Boiler, buffer, thermal store or UVC? UFH with PV

[oranjeboom]

Needing to focus on my next project after my UFH fiasco and being in a holiday rental place where the DHW is very hit and miss reminds me that I need to get some specs laid down before I start ordering stuff!

Having read so much over the past couple of years, I feel a lot more informed but also a bit bamboozled at the same time. UVC or TS, buffer tanks, what size..... So my main questions (so far):

 

1) what type of gas boiler to go for?
2) Thermal store or UVC?
3) Buffer tank needed?

 

My setup will be:

• 4kw solar PV
• Mains gas
• 2 UFH manifolds (upstairs and groundfloor - so I can choose the flow temps accordingly to get the maximum comfort levels)
• Room stats in all areas
• 2 Bathrooms (2 showers, 2 baths)
• MHVR

 

Although not going to be a passivhaus, I’m hopeful it'll be a bloody whole let better than your standard newbuild:

• 2 SIP extensions
• 300mm EPS floor insulation
• 3G windows/doors
• 100mm EWI on existing house

 

DHW requirement:

 

• 2 adults and two soon to be teenagers (so they will hardly wash reducing the DHW figures a bit)
• 2 morning showers
• 2 evening baths
• Hot water for washing up (though may get a boiler-type tap for most hot water needs in the kitchen)

 

Occupancy: Wife and kids out and I although I work from home, my office will be one of the sheds out the back so main house can either be set to off or very low temps during the day.

UFH demands:

Groundfloor: Water volume: 129L; Estimated loading: 9.45Kw
Upstairs: Water volume: 25L; Estimated loading: 2.03Kw

 

BOILER QUESTION:
Before I start getting plumbers out to fit a boiler system, I’d like to get people’s advice on whether to go for system or combi.

 

Combi 
=  don’t need a water storage  cylinder  
= no need for cistern in loft
= combi boilers don’t supply enough hot water =  better to use a separate cylinder for the hot water supply. 
= combi boilers can be quite noisy.

System

= Store hot water in a cylinder, so they can feed several outlets at once at mains pressure. 
= no need for a cistern in the loft and the expansion vessel is built in.
= Flow rates will be high as water is delivered at mains pressure, and hot water is instantaneous.

 

I'm thinking to go for a system boiler at this stage (easier/quicker install) and then to use excess PV with the cylinder.

 

THERMAL STORE OR UVC?

My thoughts so far are to have a 300L UVC fed partly by the solar PV during the day (using an Immersun type of device) with maybe E10/E7 at night and then also the gas boiler.  I would end up with two circuits – one for DHW and one for UFH. The boiler would kick in for the UFH when the UVC doesn’t require heat.

 

 

BUFFER TANK?
I'm also thinking that if I go for a UVC rather than thermal store (from what I have read on here) I would need a buffer store seeing I will have UFH. But I've read that if I opt for a TS, that can be my buffer tank also. So perhaps I should go for a TS rather than a UVC? Confused....

 

 

Hoping someone (AKA: @Nickfromwales  etc) is able to assist.....

 

Thanks, OB

 

[Onoff]

The clever people will be along shortly once they've recovered from their jet lag!

 

On a serious note I'll be following their response closely for when I come to do similar. 

 

Sunamp?

[ProDave]

For me UVC because it delivers a near constant temperature water right up to the point it runs out.  A thermal store, the DHW temp starts to reduce as you draw water, so to get enough hot water you need a large tank, or store it at a higher temperature.

 

A modulating gas boiler won't need a buffer tank. The expert from Wales will be along shortly to advise more.

[CC45]

We are v similar to what you list. Mains gas - so fitted a 24kw system boiler.  Supplies 250l uvc and a 90l buffer store.  Buffer store feeds 2 manifolds - ufh plus rads upstairs.  All rooms have their own stats.   Partly installed so far, no hurry to complete it but it worked well when we had the cold snap.

[oranjeboom]

18 hours ago, Onoff said:

 

Sunamp?

 

Had read that may be a best option where you have no mains gas etc and also the cost of the unit did not make sense, but looking at the cost of it now, they seem to have reduced it quite a bit. I will need to have a more in depth look at that again, as last time (2-3yrs ago it just did not add up).

 

16 hours ago, ProDave said:

For me UVC because it delivers a near constant temperature water right up to the point it runs out.  A thermal store, the DHW temp starts to reduce as toy draw water, so to get enough hot water you need a large tank, or store it at a higher temperature.

 

A modulating gas boiler won't need a buffer tank. The expert from Wales will be along shortly to advise more.

 

Aahhh....okay, so I don't necessarily need a buffer tank. I had read issues with short cycling etc and hence the need for a buffer. I am a bit tight on space and didn't really want to use living space downstairs for another tank.

 

9 hours ago, CC45 said:

We are v similar to what you list. Mains gas - so fitted a 24kw system boiler.  Supplies 250l uvc and a 90l buffer store.  Buffer store feeds 2 manifolds - ufh plus rads upstairs.  All rooms have their own stats.   Partly installed so far, no hurry to complete it but it worked well when we had the cold snap.

 

Thanks @CC45! And I guess the main reason you have a buffer is because you have a mix of rads and UFH?

 

[CC45]

its to prevent short cycling of the boiler - our 24kw boiler will only modulate down to 8kw which is in excess of our heating requirement, we also have an immersion in it - so in the event of boiler failure we can still have central heating / can dump excess pv into it for heating / it does make the two manifolds neater to plumb.  Like you I agonised over this and in the end I was persuaded by @Nickfromwales to put a buffer in.  The difference in cost for us between a bigger TS and a UVC+buffer wasn't a show stopper when looking at our total build cost.  

[HerbJ]

As @ProDave  recommends, the new condensing boiiers don't need a buffer tank.

 

We went through all the options and I was set on a TS but the complexity and heat losses, as outlined by posts on the Forum persuaded me that a UVC and Modulating boiler was the correct solution. 

 

We have a 300l UVC with a close coupled 18kW  WB gas condensing boiler.  The boiler is fitted with the optional diverter valve so that we can run the UFH  system (and low temp towel radiators) at 40C and the HW feed to the UVC  at 60C. it's a good solution with the UVC mostly hea ted by two electric immersion heaters supplied by out 3.99 kWp PV system.

 

The  gas boiler modulates down to about 4kW  and has enough power to recharge the  UVC  to 55C in about 40 minutes, if needed.

[Alexphd1]

wish i had mains gas!!

 

[Nickfromwales]

On 10/04/2018 at 09:00, HerbJ said:

As @ProDave  recommends, the new condensing boiiers don't need a buffer tank.

 

We went through all the options and I was set on a TS but the complexity and heat losses, as outlined by posts on the Forum persuaded me that a UVC and Modulating boiler was the correct solution. 

 

We have a 300l UVC with a close coupled 18kW  WB gas condensing boiler.  The boiler is fitted with the optional diverter valve so that we can run the UFH  system (and low temp towel radiators) at 40C and the HW feed to the UVC  at 60C. it's a good solution with the UVC mostly hea ted by two electric immersion heaters supplied by out 3.99 kWp PV system.

 

The  gas boiler modulates down to about 4kW  and has enough power to recharge the  UVC  to 55C in about 40 minutes, if needed.

Not quite  
The 18kw WB modulates to 6kw, the 12kw is the only one that drops to 4kw. The problem is you then step away from the condensing range and the efficiency drops off. 

Most boiler MI's state that you should mitigate, by design, the risk of excessive short-cycling, so if your running a passive house level of heat requirement then this would be an install that would be frowned upon by the boiler manufacturer, and may, in the worst case, cause issues with warranty claims ( if they can demonstrate that the installation is not to their stated / required standard ).

 

@HerbJ, I dont know what your delta flow is for your UFH and what your max ventilation / fabric heat losses are, but to be in optimum condensing range you should be targeting a flow of ~60⁰C to get the stated boiler efficiency ( that any design should deliver ). I accept that some folk are happy to compromise and do away with the cost / complexity / lost plant space of the buffer arrangement and will finish by saying that it will work, just its not ideal. In your instance, if I was designing it from scratch, id have insisted on the buffer with that set to sit at 50-55^oC target temp, and id have put a separate pumped circuit to the towel rads from there so the towel rads could run hotter. 

 

[Nickfromwales]

4 hours ago, Alexphd1 said:

wish i had mains gas!!

 

Id wish for a passive house, a big PV array, and tell the gas company to FO. Battery storage will soon fortify that, in hopefully the not-too-distant future. 

IMO setting fire to anything is a step in the wrong direction, as fossil fuel is only going in one direction. 

[Alexphd1]

Sorry I like my current gas. I would have gone for a nice big single gas wok burner and a 12kw system boiler probably with pv and a sunamp in new build

Edited April 10, 2018 by Alexphd1

[Nickfromwales]

On 08/04/2018 at 23:50, oranjeboom said:

Needing to focus on my next project after my UFH fiasco and being in a holiday rental place where the DHW is very hit and miss reminds me that I need to get some specs laid down before I start ordering stuff!

 

Hoping someone (AKA: @Nickfromwales  etc) is able to assist.....

 

Thanks, OB

 

 

OK, now I've slept lol. You think this would get easier wouldn't you eh?

First off, ill assume that a buffer will be integrated as you have gas and UFH and I personally would not recommend you running the UFH directly from the boiler. Maybe one day a boiler manufacturer will produce a dual burner boiler which can deliver normal service temperature flow plus very low kw at flow temps that promote full condensing. ( Dragons Den ?  ).  

Bearing in mind that you'd have to keep a TS or an UVC hot for DHW then there is a strong argument to do away with the buffer and go for a single TS. The good news is if you put a whopper of a gas boiler in, you'll get away with a smaller TS, lower set temperature ( so lower standing losses ) and nigh on constant instant DHW at high flow rates. 

23 hours ago, ProDave said:

A thermal store, the DHW temp starts to reduce as you draw water, so to get enough hot water you need a large tank, or store it at a higher temperature.

After a week of staring at a TS, I can say different. The way hot water comes out of an UVC is very much like the way it comes "out" of a TS. With the digital stats we had in, in various test locations during "Operation Doomsday Heating Device", its quite clear to see the heat effectively rising up into the top of the TS whilst the DHW PHE flow ( upper most side connection of the TS ) draws it into the DHW PHE for conversion into DHW. Accepting that particular TS design was ( sorry @newhome ) a steaming pile of dog shit, it still managed to fill a 2-seater bath ( well bloody swimming pool TBH ) with only a 1/3rd of the TS preheated and managed it with the ( 40% undersized ) boiler set to max input, so I was still reasonably impressed with its ability to turn boiler flow indirectly into DHW production even through an external PHE. The bath tap was running for a good half hour, and it would have EASILY provided 3 good simultaneous showers for 10-15 minutes as it was. If that TS was serviced by a 37-40kw oil or gas boiler, it would have done the 3 showers constant ( as in left running 24/7 ) im sure. FYI, the electric 3-phase boiler there was a 24kw and struggled, so if your considering good DHW production you really need to go bigger on the boiler for better performance and faster recovery / shorter burn times. Also remember that a 40kw gas system boiler will modulate down to ~9kw if ever necessary ( so don't think you'd be burning more gas unnecessarily because you won't )  . Fortify that lot with a conversion to integral DHW coil instead and you lose a pump, a PHE, a flow switch, the extra losses and dead leg delay, and its a winner for a DHW / space heating solution in one box. 

With the TS you'd be able to engineer the flow and return of the boiler to reside in the optimum condensing range thus guaranteeing that you get the stated boiler efficiency for the rest of its days. 

Having PV further fortifies the introduction of at least one type of cylinder of course, so your already going down that path anyhoo.

You could ditch the DHW storage cylinders and go for a high-flow combi + small buffer, but then you'd really struggle to provide 2 simultaneous GOOD showers, and wouldn't be able to store anything produced by the PV beyond the capacity of the smaller buffer........ unless.......

You have a good high flow storage combi, and you feed it with pre-heated cold water via a Sunamp. The SA would also give you the buffer medium that the ufh would want, but would add a little complexity to the design as it would need to switch from heat in ( from the boiler ) to heat out ( to the ufh ) or be very specifically designed to do both at the same time. A fail-safe would need to be installed to stop the gas boiler from ever cooking the SA, but it would make a neat two-box solution and use the PV effectively. The SA comes with a box of tricks that tells the boiler to stay off if PV has done its job too, plus you could also fit a thermic changeover valve so if the SA is hot enough, DHW flow bypasses the combi and goes directly to the taps.

This plumbing lark is so simple, I'm surprised its even discussed here tbh  

And breathe.

   

 

54 minutes ago, Alexphd1 said:

Sorry I like my current gas. I would have gone for a nice big single gas wok burner and a 12kw system boiler probably with pv and a sunamp.

LPG wok burner  No standing charges and tiny capital expenditure = more money to spend on Sunamps and PV 

 

 

 

 

[Nickfromwales]

To add, if the combi / SA option was chosen, you could go for a 12kw SA or a pair of 9kw ( so 18kw total ) and offset some of the space heating too. 6 hours of sunshine would charge that lot up and provide a LOT of storage. The gas boiler would then become a standby device for a good 60% of the year. 

Dont discount an all-electric heating / DHW solution too quickly, as its definitely a contender if PV + E7/10 is in the picture, and is deliciously simple, clean and nigh-on maintenance free, plus no annual servicing or gas / G3 tickets.   

[newhome]

1 hour ago, Nickfromwales said:

it still managed to fill a 2-seater bath ( well bloody swimming pool TBH )

 

And for the avoidance of doubt @Nickfromwales and @PeterW decided not to share that bath in case a photo ended up on BH . Will was let off given that he forgot his water wings . That bath is circa 300l BTW although I know you didn't fill it up to the top. 

 

 

[Nickfromwales]

4 minutes ago, newhome said:

although I know you didn't fill it up to the top. 

Yes, because I was worried it would end up downstairs with the weight 

[newhome]

8 minutes ago, Nickfromwales said:

Yes, because I was worried it would end up downstairs with the weight 

 

I was scared the first time I used it lol but it's passed that test already. The joists were altered to allow it and to accommodate the open plan nature of the room downstairs I believe.

 

 

[Onoff]

18 minutes ago, newhome said:

 

And for the avoidance of doubt @Nickfromwales and @PeterW decided not to share that bath in case a photo ended up on BH . 

 

 

 

First thing that crossed my mind!

 

What happened in Scotland stays in...etc 

 

 

[newhome]

1 minute ago, Onoff said:

 

First thing that crossed my mind!

 

What happened in Scotland stays in...etc 

 

 

 

Yeah cos it's like Vegas here init . 

 

[HerbJ]

4 hours ago, Nickfromwales said:

 

@HerbJ, I dont know what your delta flow is for your UFH and what your max ventilation / fabric heat losses are, but to be in optimum condensing range you should be targeting a flow of ~60⁰C to get the stated boiler efficiency ( that any design should deliver ). I accept that some folk are happy to compromise and do away with the cost / complexity / lost plant space of the buffer arrangement and will finish by saying that it will work, just its not ideal. In your instance, if I was designing it from scratch, id have insisted on the buffer, set that to 50-55^o C target temp, and id have put a separate pumped circuit to the towel rads from there, so the towel rads could run hotter. 

As @Nickfromwales  said earlier, not quite...

 

First things first, to answer your your questions:

The UFH manifolds are set at 32C on the inlet. Two UFH manifolds Ground and First Floor. No heating on Second Floor. We have deliberately kept towel radiators at same low temperature as UFH - 2 reasons, we don't really need towel radiators for heating as bathrooms have UFH - they are just to give a bit of comfort with warm towels  - and we have two small grandchildren and we didn't want hot towel rails. 

Max Ventilation/fabric heat loss is 4.3 kW , to maintain the house at 21C  with the  external temperature at -10C.     The downstairs is controlled as one zone. The First Floor bedrooms and bathrooms are individually controlled. This is for  a 337 m2  house. 

 

And now the boiler controls

 

I can set the minimum AND maximum kW output for the boiler together with maximum flow temperature and lots of other settings. The minimum output is set at 4.5 kW ( I think this is the lowest setting but I can,t find that figure in the MI) and maximum as 18 kW currently. I can  allow the control system to set the "Economic central heating temperature  and it has showing 43C, which is what it is set at. 

 

The boiler controls control the cylinder temperature with a direct connected cylinder temperature sensor. I set the cylinder temperature between  40 and 60C. i can also set the  maximum midpoint cylinder temperature  at 50C, though I have no idea how it can set this accurately as the sensor is at the bottom of the UVC and I don't use this feature??

 

Interestingly, i have just skimmed through the Installation, Commissioning and Service Instructions for the Greenstar i System  and I can find no mention of the lowest modulating level for the boiler....

   

So, as far as I can understand  I am operating this WB boiler exactly in accordance with the MI and I am happy with its performance and efficiency.  These boilers are getting more sophisticated and can be controlled to operate efficiently for most systems. 

 

We have one full year's data for the house and our NETT  gas and electricity costs were £278 -  £857 for 3654 kWh electricity and 10393 kWh gas with  £579 for 3752 kWh of PV from our 3.99 kWp system - so I'm happy with the installtion and performance to date. Also, pleased that we avoided the complexity of a TS and other unnecessary kit.

 

 

[Ferdinand]

2 hours ago, newhome said:

 

And for the avoidance of doubt @Nickfromwales and @PeterW decided not to share that bath in case a photo ended up on BH . Will was let off given that he forgot his water wings . That bath is circa 300l BTW although I know you didn't fill it up to the top. 

 

 

 

I think we need an independent witness to this claim.

 

And why has this photo not been published?

Edited April 10, 2018 by Ferdinand

[Onoff]

22 minutes ago, Ferdinand said:

 

I think we need an independent witness to this claim.

 

And why has this photo not been published?

 

1) If anyone's seen it they're trying to forget it.

 

2) If it does get published it will vie with @pocster's avatar for depravity.

 

Let sleeping dogs etc...

[Pocster]

13 minutes ago, Onoff said:

 

1) If anyone's seen it they're trying to forget it.

 

2) If it does get published it will vie with @pocster's avatar for depravity.

 

Let sleeping dogs etc...

Did you summon me ? ?

[newhome]

2 hours ago, Ferdinand said:

 

And why has this photo not been published?

 

I deffo don't have one! You'll need to check their phones for selfies . 

 

 

[ProDave]

8 hours ago, Nickfromwales said:

After a week of staring at a TS, I can say different. The way hot water comes out of an UVC is very much like the way it comes "out" of a TS. With the digital stats we had in, in various test locations during "Operation Doomsday Heating Device", its quite clear to see the heat effectively rising up into the top of the TS whilst the DHW PHE flow ( upper most side connection of the TS ) draws it into the DHW PHE for conversion into DHW. Accepting that particular TS design was ( sorry @newhome ) a steaming pile of dog shit, it still managed to fill a 2-seater bath ( well bloody swimming pool TBH ) with only a 1/3rd of the TS preheated and managed it with the ( 40% undersized ) boiler set to max input, so I was still reasonably impressed with its ability to turn boiler flow indirectly into DHW production even through an external PHE.

 

 

 

I am designing a system that will heat water when it is most efficient to do so, not all the time on demand. So I am not trying to "keep up" with demand and hope the boiler can cope, rather trying to work out the best way to store a certain amount of hot water to be used with probably no heat input during usage.

 

Heat an UVC to 60 degrees and tun the hot tap, you will get water out at 60 degrees with very little drop in temperature until it suddenly goes cold at the end (note no heat input during this time)

 

Do the same with a thermal store, and the temperature will gradually fall as you are extracting heat from a static volume of storage medium

 

So to maintain say 50 degrees HW flow for a set period with no heat input will either require a larger TS than UVC or for the stored temperature to be higher.

[Nickfromwales]

9 minutes ago, ProDave said:

I am designing a system that will heat water when it is most efficient to do so, not all the time on demand. So I am not trying to "keep up" with demand and hope the boiler can cope, rather trying to work out the best way to store a certain amount of hot water to be used with probably no heat input during usage.

 

Heat an UVC to 60 degrees and tun the hot tap, you will get water out at 60 degrees with very little drop in temperature until it suddenly goes cold at the end (note no heat input during this time)

 

Do the same with a thermal store, and the temperature will gradually fall as you are extracting heat from a static volume of storage medium

 

So to maintain say 50 degrees HW flow for a set period with no heat input will either require a larger TS than UVC or for the stored temperature to be higher.

Agreed, but where will 60oC water go? To mixer taps or thermostatic devices to be blended down. The TS comes with a factory fitted TMV, so only deliver ~50-55oC by design, thus offering reasonably similar sustain by means of blending at 'source'. 

I maintain that each solution should suit its owner, so yours suits you, but may not suit others.  A TS heated ( fortified ) by PV or E10 on a cycle would be set to heat to over 75oC so they're not worlds apart by any means. Throw in the two birds one stone appeal of the TS, plus its better DHW production "on demand", and you've got a strong case to go with it imo.