'm liable, yet I have some high standards to obtain that don't involve too much compromise! ? SWMBO requires ready and instant access to hot water at the kitchen sink and basins for regular use (I do too as in our last place hot water supply to the kitchen sink was terrible, you could go and have a cup of tea waiting). I've also got to ensure we can run 2 showers, a bath, the washing machine and dishwasher simultaneously, together with the odd basin without a drop in flow or temperature and it's got to recover immediately so anyone left waiting in the queue for any of those facilities doesn't have to wait. I'm sure these requirements aren't too unusual...are they? ?

 

Joking aside, I'm looking for a comfort/environmental/efficiency balance. Capex/Opex is again a balance here, however, right now the Capex required for ASHP on this project is too high, even after self install over MCS/RHI route. We're a family of four and we do get through a lot of hot water. Fabric energy efficiency is relatively good with a calculated required heat input of just over 7kW plus DHW.

 

From a water use perspective, I'm looking to reduce dead-leg wastage as far as possible, while also reducing overall volume of water consumption. I will be fitting flow reducers to outlets, especially the showers (x3). With our respective routines there's likely to be simultaneous use of 2 showers on a relatively regular basis, but it's mostly going to be like one shower plus washing machine and dishwasher and maybe a basin. Bath will be used daily, twice daily 2 or 3 times a week, but filling time isn't a priority and it's unlikey to affect showering.

 

By my reckoning a DHW flow capability of up to 18 - 19 l/m is going to service us fine.

 

I've been investigating options today and changed my mind about the system configuration. I've identified 2 potential spaces where I could fit in a cylinder, both spaces resolving the potential dead-legs and avoiding hot water recirculation. One of them is right in the middle of the house and can be integrated into an airing cupboard, which my wife has said she'd really like. If I installed a 210l thermal store here, I could go either heat only or system boiler rather than combi. This way I'd also retain the backup of immersion heater.

 

As for future proofing, I'm a little cynical here. I've worked in the tech industry since the mid 90s and don't think you can really 'future proof' any development. I'm making the decision based upon now and will review in 7 - 10 years, or if things move really fast, maybe even 5 years. If we're still here..otherwise it'll be a different project ☺️

[PeterW]

7 minutes ago, SimonD said:

If I installed a 210l thermal store here,

Make it bigger if possible - if you can get a 300 or 350 with a coil rather than a PHE you have no moving parts either.  

[dpmiller]

Don't discount wastewater heat recovery on the most frequently-used showers too...

[markocosic]

Central cylinder with all runs to fixtures meeting AECB guide will work nicely. (nothing 22mm unless serving bath fill taps capable of high flowrates)

 

If incoming DCW is 22l/min and you're expecting DHW of 18-19l/min then you may have a problem though.

 

What are your blended flowrates?

 

What are the DHW and DCW flowrates needed to meet these?

 

DCW temp varies. Assume 10C. Blended temps vary. Assume 40C. DHW production temps are flexible. Assume 45C for HP or 55C for gas. (being conservative)

 

40C shower uses about 86:14 in HP scenario or 66:33 in gas scenario. So you have enough DCW to satisfy your wishes or would you be putting in DHW capacity that goes unused?

 

https://www.spiraxsarco.com/resources-and-design-tools/calculators/water-mixing/water-mixing

 

 

 

 

[SimonD]

3 hours ago, markocosic said:

Central cylinder with all runs to fixtures meeting AECB guide will work nicely. (nothing 22mm unless serving bath fill taps capable of high flowrates)

 

If incoming DCW is 22l/min and you're expecting DHW of 18-19l/min then you may have a problem though.

 

What are your blended flowrates?

 

What are the DHW and DCW flowrates needed to meet these?

 

DCW temp varies. Assume 10C. Blended temps vary. Assume 40C. DHW production temps are flexible. Assume 45C for HP or 55C for gas. (being conservative)

 

40C shower uses about 86:14 in HP scenario or 66:33 in gas scenario. So you have enough DCW to satisfy your wishes or would you be putting in DHW capacity that goes unused?

 

https://www.spiraxsarco.com/resources-and-design-tools/calculators/water-mixing/water-mixing

 

Thanks, will be avoiding 22mm feeds.

 

I perhaps should have elaborated further on the figures I gave earlier. It's an existing property and currently the only tap that can provide a good indication of flow is teed off from the incoming main inside the house, running about 20 meters to an outside tap on the side of the garage. Only tap available internally gives a rubbish flow. In addition, our main is 15mm lead pipe which we know is at least 25m to the road. When I get round to doing the landscaping, I'm going to be digging up a row of hedges and when I do this I'm probably going to upgrade the mains as it currently runs more or less under the hedges. This should improve our flow rates. (I know that I should do this now but budget wise it really has to wait).

 

The measured 22l/min is not far off the theoretical 27l/min I anticipate a 15mm main can deliver (at current pressure), but may I've got that wrong?

 

I'm not expecting 18-19l right right but want a DHW system that's capable of it.

 

I'm aware of the variations in both DCW and DHW temps and the relationship between these and flow rates from combi boilers, but the blended calc figures are very helpful, thanks. Blended output of the thermal store I've identified is set to 50 degrees C at manufacture but is adjustable. The figures for volume of DHW delivery by the thermal store are calculated at 43 degrees C.

 

I think I've now arrived at an initial configuration for boiler and thermal store. Noting @PeterW's comment about heat exchanger, I've found an indirect store from Advance Appliances using coil heat exhangers. It's also one of the better performing from a heat loss perspective. Looking at boilers, Baxi 800 is at the top of my list right now as it fits the MIs for the Advance thermal store: non-modulating, 24kW if I settle on 210l cylinder. It's also IPX5D so can be installed fine in the bathroom including zone 2.

 

As an aside from the DHW, my only concern relates to the boiler heat output. I recognise I've got to look at things differently when using a thermal store, but with my house heat loss calcs calling for 7kW, I'm wondering whether at 24kW I won't be over sized and this might cost me? (The 24kW is recommended by Advance for recovery rate on the 210l tank).

[markocosic]

Don't buy a non modulating boiler. There's no reason not to have over that can dial itself down for when it is in space heating mode.

 

Personally I'd KISS with an unvented cylinder. The pressure drop on the DHW side will be lower so better flow. And they're cheaper. Go for one with a big coil in it though, and check what the power it can accept at say 70/50 flow return is rather than t he e old school non condensing 82/71C.

 

Personally I wouldn't be fussed about reheating at 24 kW (almost as quickly as it is used). Go bigger rather than faster. 210L is nothing. Reheating at anything over 12 kW ought to recharge it more than fast enough.

 

Don't set to 50C. You need (or should have according to regs) thermostatic fixtures for bath fill if you do this. (Low flowrate) Set to 48C and have no thermostatic fixtures at all. (again kiss)

 

Do look at the fixtures - something like a bristan 1901 bath filler (for gravity systems) has 3/4" inserts not little ones and can do bath fill at over 100 litres/minute of you've got more than one bar available at the fixture - whereas cheapie high pressure only fixtures all have widdly little cartridges.

 

 

In the interim before mains upgraded:

 

https://product-selection.grundfos.com/products/up-a-home-booster

 

Totally illegal to direct fit this these to the mains without asking the water board...just in case you drop the line pressure enough to suck in nasties upstream...but for adding 1+ bar at up to 30L/minute the 15-160 works well and for 0.8 bar at up to 15L/min the 15-120 works well. (e.g. the latter overcomes the 0.6 bar pressure drop in most combi boilers at their rated flow)

 

Common in Europe where regs less strict.

 

[Oz07]

Has anyone here just comsidered a boiling water tap and doing away with dhw to kitchen 

[SimonD]

3 hours ago, markocosic said:

Don't buy a non modulating boiler. There's no reason not to have over that can dial itself down for when it is in space heating mode.

 

Clearly I need to do a bit more research to understand the MIs specifiying non-modulating boiler as the preference for a well functioning thermal store. It does take me a little bit by surprise. My limited understanding is that modulation has primarily been introduced to reduce boiler cycling, which is the main source of inefficiency in the boiler. I can only surmise that the thermal store as part of the system helps to reduce cycling.

 

Aligned to this I guess is the question about how much the modulating boiler would actually add to this system efficiency wise as I suspect that looking at designing CH return temps to ensure the boiler is condensing may yield similar, if not better, results overall.

 

For me, the biggest concern around this is over-sizing the boiler as seems to be a common affliction here in the UK and will significantly affect energy consumption and efficiency of the system.

 

3 hours ago, markocosic said:

Personally I'd KISS with an unvented cylinder.

 

From reading a number of threads on various forums, it does seem like thermal store v unvented is a bit like marmite. Lots of passion either way.

 

In our last place we had an unvented system installed. The installer was an idiot who didn't seem to understand even the fundamentals of unvented systems, so we embarked on a period of rectifying the problems. The original install was worse than the gravity vented system we replaced, and once all resolved, it just about performed as the gravity system. The only benefit we saw was when showering without anything else drawing off water (should have just put a pump in). This was even with 22mm mains supply and above required pressure. No, I didn't skimp on any part of the UCV install rectification - it cost me a packet in the end - much of it down the drain. I don't necessarily think that UVC is simpler than the thermal store approach and I don't have to have G3 or get it serviced every year, or have a G3 in when something goes wrong.

 

Yes, I know there are some claimed disadvantages to thermal stores such as standing loss, and draw off temperature reduction but from what I can see, the standing losses apply just as well to UVC and the thermal store manufacturers provide pretty good data to estimate the temperature loss in relation to draw off volume.

 

3 hours ago, markocosic said:

210L is nothing.

 

I agree, but I've now run a couple of calculators from manufacturers both of which suggest 210l thanks for our house. It's interesting that the stores I've identified only go up to 250l for my planned configuration.

 

3 hours ago, markocosic said:

Don't set to 50C. You need (or should have according to regs) thermostatic fixtures for bath fill if you do this. (Low flowrate) Set to 48C and have no thermostatic fixtures at all. (again kiss)

 

I appreciate this suggestion, but again I've now read multiple manuals and articles by different thermal store manufacturers, all of which recommend draw off temps of around 50 - 52 but one even as high as 55. 48 is not much below this, and I can obviously adjust down to this no problem. ?

 

3 hours ago, markocosic said:

Do look at the fixtures

 

Thanks, will do

 

3 hours ago, markocosic said:

In the interim before mains upgraded:

 

https://product-selection.grundfos.com/products/up-a-home-booster

 

Totally illegal to direct fit this these to the mains without asking the water board...just in case you drop the line pressure enough to suck in nasties upstream...but for adding 1+ bar at up to 30L/minute the 15-160 works well and for 0.8 bar at up to 15L/min the 15-120 works well. (e.g. the latter overcomes the 0.6 bar pressure drop in most combi boilers at their rated flow)

 

Common in Europe where regs less strict.

 

TBH, I'd prefer to avoid anything like this that contravenes the regs. I'd be happy to look at a buffer tank if we do have a problem before the mains upgrade.

Edited July 25, 2021 by SimonD

[markocosic]

If your boiler can't modulate down to meet minimum load then you need to introduce thermal mass. (such as a thermal store) For example: wood burners, some oil boilers, ye olde gas boilers.

 

If your boiler can modulate down then you have no requirement for a thermal store.

 

Design CH for a maximum FLOW temperature of up to 55C (even with rads) and you can't fail to condense in operation even if the balancing is awful.

 

Well designed gravity systems will spank unvented cylinders for everything but high pressure drop showers on upper floors. They don't work well over you start adding fixtures designed for high pressure (most mixers on market today) or fitting low flow fixtures onto pipework sized for high flow (again most fixtures on market today) or indeed fitting low flow fixtures full stop (pressure drops at the velocities needed for fast response are too large on smaller pipes)

 

Also plumbers that aren't numerate, which is most working on the domestic sector, can't design gravity systems. Flowrates, velocities, and pressure drops are beyond them.

 

Mixed systems (gravity cold tank feeding the house via a booster set) spank everything. You're in total control of your supply at this point.

 

Direct mains fed unvented cylinders and direct mains fed heat exchangers (combi or thermal store) are a workable compromise in most situations. Unvented cylinders have low pressure drop on the potable water side and higher on the heating side. Thermal stores are the opposite. Your application suits the former. Just because the previous install sounds like a fudge up (I keep banging on about starting from the fixtures and blended flowrates for a reason - you probably had naff taps (low flowrate/high pressure drop) on oversize pipe (causing slow response but you dare not downsize because flowrate was poor and plumber will always blame pipe) doesn't make this one the same - and the thermal store will only be worse than the unvented in terms of pressure/flowrate/response.

 

G3 isn't worth the paper it's written on IMO. The mucking fuppets can't even fit the systems right let alone inspect for proper operation. Fair enough if you actually intend to pay for the paper each year then yes there will be a saving. Most don't service unvented cylinders or boilers to no ill effect.

[markocosic]

The reason they recommend higher setpoints is thermostatic mixers by the way 

 

Cheap thermostatic mixer showers can't create 40C water using hot at 45C and cold at say 10C. They need 10C "headroom" on the hot supply temperature. So 50C to generate 40C at the outlet.

 

Add in heat loss en route from big old uninsulated pipework or pipework direct buried in a masonry wall (e.g. shower feed) and you can find that a supply of 55C is needed to hit 40C blended.

 

Avoid poor quality fixtures and bad pipe routing and you can safely drop to 48C.

 

It's bum covering on their part not a technical requirement in all scenarios.

 

Safety and efficiency aside (the cooler the less risk, the higher generation efficiency, and the lower the losses), you also get a response benefit by running the DHW cooler. Making 40C out of 10+45 rather than 10+55 means a greater fraction of hot water and means the hot pipework purges faster; thereby improving responsiveness.

 

I work in the district heating sector and plumbers - including the NHBC whilst a prize muppet by the name of Peter Gray used to be in charge of heating - specifying 22mm pipe and 55-60C delivery temperatures then whining about response time are one of the banes of my day job. "Turn it down to 45" is a standard band aid to improve response. (and safety/efficiency) You're always fighting backside covering equipment vendors and unthinking 'more is better' specifiers though.

 

Fire over fagpacket schematic showing fixtures and runs and we can size the flowrates and pipes on this thread ?

[Nickfromwales]

Hmmm.

 

A 250L TS services by a 37kW modulating boiler would just about be suffice eg for the list of hot water dependant devices you list earlier in the thread that you wish to run nigh-on simultaneously. That it a BIG ask of DHW flow rates. That setup would promote almost instantaneous reheat of the cylinder, with a great degree of heat transfer from burner to DHW. Also, a 28mm DHW coil is an absolute no brainer, forget 22mm.

 

I would fit a bigger TS ( minimum of 300l ) or a 500L UVC. If you go UVC you 'could' go for a smaller boiler, but the cost increase is negligible for what extra oomph you're going to get, plus with a bigger max output, even at 37kW, you'll still be able to drop to 7kW-9kW fully modulated. The boiler will only ramp up to suit the demand, and, with your remit for DHW, this is the minimum system specification I would ( and already have done multiple times ) specify and install. Anything less and your wife will be chopping your two best friends off........ Thank me later  

 

To get away with a smaller boiler, if you're feeling brave, you would 100% need to configure the system for W-plan aka hot water priority where the boiler > cylinder / heating zone valve is a 3-way DIVERTER valve giving heating OR DHW, never both mixed; ergo the boiler max output would be dedicated solely to reheating the cylinder of choice ASAP.

 

The cold mains will need to be 28mm, from the mains to the UVC control group or TS cold inlet and cold manifold, where the 28mm pipe would then tee off into 2x 22mm feeds to each ( the DHW device and the cold manifold ). In terms of flow rates you are mentioning both types of DHW devices ( TS & UVC ) which are both COLD MAINS DEPENDANT. If you do not fit a minimum of a 300L accumulator ( or 2x 150L if space is difficult ) you will never get the earlier requested DHW performance, not even a chance.

 

In respect of the hot return circuit ( HRC ), you won't manage without it. That is because you will have 28mm primary hot pipework, cascading down to 22mm going to a 3/4" DHW manifold, and then your 15mm and 10mm radial feeds coming from that. The runs from the manifold may seem short enough to give the confidence to abandon the HRC, but the dead cold leg before the manifold will soon shatter that dream. Fit the HRC, insulate, insulate, insulate, and again, your wedding tackle remains to live another day. You could just run the HRC to the end of the DHW manifold, and just one branch to maybe the kitchen sink, and the ensuite basin tap.

 

Enjoy!!

Edited July 25, 2021 by Nickfromwales

[SimonD]

Wow, okay, so lots for me to digest here! Much appreciated.

 

One question that does immediately arise from both or your comments, @markocosic and @Nickfromwales is gravity fed DHW. In your views, have I overlooked this as a valid option? At least this decouples DHW from cold mains feed and can be supplemented by pump/s where necessary?

 

Thanks.

[Nickfromwales]

Just now, SimonD said:

Wow, okay, so lots for me to digest here! Much appreciated.

 

One question that does immediately arise from both or your comments, @markocosic and @Nickfromwales is gravity fed DHW. In your views, have I overlooked this as a valid option? At least this decouples DHW from cold mains feed and can be supplemented by pump/s where necessary?

 

Thanks.

The cost and annoyance of pumps off an open vented system, with additional pipework, elevated 50 gal or 75 gal tanks minimum, maintenance and then segregation of cold and hot pumped feeds throughout the house……..

vs one pipe to an accumulator with no moving mechanical parts, no electrical connection, and silent in operation. 
?

Sealed and pressurised systems will have very little heat losses by comparison. You’d also need multiples of pumps and probably 2x 50 gal or even 2x 75 gal open CWS tanks in the attic / races space. They would have ball valves which makes noise whilst SLOWLY refilling the tanks = piss you off if it’s near a bedroom. 
???

[Nickfromwales]

Oh, I forgot to say……..

 

 

 

 

 

 

???????????

[markocosic]

Hybrid: Fill a tank from mains. Pump from tank into your dcw/dhw. Rely on pump for pressure. Total independence from incoming mains so works even if incoming pressure is gash-tastic. (e.g. leaky pipe London where the water board dial down delivery pressure to 1 bar) You have a tank open to air to keep clean though.

 

Accumulator: Fill a balloon from mains. Rely on balloon to sustain pressure. Reliant on mains pressure to fill balloon and drive flow. (e.g. countryside where pressure is high but small pipes mean low flowrate) No tank open to air but won't work if incoming mains pressure is gash-tastic.

 

I wouldn't do gravity. I just said that it often works well...if the fixtures are correct...to highlight the need to start with fixtures then work backwards.

[markocosic]

You don't open vent the DCW and DHW Nick. You do a "break tank" off the mains (so that you're allowed to pump) and feed the whole house with boosted DCW.

 

Tank can be sited anywhere. They're usually in basements (of blocks of flats) not in roof spaces.

 

You can't do that with an accumulator setup because you're not allowed to pump the incoming mains.

 

Accumulator setups are cleaner if you have high pressure low flow mains.

 

Boosted setups are the only option if your mains is low pressure.

[markocosic]

Do draw the schematic and I suspect you'll be fine without dhw recirculation - depends on your desired flowrates and wait times.

[Oz07]

This is like a heavyweight battle. The Welsh Dragon has his belts on the line but the up and comer is giving as good as he gets. It would help if I understood any of what is being said!

[Nickfromwales]

1 hour ago, markocosic said:

Boosted setups are the only option if your mains is low pressure.

I have loads of them in domestic and light commercial installations working perfectly well. Zero pumps. You just size for the potential of the main, usually taking advantage of the night time and midday increases in static pressure as the main goes off peak. Non return valve captures and holds that for use therein.

 

1 hour ago, markocosic said:

Reliant on mains pressure to fill balloon and drive flow. (e.g. countryside where pressure is high but small pipes mean low flowrate) No tank open to air but won't work if incoming mains pressure is gash-tastic.

Got one install in the arse end of the green belt. Shite cold mains, so 2x 200L accumulators ( so I could fit them in eaves spaces ) on a job with a knock-though extension with them adding in another ensuite. Guy was gobsmacked at how much improved the system was. For completeness, in this house they had so poor a cold mains that the existing UVC could not push hot water upstairs to the bathroom if the kitchen tap was open. You could literally hear the water going down the pipe / air being sucked in through the bath tap when you opened it. Swapped them out to a Vaillant 937 high flow combi, after about 5 other plumbers said they had to keep the UVC and forget ever thinking about a combi. 8 years that's been in, for my mates boss.

 

1 hour ago, markocosic said:

You don't open vent the DCW and DHW Nick. You do a "break tank" off the mains (so that you're allowed to pump) and feed the whole house with boosted DCW.

Fully aware of that, but you still need segregation and a LOT of downstairs space. Still noisy, still very slow to refill / recover, and you need a large expensive pump to do what this member is trying to achieve. I'm advising as to this thread, not for general information  For the OP's solution, they would need at least 3x 50 gal CWS. So where are they going to go?

 

 You also need to treat water in the break tanks as non potable in the real world, so bathing / not for human consumption only.

Edited July 25, 2021 by Nickfromwales
Had to change my boxing gloves :D