Benefits of a thermal store?

[Ricco]

Hi all

 

I am posting for advice as my head is spinning with trying to choose the correct option for our heating system.

 

Our heating system is going to be as follows....

 

Oil fired boiler

UFH throughout

3 Towel Rads

DHW capacity for 1 bath and 3 showers

 

From the design stage i have always stated i want a thermal store and the reason behind this was for three reasons.

1. I do not want the UFH heating connected directly to the boiler due to short cycling and wanting the boiler to be able to run in the most efficient way.

2. I would rather not have 2 tanks (buffer tank and un-vented cylinder) due to it taking up more space if i can avoid it.

3. I would like PV panels at some stage in the future (when the budget permits!) and would install a smart immersion to heat water - it would make more sense to heat one body of water for the DHW and heat rather than just DHW.

 

My plumber is getting a local plumbing merchant to design the heating system and i have explained my preference for a thermal store. Originally he said that he had only installed thermal stores when there was an uncontrolled heat source like a wood burner - and would connect the UFH directly to the boiler, but with explaining to him my points above he understands my though process.

 

I have contacted several cylinder suppliers today for pre sales technical advice and have become more confused than ever. They both said to connect UFH directly to the boiler and just use a un-vented cylinder for DHW. I was told that the thermal store would only provide DHW and they are designed to amalgamate multiple heat sources but not for providing multiple circuits. But with 13 circuits in my UFH i feel the short cycling would be an issue and would feel like the boiler is never off!

 

With the options of vented or un-vented (i would rather have un-vented as i don't want a tank in the loft), direct or indirect, size and type of store i am at a loss as to what to go for.

 

Any or all advice would be very much appreciated.

 

Thanks

 

Richard

[Onoff]

Welcome. I too have an oil boiler here and am shortly going to have to do something to get my as yet just one single UFH loop running. My place is a refurb, as I go, one room at a time type deal. The boiler here short cycles as it is enough but I think that's due to mutiple heat loss issues; unlagged pipes, plasterboard tent etc. A TS, buffer etc has been discussed for me before now. For now I'm just siting a manifold centrally in the house and working from there.

 

Haven't got a clue on your problem but the brains of the collective here will be along shortly!

Edited November 21, 2018 by Onoff

[PeterW]

@Ricco this sounds like a classic “not done like this here” scenario ..!

 

what you describe is perfectly suited to a TS, although with that many loops I assume it’s a big house so it will need a big TS. You will need to get 3 sets of tappings on it - two low down for the UFH coils, 2 at a slightly higher point for the towel rails, and then the main DHW coil. This would probably need to be a 35Kw coil with 28mm tappings. 

 

This is bread and butter for the likes of Newark Copper Cylinders who have an interactive planner on the website. You could get away with an all in one with the header tank built in but only if it’s above the towel rads. A small header in the top of a wardrobe or airing cupboard would also work fine. 

 

How big is the floor area and what’s the heat load ..??

 

 

[ProDave]

Well I am the stick in the mud that has ran the UFH in our previous house direct from the oil boiler for the last 15 years without issue.  And an UVC for hot water.

 

I looked at thermal stores and while  they sound a good idea, the big problem is as soon as you start drawing DHW the temperature in the tank starts to drop so the delivered HW temperature falls. So you need hotter water in the tank than you think and you will never be able to use the full capacity of the tank before it just gets too cold.  Compared to an UVC that pretty well delivers it's full tank full at the same temperature until it runs out.

 

Standing heat losses seem higher from a thermal store, largely I think because nobody seems to make a thermal store as well insulated as  say a Telford stainless UVC.

[Jeremy Harris]

I think knowing the heating requirement and DHW requirement is the first thing to work out.  I

 

Is this a new build?  If so, then the heating requirement, even if only built to the pretty bloody awful building regs "low energy (!)" requirements won't be very high at all, and hot water probably at least equal the heating requirement, may well exceed it (hot water makes up most of our energy usage by a fair bit).

 

I hate thermal stores with a passion, for the same reasons that @ProDave has given.  You get more usable hot water from an unvented cylinder and even more from a Sunamp PCM store.

 

If the UFH is set into a heat store (cast into the ground floor slab, for example) then you probably have no need of a buffer, as you can fire the boiler to charge the floor and that will act as a buffer.  Take a look at how @TerryE is running his UFH, albeit with a direct electric heater, to get some idea as to how well this can work.  We do something similar, but with a lower flow temperature and an ASHP.  Our underfloor  heating comes on for an hour or two every day or two and charges up the floor like a big storage heater, and the house stays within about 0.5 deg C of our target temperature all the time, with nothing more complex than a single room thermostat to control it.

[vfrdave]

I have a very similar setup requirement as @Ricco

Approx 11 zones UFH across 2 manifolds on a floor area of 240sqm approx (bungalow)

A small towel/radiator circuit

DHW

The house floor is the traditional sub-floor/insulation build up/ 50 or 60mm flow screed with UFH therefore not the floor slab buffer like yours.  This is a typical NI floor construction, we largely haven't evolved to passive rafts.

 

I too was going to go with a thermal store but my head is in such a spin now based on conflicting advice and what has been said above.  I did have an earlier post on here and Im almost certain @Nickfromwales advised a thermal store most suitable despite the heat loss risk.

 

@JSHarris you reference the heating requirement which of the figures form your heat loss spreadsheet should I reference?

 

 

 

[Declan52]

13 minutes ago, vfrdave said:

I have a very similar setup requirement as @Ricco

Approx 11 zones UFH across 2 manifolds on a floor area of 240sqm approx (bungalow)

A small towel/radiator circuit

DHW

The house floor is the traditional sub-floor/insulation build up/ 50 or 60mm flow screed with UFH therefore not the floor slab buffer like yours.  This is a typical NI floor construction, we largely haven't evolved to passive rafts.

 

I too was going to go with a thermal store but my head is in such a spin now based on conflicting advice and what has been said above.  I did have an earlier post on here and Im almost certain @Nickfromwales advised a thermal store most suitable despite the heat loss risk.

 

@JSHarris you reference the heating requirement which of the figures form your heat loss spreadsheet should I reference?

 

 

 

Mine is pretty much that setup with UFH on the ground floor and 4 small rads upstairs. I have a 350l thermal store that supplies all my heating and dhw needs. 

As above you can empty it fairly quick if a zones called for heat and the  big shower was in use at the same time.  

It does lose heat as soon as your heat  source stops warming it up but by insulating all the pipework and I put extra around it using cylinder jackets I have managed to get it as low as possible.  Before I done any of that work I could have grew lots of dodgy plants in that room.  It was by far the warmest room in the house. By also cutting 20m of the bottom of the door any heat lost will go out into the  landing and heat the upstairs without turning the rads on. 

It's not a perfect setup but it works for me.  I found after a few months of tinkering that  63 degrees was my sweetspot to keep the tank warm and let it last longer.  Any higher and I lost more heat to the upstairs. 

Also having  the water coming out of my shower at 3 bar is great.  Last house was 1 bar if nothing else was running. 

[Temp]

I have a Grant oil boiler feeding a vented 300L thermal store.  3300sqf approx. house built 11-12 years ago. Could be better insulated. The store feeds UFH on both floors, DHW and towel rads. Thermal store was from..

 

http://www.heatweb.com/products/cylinders/heatbank/heatbank.htm

 

My store, boiler, UFH and towel rails share a common water system which is vented so no PRV anywhere. I dose it with Fernox F1. 

 

For DHW it has an external heat exchanger on the side of the store. This feeds a pretty high flow rate shower (Large rain type rose and body jets etc) but copes well. Down side of this set up is the extra pump needed to circulate water from the store to the heat exchanger. That pump and the exchanger itself leak quite a lot of heat into the store room it's in. Great for drying clothes in there.

 

Our boiler is large - 40kW - probably could have used a smaller boiler and a larger store but the system works ok. Manages to keep family of 4 in hot showers one after the other.

 

The boiler-store loop has a thermally controlled bypass on it because Grant recommend a minimum return temperature to stop corrosion. This only kicks in when the store is heated from cold (eg after a holiday or long weekend away in summer).

 

Oil boilers don't modulate and the store does stop it short cycling. However was quite critical to get set up. I ended up fitting smaller jets in the boiler which reduced the power and made it easier. They say the smaller jets also improve efficiency a fraction as well.

 

If doing this again I'd go for a bigger store and smaller boiler.

 

Edit: The towel rads are on their own output from the store (normally used for conventional rads). It has a pump and a pipe stat on the return. That way pump runs until the return pipe is hot then stays off until pipe (and rads) have cooled a bit. Then pump runs again. Idea was to avoid having the pump running all the time stirring the store.

 

Edited November 21, 2018 by Temp

[PeterW]

1 minute ago, Temp said:

Down side of this set up is the extra pump needed to circulate water from the store to the heat exchanger. That pump and the exchanger itself leak quite a lot of heat into the store room it's in.

 

This is the benefit of using a TS with an inbuilt coil - you remove the need for the PHE, flow switch and pump. 

[Ed Davies]

26 minutes ago, Temp said:

 Down side of this set up is the extra pump needed to circulate water from the store to the heat exchanger. That pump and the exchanger itself leak quite a lot of heat into the store room it's in.

 

Does that pump run continuously or is it switched somehow? If switched, how? Flow switch on the DHW pipe? If switched then an additional advantage of a DHW coil in the TS is quick heat meaning less wait and water waste running cold through until hot appears at the tap.

[PeterW]

22 minutes ago, Ed Davies said:

 

Does that pump run continuously or is it switched somehow? If switched, how? Flow switch on the DHW pipe? If switched then an additional advantage of a DHW coil in the TS is quick heat meaning less wait and water waste running cold through until hot appears at the tap.

 

They are switched and can cause issues with either low flow rates, or flutter when the switch detects back flow or pressure loss elsewhere in the cold system (as per issue with @newhome TS...)

 

[TerryE]

7 hours ago, JSHarris said:

If the UFH is set into a heat store (cast into the ground floor slab, for example) then you probably have no need of a buffer, as you can fire the boiler to charge the floor and that will act as a buffer.

 

You will have to do the UFH calcs before going this route, IMO. It's a brilliant solution if you have a passive class house, but might not work for a typical minimum compliance thermal design.  There are very few days a year when we need much more than 1 kW average to heat our reasonably large 4 bedroom house, so using a 3kW heater to heat the slab only needs to be on for 8 hrs a day.

 

However, a more typical grand design with acres of glass at a U value of roughly 1 will require maybe 4 times that, so a large ASHP or gas boiler is pretty essential.  You will need a buffer tank or TS simply because you can't pump the output of a typical boiler directly into the slab, and as you say you need to have a reasonable cycle time.  I would have thought that a buffer tank + SunAmps would be a better fit in your case.

[Temp]

5 hours ago, Ed Davies said:

Does that pump run continuously or is it switched somehow? If switched, how? Flow switch on the DHW pipe? If switched then an additional advantage of a DHW coil in the TS is quick heat meaning less wait and water waste running cold through until hot appears at the tap.

 

Yes there is a flow switch. The one fitted has a very low flow detection point but they are hard to get hold of. I took the precaution of getting a spare awhile back because others available online all seem to have a higher detection point.

 

[Temp]

8 hours ago, JSHarris said:

If the UFH is set into a heat store (cast into the ground floor slab, for example) then you probably have no need of a buffer, as you can fire the boiler to charge the floor and that will act as a buffer. 

 

+1 for the UFH. Might need one for the DHW if the boiler is small. Not all our floors are pipe in screed.

 

Do check the minimum return temperature spec for your oil boiler. Grant say..

 

"The boiler should not be allowed to operate with return temperatures of less than 40° C when the system is up to operating temperature."

 

The UFH return temperature could be lower so you would need a mixer to warm up the return a bit.

 

Edited November 21, 2018 by Temp

[Ed Davies]

20 minutes ago, Temp said:

Yes there is a flow switch.

 

Ta. So how quickly after the switch triggers does hot water start coming out of the heat exchanger? It must take a non-zero amount of time for the primary circuit to get heat into the exchanger and then for the exchanger itself to warm up.

 

Reason for asking is I'm thinking of a similar arrangement for preheat of water into a Sunamp. Maybe I'd prefer not to just use a coil in the buffer tank as there might be other heat sources (direct from solar thermal) which could be used. Not sure, rather depends on this sort of consideration. (And it sounds like the sort of thing the OP would like to consider, too.)

Edited November 21, 2018 by Ed Davies

[Jeremy Harris]

1 minute ago, Ed Davies said:

 

Ta. So how quickly after the switch triggers does hot water start coming out of the heat exchanger? It must take a non-zero amount of time for the primary circuit to get heat into the exchanger and then for the exchanger itself to warm up.

 

Reason for asking is I'm thinking of a similar arrangement for preheat of water into a Sunamp. Maybe I'd prefer not to just use a coil in the buffer tank as there might be other heat sources (direct from solar thermal) which could be used. Not sure, rather depends on this sort of consideration. (And it sounds like the sort of thing the OP would like to consider, too.)

 

 

I arranged our PHE to be on the first floor, with a 28mm thermosyphon feed from the buffer below.  The pump is on a flow switch, triggered by hot water demand, but the PHE will often already be slightly warmer than the buffer, as it is right at the top of the system.  This means there's no delay with preheat in practice, as the PHE is always nice and warm.

[Ricco]

Appreciate all the responses so far..

 

To answer a few questions from @JSHarris and @PeterW we are building a 269m2 2 storied house with block construction that is being built with reasonable standards of insulation (150mm PIR insulation in floor, 200mm full fill pumped cavity, 60mm PIR on the slab upstairs and 400mm of wool in the loft. The windows are all tripple glazed with a u-value of 0.9 max and doors with a u-value of 1.2) 

We are having MVHR and are taking some air tight measures.

 

The UFH will be in a 50mm - 60mm cemfloor liquid screed.

 

I will look at the SAP calculations later and see if i can find the "as designed" heat load of the house.

 

DHW will be for 2 adults and 2 children (3 showers and 1 bath) but want to ensure we have the capacity as the kids get older.

[PeterW]

Right .... so brand new, I would bin the Oil Boiler and go all electric. Sunamps off E7 electric for the DHW and ASHP into the floor as the heating.  

[vfrdave]

2 hours ago, PeterW said:

Right .... so brand new, I would bin the Oil Boiler and go all electric. Sunamps off E7 electric for the DHW and ASHP into the floor as the heating.  

This sounds like an expensive solution but I am uninformed to sunamp costs and running costs. Can anyone provide more info.

[Nickfromwales]

19 minutes ago, vfrdave said:

This sounds like an expensive solution but I am uninformed to sunamp costs and running costs. Can anyone provide more info.

I found @readiescards a 9kW IVT HP for £700, and the Sunamp for DHW would be £2k to £2.3K depending upon size 9 or size 12

 

[vfrdave]

2 minutes ago, Nickfromwales said:

I found @readiescards a 9kW IVT HP for £700, and the Sunamp for DHW would be £2k to £2.3K depending upon size 9 or size 12

 

More expansive than a boiler and oil tank along with thermal store but is there less plumbing required also?

[vfrdave]

@Nickfromwales are the sunamp an approved installer job as well?

[Nickfromwales]

10 minutes ago, vfrdave said:

More expansive than a boiler and oil tank along with thermal store but is there less plumbing required also?

Theres no way in a million years that the above is more expensive than an oil boiler ( circa £2k+ ) and a tank + lines + slab + installation, plus at least £800-£1k for a decent TS, prob moe like £1.2k tbh. Have you been at the cooking sherry ?

[vfrdave]

@Nickfromwales I will concede it was a back of a fag packet price with a basic boiler and not considering slab/install etc, plus my head like @Ricco is in a spin with all this.

Does the sunamp require approved installer?

Fancy some consultation in NI?

Edited November 21, 2018 by vfrdave

[Nickfromwales]

2 minutes ago, vfrdave said:

@Nickfromwales I will concede it was a back of a fag packet price with a basic boiler and not considering slab/install etc, plus my head like @Ricco is in a spin with all this.

Does the sunamp require approved installer?

Fancy some consultation in NI?

In honesty, i think you need an accreditation from SA to install. The physical part of the install is dead straight forward, but the planning and design is uber critical.

If there's a wee dram of cooking sherry left over, i may consider it