UFH with existing Radiators and system boiler

[Nickfromwales]

6 minutes ago, CC45 said:

I've seen a lot of UVC going in.

That's not exactly the benchmark for deciding on what's best for your house . Each install is reasonably unique to the occupants and DHW use, so there is no one-size-fits-all solution afaic, unless it's an guesstimate-sized solution unsympathetic to the actual needs of the property. 

[CC45]

Telford is only 10 miles down the road - I'm meeting them on site tomorrow at 3pm to discuss the system.  I will report back on what they say.  Good to know they do a good job.  Thanks Nick.

[Nickfromwales]

Sam Fosket is the guy I speak to regarding design / procurement etc, after a brief chat with the tech guys or Rodger ( the MD iirc ) for any fine-tuning. 

Excellent service and really happy so far with both product and price. 

A word of warning, DO NOT sign for the cylinder unless you have physically unwrapped and FULLY inspected it. Once you sign for it you own it, and they don't make exceptions to that rule.   . 

[CC45]

Hi Nick,

 

Telford rep called around - nice chap but didnt really clarify the situation. I've phoned WB & they said that a 21kw boiler (can deal with the hot water reheat requirement time) can modulate down to 7.2kw & that there wouldnt be an issue with short cycling (heating demand of 5kw).  The 18kw was ~ 6kw min.  Phoned Sam Fosket at Telford Cylinders - and he didnt think short cycling would be an issue because the house is well insulated / airtight so the room temps would only decline slowly and therefore not be calling for heat that frequently - you were right he was good.  I've settled for an 18kw boiler and a 250l uvc.  If needed there is room for a buffer cylinder if required. Will use Telford for the cylinder and WB for the boiler.  Hope this helps @Silage with his deliberations.

[Nickfromwales]

@CC45 the following should be of interest . 

Link

[Nickfromwales]

55 minutes ago, CC45 said:

Hi Nick,

 

Telford rep called around - nice chap but didnt really clarify the situation. I've phoned WB & they said that a 21kw boiler (can deal with the hot water reheat requirement time) can modulate down to 7.2kw & that there wouldnt be an issue with short cycling (heating demand of 5kw).  The 18kw was ~ 6kw min.  Phoned Sam Fosket at Telford Cylinders - and he didnt think short cycling would be an issue because the house is well insulated / airtight so the room temps would only decline slowly and therefore not be calling for heat that frequently - you were right he was good.  I've settled for an 18kw boiler and a 250l uvc.  If needed there is room for a buffer cylinder if required. Will use Telford for the cylinder and WB for the boiler.  Hope this helps @Silage with his deliberations.

Short cycling isn't the biggest problem, it's the fact that the boiler will almost never get into condensing range as it'll be running at full modulation for all of its life. . So your high 90's efficiency boiler won't ever be running at that max percentile. If your staying there long term it's one to consider. 

Boilers need to pulse higher temp heat into a load rather than just constantly trickle the heat in. Connecting directly, with the boiler trying to match the heat on a near-linear basis, also means the boiler pump, fan, and gas valve are all constantly engaged and labouring.

Once this is in and working it'll be unlikely that it's ever revised, but if you can live with the reduced efficiency and the reduced longevity of the boiler then it's a simpler install. Strange that WB said that tbh, as for a boiler not to short-cycle, the typical load should be equal-to or greater-than the min kW output. ?

[CC45]

High @Nickfromwales, ok, good explanation.  Got the link - I'm in but I just want to check with the plumber doing the installation on Tuesday that he's happy with a Telford cylinder (he normally uses megaflow tanks).  He may prefer it to be a pre plumbed one since he lives Q far away and is doing the installation as a favour.  If we put a buffer tank in (got the room for it & more than happy  to do it) - would 80 or 100litres be enough?  I assume the buffer should have an immersion (just in case / if we put pv in) and I assume it would need tappings for the ufh and the rads? (do these need separate ones? - rads higher up the tank for hotter temps and the ufh lower down?).  The rads would need a pump as well since the boilers pump would only be used to pump to the buffer tank.  Would a TS be easier?

 

I'm < 30 mins from Telford tanks - does that help with the group deal (pretty sure they wont deal direct)?

 

 

[Nickfromwales]

@CC45  They only go through resellers.

A TS would give you a single cylinder solution, but if you go that way you would be better off slightly larger so you don't have to heat it quite as hot as you normally would. They can be sized pretty close to the demand tbh but then need to be heated to higher temps ( close to 80oC) to get the sustain, and cylinder heat losses become problematic at those temps. Going larger means you can run at a lower set temp, lower standing losses, and a bigger thermal battery to charge off Pv, which are advantages I'd favour even though I knew I'd never otherwise 'need' the additional volume. Pv via an immersion would exceed the set temp ( say 60oC ) and roof the TS out at around 70-75oC, so any excess would be stored in this redundancy aka headroom. An UVC would have to have the immersions set lower ( so less headroom to store Pv ) as they're not designed to be sat that high in normal service due to regs on safety for the max temp of DHW available. TS's all have factory supplied blending valves ( TMV's ) on the DHW outlet to control max DHW temp to the outlets to allow them to be run much hotter, so that's already covered by the TS design. 

A buffer would only be kept warm during the heating seasons and would otherwise be cold, but the UVC would be warm / hot near enough 24/7, only losing a certain amount of heat between being topped up to the set temp every timed hot water event, so if your going to keep a cylinder warm / hot / warm anyway then I'd prob go for a TS. Pv would obviously maintain the upper temp range of an UVC during the day regardless of the DHW time clock settings, so another reason maintaining a larger cylinder wouldn't be so much more inefficient. 

A TS is basically an instantaneous water heater, so you'll get better DHW production too as a TS heated on demand by a boiler will give nigh-on constant high flow DHW, much like a huge combi, whereas an UVC will drain DHW off faster than the coil can replace it, even more so with the smaller kW boiler.  That could be mitigated against by choosing and oversizing an UVC but then still leaves you with no buffer. For sizing, add the two stated volumes together ( buffer and UVC ) and your at 350  litres or thereabouts anyway so a 350-400 litre TS would be best ( if low temp storage and Pv is factored in ) or 250-300 litre if just the boiler is being allowed for the heat source and deffo no Pv. Do you think it's likely you'll ever have Pv? It's quite critical info when deciding upon a design / sizing etc. A TS of this size could have 2 x 3kw immersions fitted, one above the other, so if you wanted to further reduce ( summer overheating ) standing losses you could, in the summer, divert Pv into the upper immersion set to a lower temp, and then, in the winter, divert Pv into the lower immersion set at a much higher temp when space heating is also required. A simple seasonal manual changeover switch would facilitate that for just a few tenners. 

The tappings can be specified upon ordering the cylinder, but yes, indepenadant sets for the rads and Ufh on the mid section of the tank will be required. For the rads you'll have to blend the flow to get 'safe' temp water to them or they could be get uncomfortably or dangerously hot, however that's quite easily achieved by just fitting a TMV across the flow and return tappings immediately before the radiator circuit pump. Then you can user define the surface temp of the rads and get them down to a manageable temp which is a very handy feature. For eg; If you go for an UVC and rads + Ufh then your plumber would likely set the boiler flow temp equal to or higher than the cylinder stat set temp ( say 60oC ) so as to get it up to temp quickly and get the stated volume of DHW vs size. The downfall with that is the boiler flow temp would then dictate the surface temp of your radiators, so in a well insulated house they may well get way too hot and cause discomfort but you'd have no means of attenuating that ( basically the same reason you'd need the TMV on the rad pump off the TS as just mentioned above, but then you'd have such control ). With an ashp I'd rbe commend the two tank setup, but with gas I'd go for the TS. 

This all sounds quite complicated when delivered quite concentrated in this breif, but it's actually quite straight forward. Make the physical side of the install and the operating disciplines separate and it becomes more digestible, but fitting a system you can fine tune will give more efficiency long term IMO so is worth the extra consideration at least. 

 

Nuts and bolts :-

• Boiler to TS controlled by the cylinder stat for max boiler efficiency per 'burn'
• Internal TS DHW coil for DHW production ( via supplied TMV ) will run 2 showers simultaneously with ease
• Ufh off dedicated tappings direct to manifold ( with supplied pump and TMV ). No secondary pump required for flow between the TS and the manifold
• Rads off second set of dedicated tappings ( via customer supplied TMV and additional pump )
• 1 x 2-port motorised valve on each flow to stave off unwanted convection heat circulation when there is no demand. 3 in total, 1 for boiler, 1 for rads circuit and 1 for Ufh circuit. 

 

Hows that for breakfast reading ?

[CC45]

Hi Nick, just back home for dinner so just a quick response for now.  Dont think we will put pv on - done the calcs and the payback is very dependant on what % of generation is used, and my calcs suggest 10 to 15 yr payback - too long for us so I would discount pv. If the fit hadn't reduced we would put it in for sure.  The choice seems to therefore be a 250 uvc + 100 buffer or ~ 250 or 300 TS.  Back to site now, will read again later on.

 

Cheers for your advice - appreciated.

[Nickfromwales]

?

[PeterW]

1 hour ago, CC45 said:

The choice seems to therefore be a 250 uvc + 100 buffer or ~ 250 or 300 TS.

 

If you go TS then oversize it as they don't hold as much at a specific temperature as a UVC

 

350 or 400 litre TS isn't as big or bulky as you think and will balance most things out especially  as you want to pull rads from it too. 

[Silage]

Looking at repositioning the boiler outdoors. Any advice on good makes. Current indoor is a Grant Vortex Pro - not cheap though. No solar planned now as run out of funds so just UFH and rads plus DHW.