CH/DHW design

[chrisb]

Some may have seen my thread(s) elsewhere about our extension. Relevance here is to UFH installation, and also new space for boiler +/- something to store hot water in.

 

Reading through the threads here and on the old place, I think I have four options, and I think I know I'd be best with a thermal store, but I'm losing the capacity to think objectively and would welcome some advice please.

 

So we have what is now a 2.5 bedroom semi and will become a 4.5 bedroom semi. Currently one shower-over-bathroom and another shower-only room. We are a family of 4, with 2 young boys.

Current showers are thermostatic (when not scaled up) bar mixers, both separately pumped. We also have a pump on the DHW for all but the shower room sink, otherwise 1. My fancypants kitchen sink tap only dribbles, and 2. the utility DHW isn't much better. The existing plumbing at the time made it easier to also pump the bathroom sink and bath DHW.

 

DHW is fully pumped open vented with cylinder in airing cupboard in main bedroom.  I converted this from gravity DHW, pumped CH a few years ago and never rearranged the feed and expansion pipes properly. Basically, I want / need to replace it, both for plumbing and space reasons. The boiler is a Potterton Kingfisher, so that's leaving too.

 

Current heating is by panel rads in back bedroom and 1/2 size bedroom (both childrens rooms) and towel rail in each bathroom. Our bedroom has the rad disconnected as I never bothered replacing it, and we never have it on anyway. Lounge has a panel rad (with a sofa in front of it). Kitchen has a wet fan heater (temporarily installed for now, and highly effective). Also tiny rad in hallway (possibly a bit undersized, but can't easily fit a bigger one).

 

We can run out of hot DHW fairly easily if we both shower and bath kids, or do big washing up in a short timeframe, so I'd be keen to improve that if possible.

No water softener now, but I think essential to go into replacement solution.

Mains water I have just measured to be 3 bar static pressure with ~20l flow at 1 bar dynamic pressure. Seems pretty good?

 

Large ground floor rear and two-storey side extension going on, so I'm expecting heat losses to reduce due to improved insulation in new external walls and floors. UFH is going into the slab in all new floors, and I'm hoping to take up the existing kitchen floor to do the same there. (In an ideal world, I'd take up all the ground floor, but not going to happen now).

 

Option 1: Keep open DHW and CH, with buffer for UFH. Relocate DHW cyl to utility / garage in new extension. Keep pumps for showers and some DHW. New boiler.

Option 2: New combi boiler feeding rads & buffer for UFH. 

Option 3: New boiler feeding rads & buffer for UFH & UVC for DHW.

Option 4: Thermal store heated by new boiler. Feed UFH from bottom and rads and DHW either with coil or FPHE from top. Possibly adding PV or ASHP in future.

 

Any more thoughts or advice on this please?

Thanks, Chris

[SteamyTea]

1 hour ago, chrisb said:

Possibly adding PV or ASHP in future.

After a quick skim though, make that choice now, it will make the rest of the system fall into place.

Then you can think about a couple of Sunamps for your DHW.

[Nickfromwales]

With 1 bar dynamic at 20l / min you would really be better off with a mains pressurised DHW solution. Pumps wear out, are noisy, and aren't exactly great when they're working as you have to segregate the different systems eg pumps for baths and showers but nothing else . 

Relocate the cylinder to the utility, or better still the airing cupboard so you can make use of the otherwise wasted heat loss. 

The home your describing isn't going to be 'low energy' so I doubt an ashp would ever be an economical option tbh, so best to decide that now as it would completely change the advice you get / design remit. It would be unsuitable for space heating via rads, unless they were oversized accordingly, which would take up valuable space. Ashp's only work well ( efficiently ) at low flow temps so unless you were VERY well insulated and reasonable airtight your heat losses will make that not a viable option. 

Future addition of Pv will always be easily integrated with either a TS or an UVC, so the only thing I'd add to that now would be to ensure you have at least one immersion heater. 

UVC or TS ? 

A TS would give you a single cylinder solution, and, if fed by a suitably sized gas boiler, would give you nigh-on constant DHW.

With an UVC and Ufh you'd also be looking for a buffer to compliment that arrangement, so more space taken up, more losses and more complexity. An UVC also needs to recharge between depletion so needs to be sized slightly larger to cope. 

A TS typically has higher standing losses due to its higher set temp, so I'd invest the ashp money in the Pv and use that to heat the cylinder and offset the losses. If your deffo going for Pv then I'd say go for a hot return circuit too to give more convenience eg getting hot water out of the taps almost instantly, which would also negate part / totally re-plumbing to suit the new cylinder. If the cylinder would end up central to the bathrooms it may not be necessary but that can be decided after working out what your decision are so far. 

[chrisb]

Thanks @SteamyTea & @Nickfromwales. Sounds like ASHP is out - I thought that might be the case. Whole front roof is south facing, so PV is an obvious option, but I don't want to commit the capital now as it might mean no kitchen in 6 months time. Not sure i'd hear the last of that. I can add later when funds are certain. I'll run some cables for this purpose at first fix.

 

Logically, I think my next questions are which model of gas boiler, in what size, and how big should the TS be? Also, should I look out for certain features? FPHE vs internal coil for DHW etc?

 

@SteamyTea we're on mains gas, and as much as I love the idea of a sunamp (or two), the numbers just don't stack up right now as the repayment time would be excessive I think.

 

Ref Hot return circuit, the new cylinder will end up no more than 3m from the kitchen and 5 m from the bathroom, so its probably not worth it. Kitchen and utility are currently the longest to wait for hot water, and the distances to those will be halved, so not really an issue IMO.

 

Thanks

Chris

[Nickfromwales]

Size the TS as small as you can, as a suitable gas boiler can easily match your max demand eg space heating at nominal draw plus DHW being run simultaneously at typical max flow. The only reason to oversize it would be to dump excess pv into it, so best to commit to pv so you can design now accordingly. 

The last 500ltr TS i fitted was in a 4 bathroom house and I complimented that with a Vaillant 637 ( 37kw ) system boiler. Even though it will very rarely see full demand it is capable of regularly delivering as little as 9kw whilst fully modulating, however you need to avoid full modulation to get the operating / return temp into the realms of the optimum for condensing to occur. 

By using a TS you can set the boiler to come on at the premium flow vs return temp to maximise efficiency. That basically means the boiler pulses heat into the TS, controlled by the cylinder thermostat, rather than 'trickle' heating linear to the current demand. 

That said I'd say you can get away with a 300L TS, and a 30kw boiler. 

[chrisb]

Now that the UFH pipe is in, and slab about to go down, I'm now beginning to think about this again a bit more seriously.

UFH Load as calculated by Wunda is max 5kW. I also have 2 small towel rails, three small rads and one large rad covering non-UFH rooms. I'm hoping that the plinth fan convector is now redundant with the UFH now in (although my wife loves it for drying clothes).

 

Current mains water supply is in 15mm copper at least under the slab in the kitchen (we found it). It runs inside a 4" pitch pipe as a duct to the front of the house and to the road. The last 300mm was running direct in concrete before popping out under the sink. it has plenty of corrosion and dents. I'm likely to replace it with 32mm MDPE when we have the drive relaid (planning condition, so not too far off). I guess mains flow will increase afterwards?

 

This is what I'm thinking so far:

• 300L TS on @Nickfromwales advice - no idea which brand, material, model etc
• Tappings to UFH manifold from the bottom to get low temperature? Do I need a pump TS-->UFH, or will the UFH pump do this? Do I need a 2 port valve here to stop convection when UFH is off?
• Tappings for rads from the middle of the TS, with a TMV, pump and 2 port valve to stop convection?
• DHW coil in the top with TMV. Should I have a pair of tappings here too, incase I want to convert to FPHE later for increased DHW flow?
• Tappings for boiler - not sure of best place for these?
• 2x 3kW Immersion - bottom and mid-position, partly to give some redundancy, but also for future PV.

 

Additional questions:

• Should I be hydraulically separating the UFH circuit from the rads to prevent muck going down the UFH pipes?
• How many thermostats should I have and what position should they be in?
• As the TS will go in the garage, and the rads are upstairs, I think I'll need to keep the F&E tank in the loft to maintain vented status and thus not require G3 compliance?
• The 30kW boiler: should it be heat-only, or system? I think system won't do for vented TS, so it should be heat-only, with a pump to circulate water through the TS. Am I correct?
• Recommendations for the boiler would be great.

Thanks all

Chris

 

[Nickfromwales]

1 hour ago, chrisb said:

Should I be hydraulically separating the UFH circuit from the rads to prevent muck going down the UFH pipes?

Not really necessary if you fit a magnetic filter on the return of the boiler, and maintain the correct dilution of corrosion inhibitor. 

 

2 hours ago, chrisb said:

How many thermostats should I have and what position should they be in?

This should be reflective of the number of spaces making up a zone. A zone can have multiple Ufh loops heating it, for eg an open space kitchen diner could have 4 Ufh loops but only be controlled by a single thermostat. That thermostat would open and close those 4 of the manifold actuators in unison to govern the room temp. Most important would be making sure they're not in direct sunlight or in any draught. 

 

2 hours ago, chrisb said:

As the TS will go in the garage, and the rads are upstairs, I think I'll need to keep the F&E tank in the loft to maintain vented status and thus not require G3 compliance?

A sealed system would require G3, so yes, going open pipe ( vented ) would negate that. You don't need to have an open system and you will suffer a lot more conviction / evaporation heat loss with a TS that's connected to an f&e tank and its associated pipework. It will self - maintain though, and never need any 'topping up' manually as you have to do when sealed and pressurised. 

 

2 hours ago, chrisb said:

The 30kW boiler: should it be heat-only, or system? I think system won't do for vented TS, so it should be heat-only, with a pump to circulate water through the TS. Am I correct?

Yes.

 

2 hours ago, chrisb said:

•  
• Recommendations for the boiler would be great.

The last heat-only I did was a Vaillant 4-series. Cracking bit of kit. 

 

 

 

[Nickfromwales]

2 hours ago, chrisb said:

300L TS on @Nickfromwales advice - no idea which brand, material, model etc

You'll struggle to find a better deal than Telford. Stainless steel and iirc a 10 year warranty on the TS's.

 

2 hours ago, chrisb said:

Tappings to UFH manifold from the bottom to get low temperature? Do I need a pump TS-->UFH, or will the UFH pump do this? Do I need a 2 port valve here to stop convection when UFH is off?

Ufh pump will pull on its own. And yes to the 2-port ZV. 

 

2 hours ago, chrisb said:

Tappings for rads from the middle of the TS, with a TMV, pump and 2 port valve to stop convection?

Yes, yes and yes. 

 

2 hours ago, chrisb said:

DHW coil in the top with TMV. Should I have a pair of tappings here too, incase I want to convert to FPHE later for increased DHW flow?

If you're really worried about flow go for the 46kw 28mm coil and TMV. Tbh, I had a 400L running the smaller 22mm coil and it was running 2 showers simultaneously whilst still being able to draw hot off at the kitchen sink / washbasins etc. 

 

2 hours ago, chrisb said:

Tappings for boiler - not sure of best place for these?

The cylinder manufacturers will put them where they need to go automatically. 

 

2 hours ago, chrisb said:

•  
• 2x 3kW Immersion

Yup. One for Pv and the other / both for peak DHW boost. 

[chrisb]

8 hours ago, Nickfromwales said:

11 hours ago, chrisb said:

How many thermostats should I have and what position should they be in?

This should be reflective of the number of spaces making up a zone. A zone can have multiple Ufh loops heating it, for eg an open space kitchen diner could have 4 Ufh loops but only be controlled by a single thermostat. That thermostat would open and close those 4 of the manifold actuators in unison to govern the room temp. Most important would be making sure they're not in direct sunlight or in any draught. 

 

My bad - I meant thermostats on the TS, not in the rooms for UFH. Eventually I'd like to do some more intelligent monitoring/control with a Pi or similar, but will need sensible stuff at first so it 'just works'.

 

8 hours ago, Nickfromwales said:

11 hours ago, chrisb said:

As the TS will go in the garage, and the rads are upstairs, I think I'll need to keep the F&E tank in the loft to maintain vented status and thus not require G3 compliance?

A sealed system would require G3, so yes, going open pipe ( vented ) would negate that. You don't need to have an open system and you will suffer a lot more conviction / evaporation heat loss with a TS that's connected to an f&e tank and its associated pipework. It will self - maintain though, and never need any 'topping up' manually as you have to do when sealed and pressurised. 

 

I think I understand this, but just to be sure:

Open Pipe/Vented means having a F&E tank in the loft, with a feed pipe from the bottom of that tank to the bottom of the TS. Also, a pipe from the top of the TS runs up and discharges over the F&E tank. This arrangement means that the TS, rads, UFH and boiler are all running at low pressure as defined by the height of the F&E tank above them. A system boiler cannot be used with this type of system. Am I on the right track?

 

The alternative would be to pressurise the whole system with a system boiler and fill loop, but this results in needing G3 compliance.

[Lesgrandepotato]

Will your Solar set up be able to run a 3kw immersion? As i've found out this week smaller immersions 1kw sized etc have a different boss size.. if the immersion is too large then you'll be importing from the grid to make it work and the PV will cost more in electric than it would in gas. 

[Jeremy Harris]

12 minutes ago, Lesgrandepotato said:

Will your Solar set up be able to run a 3kw immersion? As i've found out this week smaller immersions 1kw sized etc have a different boss size.. if the immersion is too large then you'll be importing from the grid to make it work and the PV will cost more in electric than it would in gas. 

 

 

Doesn't matter.  Any diversion device will just modulate the power down to a few watts, as required to keep the system from importing.  Some can run two 3 kW immersions, if need be.  Mine can switch up to 20 A OK, so would be OK with a 5 kW immersion, if such a thing was available.  If the excess PV was only 100 W that's all that would be sent to the immersion.

 

There's no need for a small immersion, unless you intend doing something like running an off-grid system with a low level background generation capability (something like a small hydro plant that delivers a fairly constant low power).

[PeterW]

If you’re going to go pressurised then consider a buffer and UVC. Easier than sorting the header tanks etc 

[Nickfromwales]

1 hour ago, chrisb said:

 

The alternative would be to pressurise the whole system with a system boiler and fill loop, but this results in needing G3 compliance.

Yes to all in the previous post. Read about @JSHarris heating controls in his blog. He uses a simple room stat that has a 0.1oC hysteresis which works well. 

You do need G3 when you pressurise a hot water cylinder, but you'll get less losses over time plus no header ( F&E ) tank. As the F&E setup will continuously expand and evaporate it takes in fresh water. That means you'll have to keep the inhibitor level monitored and top up more frequently. Other than that there's no real downfall, but I do think it's a step backward TBH, and who wants a header tank these days ? 

11 minutes ago, PeterW said:

If you’re going to go pressurised then consider a buffer and UVC. Easier than sorting the header tanks etc 

That's the more common solution, and the uvc can be stored at lower temp too compared to the TS so far less latent ( waste ) heat loss. 

[chrisb]

I'm coming round to the idea of a pressurised system with buffer and UVC.

 

So, how big a buffer and UVC?

 

Can the rads come off the buffer so I can regulate the temperature with a TMV?

 

I'm guessing the 30kW boiler is still valid, but a system boiler now.

[chrisb]

I think I've worked out that I need a buffer 100l or perhaps slightly larger. 

I've picked up a 250L UVC from eBay brand new at a bargain price.

 

For the buffer, is there any reason why I cannot use a UVC, but connecting to the top and bottom ports, ignoring the coil? eBay UVCs seem fairly available, but not small buffers......

[CC45]

Our buffer tank (to feed ufh & rads upstairs) is 90l direct.  Telford cylinders.  Nickfromwales did the deal & they were happy for me to pick them up (we live local) so saved a few £ again.

Edited November 11, 2017 by CC45
Spelling!

[SteamyTea]

12 hours ago, CC45 said:

 Nickfromwales did the deal & they were happy for me to pick them up (we live local) so daved a few £ again.

Who is this Dave, and can he save me money?

Or did you mean Nickfromwales, then it would be "so nicked a few £ again"

[Nickfromwales]

8 minutes ago, SteamyTea said:

Who is this Dave, and can he save me money?

Or did you mean Nickfromwales, then it would be "so nicked a few £ again"

Wouldn't be the first time 

[ProDave]

Throughout my whole build I have been Daving loads of money.