UFH with existing Radiators and system boiler

[Silage]

Into detail design stage for an extension to existing stone cottage. Basically a timber clad 2 storey box with a glass link to replace single storey hotchpotch that is there now. MBC have quoted for a passive slab (ouch£!) but due to size restiction will go for u=0.14 walls (open panel , 300mm finished) and u=0.14 flat roof by MBC if I can deal with the + VAT price.

 

Existing gravity DHW gravity fed with pump for shower, supplied by a 2009 year oil fired Grant 15/28 kw utility condensing system boiler that also runs the rads. 

 

The rads will have to stay in the cottage but how to run UFH in the extension.. Based on an MBC house build nearby I'm pretty sure I won't need any heating on 1st floor apart from electric towel rads and/or electric mat for en-suite (although they don't have a link to a drafty cottage so slight concern!) 

 

Yes I'd like to do away with the oil boiler but not sure I can considering the poor performing cottage and potential costs. I'd like to go mains pressure also so UVC, (mains flow rates of 40 L/m). If so then potential for solar PV or solar Thermal if worth the investment. Do I fit a buffer for UFH to stop short cycling?

 

Any thoughts? Any alternative schemes?

[Mr Punter]

I would concentrate on improving the cottage.  Insulate and improve airtightness.  Control ventilation maybe with MVHR.  In many ways this is much more of a challenge than new build.

[Nickfromwales]

12 hours ago, Silage said:

Do I fit a buffer for UFH to stop short cycling?

If you fit a Thermal store, it'll double up as a hot water tank ( via an integral instantaneous coil ) and also provide a means of buffering the heat delivery from the oil boiler. Plus, you can dump all your excess pv into it. ST is a one trick pony so unless you are at passive levels then I'd not even bother considering it. Pv is SO much more useful in comparison. 

Alternative is a small / medium buffer and an UVC. 

Cons with a TS is you need it hot 24/7 for the months that you require heating ( to get DHW ). 

Pros with a TS is you only need one tank. 

Edit to add : also less plumbing, controls and associated expenses with the TS too. 

 

[Silage]

8 hours ago, Mr Punter said:

I would concentrate on improving the cottage.  Insulate and improve airtightness.  Control ventilation maybe with MVHR.  In many ways this is much more of a challenge than new build.

 

Thanks Mr P. I' be done quite bit of work upstairs with insulation between rafters and insulated plasterboard, more to do but time and £. It's a stone cottage (15" rubble filled walls) though so no external cladding possible or desirable, internally small so reluctant to lose any more space. Airtightness not good so I'll look at that down the line particularly suspended floor section which is cold, I was wondering if I could extend the MVHR across the new build to the old. Anyhow priority is 2 storey space so efficient extension (at least until connected to the cottage, I need to think about how to minimise losses). 

[Silage]

On 24/05/2017 at 10:14, Nickfromwales said:

If you fit a Thermal store, it'll double up as a hot water tank ( via an integral instantaneous coil ) and also provide a means of buffering the heat delivery from the oil boiler. Plus, you can dump all your excess pv into it. ST is a one trick pony so unless you are at passive levels then I'd not even bother considering it. Pv is SO much more useful in comparison. 

Alternative is a small / medium buffer and an UVC. 

Cons with a TS is you need it hot 24/7 for the months that you require heating ( to get DHW ). 

Pros with a TS is you only need one tank. 

Edit to add : also less plumbing, controls

 

 

@Nickfromwalesthanks for the reply. So a thermal store such as a McDonald Thermflow then (recommendations welcomed), the oil boiler heating the primary water via a coil and the rads and UFH drawing from this primary water via pumps (so 3 pumps in total inc system boiler pump). Immersion heater fed from PV. The DHW being mains pressure from the instantaneous coil heated by the primary side - what sort of flow rate do you get for showers though, it has to be an efficient coil for a power shower (x2?) doesn't it? 

 

Do you mean you need the TS hot for DHW even when you don't require heating? I guess this isn't too efficient due to standing heat loss but then I guess a conventional DHW cylinder has a similar issue? Standing heat loss may be more useful (or better tolerated) in the existing cottage downstairs bathroom than the highly insulated extension. 

[Onoff]

On 2017-5-24 at 10:14, Nickfromwales said:

Cons with a TS is you need it hot 24/7 for the months that you require heating ( to get DHW ). 

 

That's no different the is it to say my current set up with an old hot water cylinder in the loft? If I want DHW "on tap" I have to keep the water hot in the cylinder.

[Triassic]

3 hours ago, Silage said:

 

 

@Nickfromwalesthanks for the reply. So a thermal store such as a McDonald Thermflow then (recommendations welcomed), the oil boiler heating the primary water via a coil and the rads and UFH drawing from this primary water via pumps (so 3 pumps in total inc system boiler pump). Immersion heater fed from PV. The DHW being mains pressure from the instantaneous coil heated by the primary side - what sort of flow rate do you get for showers though, it has to be an efficient coil for a power shower (x2?) doesn't it? 

My last mixed UFH and radiator system only had two pumps, the boiler pump for the rads and a separate UFH manifold pump for the UFH.

[dpmiller]

I'm looking at a Copper Industries Maxipod (a near clone of the McDonald)and you'd definitely need the third pump as the TS is controlled indepenent of the heating circuits.

[Nickfromwales]

14 minutes ago, dpmiller said:

I'm looking at a Copper Industries Maxipod (a near clone of the McDonald)and you'd definitely need the third pump as the TS is controlled indepenent of the heating circuits.

All depends if it's a system boiler . They have they're own pump inside already, so that would do TS & rads, and the UFH manifold would take care of the UFH circulation. I've never found it necessary to fit a pump between the TS and the pump on the UFH manifold as the manifold pump pulls through on its own. I think many installers who are doing these mixed installations fit one as a knee jerk but they're really not necessary. 

[PeterW]

Some TS units need the 3rd pump for the PHE unit on the side. Others use an in tank coil so no pump needed. 

 

Newark Copper Cylinders will make what you need - can be a lot cheaper than the others even for a bespoke cylinder. 

[dpmiller]

Motor valves then Nick surely. I'd be wanting to divorce the Boiler (heat source) from the rad circuit to try and maximise stratification. Although, I'm thinking of my own scenario which will be a bit belt and braces (ASHP and boiler stove plus immersun on PV) and I want to keep the top of the tank hot for the DHW , tapping off lower down for UFH as oversized rads.

[Nickfromwales]

Stratification won't be an issue, and the DHW coil won't suffer trust me. The coil, referencing the Telford setup, fills most of the inside of the cylinder ( 46kw 28mm coil vs 300-400ltr TS ) so DHW production is instant, and if served by the boiler on demand, will also be constant ( unlike an UVC ). 

Yes to motorised valves, to prevent convection heat loss down the connective pipework as well as control. The last TS job I did with mixed space heating had a manifold about 18m away from the TS with pipework ( 36m of it ) dropping underground through an umbilical trench, from the new plant location in the garage to the house, and then rising to first floor airing cupboard where it dropped back down to ground level to heat the new summer room. This had no additional pump, just the one at the manifold, and I was getting convection circulation within minutes. A 2-port zone valve soon sorted that, but truth be told it was such an adverse run I never though it would draw naturally, but it did, and bloody effectively too

Having the rads off the boiler could reduce efficiency a little as it would be harder to ensure that the boiler stayed in optimum condensing temp range, so that's one argument for said divorce, but the flow being split between TS and rads causing problems  ( i.e. reducing the flow rate through the TS thus reducing stratification ) is not going to be problematic at all. 

My experience is from designing AND fitting these systems in the real world, so the feedback is from these working A1 rather than hypothetical . The TS's perform far better, for DHW, than they should do ( according to statistics ) so I'm a big fan, when an UVC won't fit the bill of course. 

With a cold mains accumulator on each of my last two similar installs, I had 2 mixer showers and the bath filling at the same time. Not blasting out, but more than sufficient to use them. Remember that DHW is propelled by your cold mains incoming flow / pressure, so poor flow in will equal poor DHW performance, regardless of what DHW device you choose. 

[PeterW]

18 minutes ago, Nickfromwales said:

 My experience is from designing AND fitting these systems in the real world, so the feedback is from these working A1 rather than hypothetical . The TS's perform far better, for DHW, than they should do ( according to statistics ) so I'm a big fan, when an UVC won't fit the bill of course. 

 

So I've currently got a TS, and I'm also planning a UVC and gleaning info from here and other places has driven both of those decisions. 

 

If you have a mix of heating temp requirements (ie rads and UFH) then a TS is perfect assuming you have the boiler to keep up with it or have a suitably large store. They are also great for taking ST inputs. 

 

If you've got low grade heat such as ASHP then most likely you've got UFH so the best option is a very large UVC as the recovery time will be much longer. 

 

 

[Silage]

Really interesting observations, thanks @Nickfromwales, @dpmiller, @PeterW. As ever differing options regarding rads off the TS or direct from SB, most of the manufacturers talk of the advantage of running from the TS to minimise cycling although low return temperature maybe easier (if not controllable) to achieve through the rads direct? Some manufacturers are also claiming advantages form PHE for DHW over an internal coil like the Heatweb Heatbank Excel, although as @PeterW says another pump needed! Also noted that TMSThermal store are claiming better stratification through use of a mixing valve. Any more users out there? Would an Immersun be an advantage if feeding in Solar PV. 

[Silage]

Just opening up an old thread as it's close to crunch time, design sign off end of this month, build mid October. Have been wondering if I'm doing the right thing keeping the oil fired system boiler, I need a new oil tank anyhow and new flue. Boiler is 2009. Considering part of the existing cottage is early 1800's stone built and has rads (4 downstairs, 4 upstairs) would an ASHP be viable? The MBC extension would be ok I'm sure, although connected to the cottage so a little more heat loss than new build. DHW is the other consideration. 

[Nickfromwales]

Unless you've got or intend to fit oversized rads then I'd stick with oil tbh. Ufh is a low temp system that's far better suited to an ashp's output, whilst still achieving a reasonable CoP. 

[Silage]

@Nickfromwales you're dead right. I was dreaming a little, still in a dilemma over TS vs UVC. 

[Nickfromwales]

If your on oil it's a no-brainer...a TS would be best. 

[CC45]

Hi @Nickfromwales, what if you've got mains gas? Chap from Telford cylinders is visiting tomorrow and Im still unsure whether a uvc or ts is best for us? Rads upstairs + ufh downstairs.  Heating demand is low ~ 5kw.  Plumber likes uvc.

 

Cheers.

[Nickfromwales]

@CC45

What's your occupancy pattern and typical DHW needs? 

[CC45]

2 of us, mrs loves spending time in the shower. 4 bed house. We both work full time.  Id say I was frugal on both heating and hotwater but her in doors isnt. 

 

Diolch.

[CC45]

I have been leaning towards a uvc with possibly a 100l buffer tank for the heating.  The rads would need a separate pump then I guess.

 

Too many options really - every other system has been on a combi.

[Nickfromwales]

Have you selected a boiler ? You need to know how low it'll modulate eg to best match your low heating demand. For max efficiency it's either a TS, or a buffer and uvc but the latter needs room for the additional tank. 

Is your plumber 'up' on best dealing with the low energy requirements you have ? DHW is an aside, and much easier to spec. 

 

[CC45]

No boiler chosen yet - likely to be a well known make - will look at some specs tomorrow. I'm not too sure if the plumber is 'into' the highly insulated house scene.  It will be interesting to see what Telford say tomorrow - I dont think they do TS. I will contact Chelmer heating as well - they do seem to cater for both TS & UVC. I'm local to Telford so that would be convenient.  For ages I thought TS was the answer but in recent times I've seen a lot of UVC going in.

[Nickfromwales]

Chelmer = ???

Telford = ???

Telford most certainly do TS's, I've bought loads from them, mostly bespoke too