Heat battery for just underfloor heating?

[Tilbert]

Hello, I’m a first time poster here. Hopefully my question is not too daft…

 

I’m converting a barn, which is attached to our house, into our new kitchen. 
It’s about 72 square metres and will be just a kitchen. 
I’m planning on a wet underfloor heating system and considering how to heat it. 

I’m trying to figure out if I could put in a heat battery just to power the underfloor, so to draw electricity from the grid overnight at cheap rate into the battery and then release to heat the underfloor.

Any words of wisdom greatly appreciated.

 

we are on oil for the rest of the house (no gas in our village). I initially looked at air source but not impressed by the ongoing running costs.

 

Thank you
 

[ProDave]

What is your issue with an ASHP?  Heating just low temperature UFH it will use less than one third the amount of electricity than a heat battery will.  So your off peak rate would have to be less than 1/3 the peak rate to make it cheaper.

 

If the rest of your house is oil, why do you want off peak?  a penalty of off peak is the day rate is higher, so you have to use a lot of off peak to make it worthwhile.

 

The cost of buying an ASHP must be very similar to the cost of a heat battery so would seem a better and simpler choice to me.

[JohnMo]

Plus you can pick up a heat pump for the same or less money than a heat battery.

 

Why not connect to the oil system, you already have?

[MikeSharp01]

Welcome to the forum. 72mSq is a big Kitchen (9x8m) depending on the slab buildup and the insulation / air tightness of the overall build, you will want to be heating a lot of concrete and lots of insulation, it might be worth looking at the Willis heater option. These are small, a few Kw, water heaters that you can run over night on the cheap rate to warm the slab and let it cool during the day. @TerryE has done this successfully and done the sums, for his house, to show the cost / value outcomes against ASHP anyway.    

[SteamyTea]

38 minutes ago, MikeSharp01 said:

These are small, a few Kw

kW maybe.

 

If the floor is concrete, and sitting on a lot of insulation, there will be no need for an extra store.

Low temperature, high heat capacity systems are not used like a traditional system.

Edited May 7, 2023 by SteamyTea

[Nickfromwales]

1 hour ago, Tilbert said:

Hello, I’m a first time poster here. Hopefully my question is not too daft…

 

I’m converting a barn, which is attached to our house, into our new kitchen. 
It’s about 72 square metres and will be just a kitchen. 
I’m planning on a wet underfloor heating system and considering how to heat it. 

I’m trying to figure out if I could put in a heat battery just to power the underfloor, so to draw electricity from the grid overnight at cheap rate into the battery and then release to heat the underfloor.

Any words of wisdom greatly appreciated.

 

we are on oil for the rest of the house (no gas in our village). I initially looked at air source but not impressed by the ongoing running costs.

 

Thank you
 

Hi.

If you cannot attach to the oil heating system, which is 100% what you should be doing btw, then you may as well just install an in-screed direct electric heater wire and heat the slab up instead of a stupidly expensive SA / Thermino as the slab will have inherent storage and release characteristics already.

 

Before we advise any further, we would need to know how much insulation and how thick the slab / screed will be, please?

 

You'll get probably less than 3 hrs of space heating from the largest SA unit, the 12-14kWh one, when fully charged, if you're lucky. Every one of these I fitted for space heating, with plans and designs approved directly by SA, ended in very expensive arguments / tears. The ones that didn't blow up, that is  

[Tilbert]

Thank you all for your comments, certainly helpful.

The slab will be constructed of concrete, then insulation, then screed on top.

We’re insulating the walls with 100mm celotex and roof 100mm, so hoping the room will be well insulated. (It’s fairly high at circa 6metres to the apex).

 

In my mind I had thought of moving away from oil for green reasons, and also there is some difficulty in running pipes from the existing boiler to the UF heating - but if this is the most practical and good for low running costs may persevere with this.

 

I’d initially thought of ASHP, but been put off by comments on very high and unexpected running costs.

 

 

 

[JohnMo]

21 minutes ago, Tilbert said:

ASHP, but been put off by comments on very high and unexpected running costs

Running UFH from an ASHP shouldn't be expensive in fact it's possibly one of the cheapest forms of heating when coupled to UFH. Low flow temperature leads to very good CoP.

 

Attached is section out of our technical manual which gives, flow temperature, heating power and CoP, for reference.

 

By the way you should be adding way more insulation, I doubt you even meet building regs minimum requirement, with 100mm.

 

For UFH you ideally need around 150mm or more.

[Nickfromwales]

I'd review the floor construction methodology, and do as a lot here have done, including what I do for my clients.

Forget the screed, put the insulation down low on the ground, and then have a constructional, heated slab with steel mesh reinforcement for anti-cracking. The ufh pipes get cable-tied to the steel, and then the concrete pour is your finished interior floor ( less final coverings ).

A small, cheap ASHP on this would be very cheap to run. Coming off oil will have its challenges also, as the oil boiler could never see just the UFH independently or it'll "poop the bed". At the minimum, you'd need to install something like a 150L - 200L buffer tank to accept the huge amount of heat the oil boiler will produce before it realises it's not even needed. If you run the UFH and radiators simultaneously, then you may JUST get away with it, but maybe not even then.

Oil boilers like to light, burn at 100% ( or 0%, nothing in-between with oil btw ), and burn for as long as is possible, and this is because they do not modulate. This means that it would produced huge amounts of heat, the UFH would be taking 5-10% of that at most, and then the boiler would cycle on/off repeatedly ( referred to as "short-cycling" ) which would eventually ruin the boiler. 

A decent 5kW ASHP is around £2.5k FYI, or less if you go for a lesser-known brand.

 

Rethink the floor, because, as stated, it shouldn't be heated with that poor a level of insulation, sorry! For your stated floor detail, you should be having rads on the walls instead.

[Tilbert]

Thanks JohnMo and Nickfromwales, this helps and makes the ASHP more attractive. 

[ Also I realise that off-peak electricity would be a ‘no-go’ for me as I still have the rest of the house to consider and this would pull mainly daytime from the grid. ]

I’ve read through some of the other posts on the forum too and with your comments am rethinking the floor.
We’re digging out the existing floor and the original plan was to replace with;

Screed on top (70mm) - with UFH in here

Insulation below this (100mm)

Concrete slab (100mm)

DPM
Sand for blind
MOT as base (100mm) - the barn is 350 yrs old & we have no foundations

 

So, if I understand the theory, this should be;

Concrete slab on top (100mm) - with UFH in here + reinforcing mesh 

DPM

Insulation (100mm)

DPM

Sand for blind
MOT as base (100mm)

       Hopefully I have this correct…?

Is the main reason for constructing this way to maintain steady heat in the concrete slab for longer ?

 

 

As an aside, an earlier comment suggested we have more insulation. On the wall we plan to have 100mm & roof 150mm.

 

BTW, this forum is massively helpful, thanks so much…!

 

 

[SteamyTea]

5 minutes ago, Tilbert said:

 

As an aside, an earlier comment suggested we have more insulation. On the wall we plan to have 100mm & roof 150mm

Assuming the areas are the same, you would use less energy with the 150mm under the floor. The reason is that the floor will be at a higher temperature than the air in the building.

The truck is that it is all in the formula.

 

W=U-Value X Area X Temperature Difference.

 

Increase any one of them and you have a greater power loss.

[Nickfromwales]

1 hour ago, Tilbert said:

So, if I understand the theory,

 

1 hour ago, Tilbert said:

Insulation (100mm)

Look, you're new here, so I won't shout  

You need way more than 100mm of insulation. That's not going to cut it. You need a minimum of 140mm to be anywhere near "energy efficient". To clarify, British Building Regs is the worst standard to build to without getting a bollocking off the BCO. That is NOT what you should aim for.

Dig out a bit more, maybe see if you can get away with less type1, whatever. But the maths are;

FFL of (x) thickness

less 100mm concrete c/w mesh & UFH pipes

less 140mm of PIR rigid insulation

less (x) depth below that to get to a suitable substrate to start the process.

That lot added up is what you need to get down to.

😎

[Radian]

2 hours ago, Tilbert said:

As an aside, an earlier comment suggested we have more insulation

 

Say the 100mm insulation has a U-value of 0.2 W/m2, then with the UFH flow at 35^oC and the ground beneath at 5^oC, then the loss in Watts will be 0.2 * 72 * 30 = 432W. Nearly 0.5kWh constantly leaking away to the Earth.

Do the same for the roof with 150mm of insulation  when 5^oC outside and 20^oC inside:  0.133 * 72 * 15 = 144W.

To get anywhere near the same low loss through the floor you'd need a minimum of 250mm giving U-value of 0.08 * 72 * 30 = 173W.

 

[Marvin]

Hi @Tilbert

 

As @Radian's reply shows, the devil is in the detail!

 

To do with prices, heating, power, thermal resistance, and on and on, the details give the answers.

 

Insulation v heating and cooling: Super insulation beats super heating and cooling.

 

Generally:  insulate once and pay once.

 

Heating and cooling pay once, and then pay about one tenth more next year, and one tenth more the next year and..... (ignoring inflation) until you move out or die.

 

Good luck.

[Dave Jones]

10 hours ago, Tilbert said:

Thanks JohnMo and Nickfromwales, this helps and makes the ASHP more attractive. 

[ Also I realise that off-peak electricity would be a ‘no-go’ for me as I still have the rest of the house to consider and this would pull mainly daytime from the grid. ]

I’ve read through some of the other posts on the forum too and with your comments am rethinking the floor.
We’re digging out the existing floor and the original plan was to replace with;

Screed on top (70mm) - with UFH in here

Insulation below this (100mm)

Concrete slab (100mm)

DPM
Sand for blind
MOT as base (100mm) - the barn is 350 yrs old & we have no foundations

 

So, if I understand the theory, this should be;

Concrete slab on top (100mm) - with UFH in here + reinforcing mesh 

DPM

Insulation (100mm)

DPM

Sand for blind
MOT as base (100mm)

       Hopefully I have this correct…?

Is the main reason for constructing this way to maintain steady heat in the concrete slab for longer ?

 

 

As an aside, an earlier comment suggested we have more insulation. On the wall we plan to have 100mm & roof 150mm.

 

BTW, this forum is massively helpful, thanks so much…!

 

 

 

id do.

 

150mm hardcore

300mm EPS insulation

75mm fibre screed (no need for metalwork).

 

ASHP as others have said IF you can get the barn airtight. If you cant it will struggle and as you have read be expensive to run.

 

Guessing the barn doesnt have a DPC so you will need to get over this, painted on etc.

[SteamyTea]

11 hours ago, Radian said:

loss in Watts will be 0.2 * 72 * 30 = 432W. Nearly 0.5kWh constantly leaking away to the Earth.

 

[Nickfromwales]

7 hours ago, Dave Jones said:

 

id do.

 

150mm hardcore

300mm EPS insulation

75mm fibre screed (no need for metalwork).

 

ASHP as others have said IF you can get the barn airtight. If you cant it will struggle and as you have read be expensive to run.

 

Guessing the barn doesnt have a DPC so you will need to get over this, painted on etc.

200mm of PIR means virtually the same value, but with 100mm less digging / removals / muck-away  I'd stick with 100mm of concrete and anti-crack mesh, but it'll depend on whether you want to turn the heating on / off vs up/down (eg winter constant on)

[Dave Jones]

cost wise 200mm celotex is £650 more expensive than 300mm eps, but your right if you have muckaways it may be worth paying the extra for celotex.

Edited May 9, 2023 by Dave Jones

[Tilbert]

3 hours ago, Nickfromwales said:

200mm of PIR means virtually the same value, but with 100mm less digging / removals / muck-away  I'd stick with 100mm of concrete and anti-crack mesh, but it'll depend on whether you want to turn the heating on / off vs up/down (eg winter constant on)

If I go for ASHP, then guess am more likely to leave heating on constant, rather than on/off - up/down.

 

Depth wise to FFL we’d planned to be at circa 400mm, so could possibly do : 

concrete inc mesh & UFH pipes - 100

insulation 200mm
mot - 100mm

 

 

 

[Tilbert]

18 hours ago, Radian said:

 

Say the 100mm insulation has a U-value of 0.2 W/m2, then with the UFH flow at 35^oC and the ground beneath at 5^oC, then the loss in Watts will be 0.2 * 72 * 30 = 432W. Nearly 0.5kWh constantly leaking away to the Earth.

Do the same for the roof with 150mm of insulation  when 5^oC outside and 20^oC inside:  0.133 * 72 * 15 = 144W.

To get anywhere near the same low loss through the floor you'd need a minimum of 250mm giving U-value of 0.08 * 72 * 30 = 173W.

 

Thanks. Is it advisable to try to match the heat loss of the floor to the roof & walls ?

 

My poor tiny brain is starting to ache….  . What does the 72 & 30 relate to in the above calculation please.

 

Is there a formula or method to work out the ‘recommended’ insulation thicknesses using the overall area to be heated - or is it a case of separate calculation for the floor then one for the wall and one for the roof.

 

 

 

[Radian]

1 minute ago, Tilbert said:

Thanks. Is it advisable to try to match the heat loss of the floor to the roof & walls ?

 

 

Not necessarily, but it's a good starting point. Why go to so much trouble in one area only to undo the good work in another.

 

3 minutes ago, Tilbert said:

What does the 72 & 30 relate to in the above calculation please.

 

72 was your sq.m floor area (and by inference roof area). 30 was the temperature differential assuming flow temperature of 30^oC in the UFH pipes and a soil temperature of 5^oC. Just an approximation but a fairly reasonable one that demonstrates what @SteamyTea was pointing out about the differential being so much greater when you're insulating hot pipes:

 

20 hours ago, SteamyTea said:

W=U-Value X Area X Temperature Difference.

 

As SteamyTea posted earlier, this formula calculates the heat loss (power) across a given area of insulator for a given temperature at each side. You need a separate calculation for each element and a way to stack up the various layers of building materials to arrive at a combined heat loss. Here's a handy online tool that you can use for free to do this.

[Nickfromwales]

48 minutes ago, Tilbert said:

Thanks. Is it advisable to try to match the heat loss of the floor to the roof & walls ?

 

My poor tiny brain is starting to ache….  .

Stay with us, you'll be fine.

 

The point being made, is the chain is as strong as the weakest link, so in that room it would be wise to investigate how well the walls and roof retain heat in the room, once the floors produced it.  

Take this one step at a time, and the floor is the chunky one-time-only job, so let's stay focussed on that for now. Draughtproofing can come later, internal insulation can come later, beefing up the roof could come now if it only means buying a bit more fluffy rockwool and a stinking day fitting the wretched stuff.

You don't need to 'match' anything right now, just do the floor as above and then work your way up  

1 hour ago, Tilbert said:

If I go for ASHP, then guess am more likely to leave heating on constant, rather than on/off - up/down.

 

Depth wise to FFL we’d planned to be at circa 400mm, so could possibly do : 

concrete inc mesh & UFH pipes - 100

insulation 200mm
mot - 100mm

This will serve you very, very well. The ASHP is a bone of contention as it's just as large a spend if not larger than just adding a buffer tank and using the oil. You could ( if the oil boiler is NOT a combi and you have a cylinder ) swap the existing hot water cylinder to a thermal store ( buffer tank that has a coil inside which heats the hot water instantaneously ) which would utilise the same footprint. From that you could feed the UFH manifold and the radiators, and be without the short-cycling issues.

ASHP, if plumbed through a wall and is a simple install, could be the least disruptive option. Without being there to survey it, it's quite tough to call it in fairness.

[Tilbert]

2 hours ago, Nickfromwales said:

Stay with us, you'll be fine.

 

The point being made, is the chain is as strong as the weakest link, so in that room it would be wise to investigate how well the walls and roof retain heat in the room, once the floors produced it.  

Take this one step at a time, and the floor is the chunky one-time-only job, so let's stay focussed on that for now. Draughtproofing can come later, internal insulation can come later, beefing up the roof could come now if it only means buying a bit more fluffy rockwool and a stinking day fitting the wretched stuff.

You don't need to 'match' anything right now, just do the floor as above and then work your way up  

This will serve you very, very well. The ASHP is a bone of contention as it's just as large a spend if not larger than just adding a buffer tank and using the oil. You could ( if the oil boiler is NOT a combi and you have a cylinder ) swap the existing hot water cylinder to a thermal store ( buffer tank that has a coil inside which heats the hot water instantaneously ) which would utilise the same footprint. From that you could feed the UFH manifold and the radiators, and be without the short-cycling issues.

ASHP, if plumbed through a wall and is a simple install, could be the least disruptive option. Without being there to survey it, it's quite tough to call it in fairness.

Thanks for your guidance, certainly helping to think it all through. 
I’m tempted with the ASHP as it’s a simpler install and there’s a difficult run from the oil boiler (some likely ups, downs and arounds… just to get to the UFH manifold). 
I’m not really sure how to calculate the ongoing running costs of oil vs ASHP, I have some data from a supplier but hard to factor in all the variables.

[Tilbert]

3 hours ago, Radian said:

 

Not necessarily, but it's a good starting point. Why go to so much trouble in one area only to undo the good work in another.

 

 

72 was your sq.m floor area (and by inference roof area). 30 was the temperature differential assuming flow temperature of 30^oC in the UFH pipes and a soil temperature of 5^oC. Just an approximation but a fairly reasonable one that demonstrates what @SteamyTea was pointing out about the differential being so much greater when you're insulating hot pipes:

 

 

As SteamyTea posted earlier, this formula calculates the heat loss (power) across a given area of insulator for a given temperature at each side. You need a separate calculation for each element and a way to stack up the various layers of building materials to arrive at a combined heat loss. Here's a handy online tool that you can use for free to do this.

Great thanks, I’ll give that tool a try.

Would 30 degrees be a typical flow temperature.

 

[SteamyTea]

1 hour ago, Tilbert said:

I’m not really sure how to calculate the ongoing running costs of oil vs ASHP, I have some data from a supplier but hard to factor in all the variables.

What the SCoP is for.  It tells you the expected performance over a typical metrological year.

Then divide the price of electricity by the SCoP to get the running cost.

 

The heat demand of the house is blind to the technology used to heat it.