Heat battery for just underfloor heating?

[Radian]

34 minutes ago, Tilbert said:

Would 30 degrees be a typical flow temperature.

 

There's no real typical value. Only for a given type of floor buildup. Thick concrete slabs tend to be run at lower temperatures for longer. These are well suited to ASHP.

My own UFH is in a thin screed and fired by a gas boiler. I run it hotter for less time and expect a faster response from it.

[SteamyTea]

14 minutes ago, Radian said:

56 minutes ago, Tilbert said:

Would 30 degrees be a typical flow temperature.

 

There's no real typical value

While not typical, @JohnMo has a chart with pipe spacing and temperatures on it.  So should be able to design for 30°C, might just mean that most of the floor is sitting on pipe, but you get the idea.

[JohnMo]

[SteamyTea]

Something like this then.

35 W.m²

 

[Tilbert]

Thank you all for your great input.
 

I’m aiming on the ASHP and floor construction with the slab on top and UFH in the slab.

The graphs and calculations are a bit over my head (tiny brain…  ) - but I’ll try again when I have a clear head (feels a bit like going back to school….  …. homework time…..!!
 

All is greatly appreciated 

[JohnMo]

49 minutes ago, Tilbert said:

homework time

Every day is a school day.

 

To make sense of the attached you need to know your calculated heat loss, then divide that by the floor area, that gives you W/m2. then its just a matter of drawing a straight line between your preferred flow temp (high 20s to low 30s) and the W/m2 and that will give you ufh pipe spacing you need as minimum

[Tilbert]

Hello all again, I meant to post sooner but we had planning delays …..!

 

Seem to have some progress now and the floor construction we are planning is :

tiled floor finish - 20mm

concrete slab with UFH pipes tied to rebar mesh reinforcing - 150mm *

Kingspan TF70 pir insulation - 150mm

concrete - 50mm

MOT blinding - 100mm

ground (we have no existing foundations).

 

So overall depth will be circa 470 - 500 mm, which is about as thick as we can go.

 

* the slab is 150mm as we’re having a mezzanine floor built on top, freestanding in metal and structural engineer says 150mm needed.

 

I did some U-value calculations and came out at:

floor - 0.153 w/m2k

roof - 0.15       “

walls - 0.18      “
New French doors - 1.5 w/m2k

We have 2 old existing windows and a door, but no idea how (or if) to work these out.

 

Any suggested amendments or observations, greatly appreciated.

 

Big thanks all once again 
 

[JohnMo]

Question do you need the 50mm concrete below the insulation, could that be insulation? EPS or PIR.

 

Your French doors U value is quite poor, we have double glazed French Doors and had Krypton fill double glazed unit and good frames and an overall U value (frame and glazing) of 1.1. So some improvement could be made there.

 

The old one if double glazed will be circa 2 to 2.5 U value, if single glazed 5 at best guess.

[joth]

On 11/05/2023 at 11:33, Tilbert said:

Thank you all for your great input.
 

I’m aiming on the ASHP and floor construction with the slab on top and UFH in the slab.

The graphs and calculations are a bit over my head (tiny brain…  ) - but I’ll try again when I have a clear head (feels a bit like going back to school….  …. homework time…..!!
 

All is greatly appreciated 

 

Are you thinking specifying the ASHP exactly to meet the needs of this one room of UFH, or oversize it for future connection to other rooms or providing DHW? If so, you may find it can't modulate down sufficiently to meet the needs of this one room, so you may need a buffer tank. 

Alternatively buy the smallest cheapest unit you can now with a eye to selling and replacing it if needs change. 

 

 

[Adrian Walker]

On 07/05/2023 at 21:55, Tilbert said:

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.

 

 

 

Make it an insulated slab, foamed glass or similar then concrete on top. Do you really need a screed?

[Tilbert]

On 25/09/2023 at 11:17, JohnMo said:

Question do you need the 50mm concrete below the insulation, could that be insulation? EPS or PIR.

 

Your French doors U value is quite poor, we have double glazed French Doors and had Krypton fill double glazed unit and good frames and an overall U value (frame and glazing) of 1.1. So some improvement could be made there.

 

The old one if double glazed will be circa 2 to 2.5 U value, if single glazed 5 at best guess.

thanks, and good idea on the Krypton, will ask suppliers.

The structural engineer has specified the 50mm

 

[Tilbert]

On 25/09/2023 at 12:08, joth said:

 

Are you thinking specifying the ASHP exactly to meet the needs of this one room of UFH, or oversize it for future connection to other rooms or providing DHW? If so, you may find it can't modulate down sufficiently to meet the needs of this one room, so you may need a buffer tank. 

Alternatively buy the smallest cheapest unit you can now with a eye to selling and replacing it if needs change. 

 

 

I think we’ll need the ASHP just for this one room, no DHW needed. Not sure what a buffer is or if needed in this case.

thanks again for reply
 

[JohnMo]

1 minute ago, Tilbert said:

Krypton, will ask suppliers.

Ask them for a full door/window U value calculation (U_w), if they don't/will not supply be careful. They ae not telling you something - like the window frame is rubbish thermally, so will make the U_w rubbish no matter the U_g is.

 

The only figure that important is U_w but most suppliers only want to give out U_g.

 

U_w - mean whole window or door set U value

U_g - glazing U value only

[Tilbert]

On 25/09/2023 at 15:49, Adrian Walker said:

Make it an insulated slab, foamed glass or similar then concrete on top. Do you really need a screed?

 This is the layout from Structural engineer design.

thanks

[JohnMo]

5 minutes ago, Tilbert said:

Not sure what a buffer is or if needed in this case.

thanks again for reply

Towards the bottom of the page - what a buffer is and what it does

 

[Temp]

On 07/05/2023 at 21:55, Tilbert said:

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

 

The issue with ASHP is that electricity has historically been three times the cost of mains gas per kWh. So an ASHP needs to be operated with a COP of 3 to be comparable to gas. This is possible if the ASHP is sized and installed correctly. In fact it's possible to get higher than 3 resulting in cost savings.

 

However if the installer gets it wrong and the COP averages only say 2 then the bills could be 50 % more or worse. In some bad cases  they could double. 

 

To maximise the COP the ASHP must be large enough and coupled to a low temperature system (eg UFH under tiles or over sized rads). If its under sized or connected to rads designed for the much higher flow temperatures produced by gas boilers then COP can suffer.

 

This proplem is reducing as new designs of ASHP are getting better at producing high flow temperatures. 

 

Others know more about the state of the art than me.

[ProDave]

1 hour ago, Temp said:

However if the installer gets it wrong and the COP averages only say 2 then the bills could be 50 % more or worse. In some bad cases  they could double. 

But a COP of 2 is better than effectively a COP of 1 if a heat battery and a resistance heating element is the alternative being considered in this thread.

[JohnMo]

I ran my ASHP yesterday more on purpose (finishing off commissioning as ASHP was installed at the start of summer) than a need. Ran it for about 7 hours, the electric meter recorded about 2 kWh of additional electricity use, at yesterday's temp the CoP would have been circa 6.2. Just looked back at my gas usage for last year, it was a bit colder, but a typical day in late September used 21 to 25kWh of gas. So 7p X 21, so about £1.50 at today's costs, while the ASHP was £0.60. 

 

If you have PV and run the ASHP in the shoulder months, it's cheaper than gas. If no PV it compatible to gas down to about zero degs.

[Temp]

1 hour ago, ProDave said:

But a COP of 2 is better than effectively a COP of 1 if a heat battery and a resistance heating element is the alternative being considered in this thread.

 

Definitely, I should have said.   

[Adrian Walker]

On 28/09/2023 at 15:23, Tilbert said:

 This is the layout from Structural engineer design.

thanks

Why not someone similar to this? https://www.mikewye.co.uk/product/foam-glass-insulation-aggregate/

[JohnMo]

6 minutes ago, Adrian Walker said:

Why not someone similar to this? https://www.mikewye.co.uk/product/foam-glass-insulation-aggregate/

You need a deep foundation for that, nearly 4 times as deep as PIR!

[Tilbert]

Aaaghhh…. and Hello….!

 

I’m hoping for some inspiration from you good people on this helpful forum…..

We seem to have run into a dilemma on the slab construction from the UFH company we’re using. All along I had proposed the above slab layout with the UFH pipes in the 150mm slab. But, just as our builder is about to start the first layer of MOT, they sent the following email:

 

 

Hi, 

We have had a look at the proposed floor construction for your extension. The optimum depth of screed is 65-75mm and it looks like you are proposing 150mm plus floor finish which not something we would be able to recommend. The absolute maximum we would occasionally see is 100mm. Having such a huge slab double the recommended depth will have some impact the performance of the system / running costs and make it difficult to control. 

We would strongly suggest that you build the floor up by either increasing the thickness of the concrete slab (below the insulation) or increasing the thickness of the insulation. I appreciate that insulation is expensive however you will save money on Screed and on the running costs in the long run. 

 

 

Aaaghh… so am now worried that the UFH method we had planned to use will not work. The structural engineer had said the 150mm needed, but I can run this alternative by him to see what he thinks…

 

What do the great minds on here think ?

 

Big Big Thanks, as ever

[Mr Punter]

On 29/09/2023 at 16:26, Adrian Walker said:

Why not someone similar to this? https://www.mikewye.co.uk/product/foam-glass-insulation-aggregate/

 

 

Materials alone will be an extra £150/m2.

[Mr Punter]

Ask the SE to just do 100mm concrete slab onto compacted MOT.  Why is it reinforced?  You can then have insulation and 50mm liquid screed or 75mm traditional screed.

[SteamyTea]

Can you get more insulation in and end up with the same floor levels?