Buffer tank and secondary pumps. Do I need them?

[joth]

On 25/12/2025 at 17:56, JohnMo said:

How do you do your schedule?

It's controlled by Loxone with load and weather compensation, but during periods of very cheap energy i push right down the turn on threshold, bump up target temperature very slightly, and boost the curve based on the difference between the floor slab temperature and target temperature 

Means i can squeeze out a day's heating demand in about 4 hours operation. The DHW schedule is a bit more hard coded as I want that to happen last thing during the cheap rate block

 

[joth]

5 hours ago, jimseng said:

I don't have to run the UHF at 35, I can set any level and I intend to collect lots of data and learn how to get the most out all my energy demands and sources

To get best energy efficiency you'll want to use weather compensation, so you set a curve of flow temperature for given outdoor temperature. Perhaps 35 or more on vary coldest days tapering down to around 20 when the outdoor temperature approaches 20.

The only downside of running a very low flow temperature is risk of the heat pump short cycling, which reduces its efficiency greatly, but a large ufh area makes this very unlikely so long as most looops remain open. 

[JohnMo]

42 minutes ago, joth said:

squeeze out a day's heating demand in about 4 hours operation.

So is your heat pump oversized to allow this?

[joth]

On 26/12/2025 at 20:55, JohnMo said:

So is your heat pump oversized to allow this?

No, it was oversized to facilitate fast DHW reheat times, and because the MCS installer was skeptical our enerphit would succeed. PHP said 4kW demand but we installed 8kW.

This supporting the fast cheap rate heating cycle is a happy coincidence, as in 2019 I really wasn't specifying this based on the existence of such deals. 

 

[JohnMo]

7 hours ago, joth said:

No, it was oversized to facilitate fast DHW reheat times, and because the MCS installer was skeptical our enerphit would succeed. PHP said 4kW demand but we installed 8kW.

This supporting the fast cheap rate heating cycle is a happy coincidence, as in 2019 I really wasn't specifying this based on the existence of such deals. 

 

I'm in the process, of really considering replacing my 6kW ASHP, that doesn't modulate well with a 4kW one, that does modulate well. 

 

Will then either repurpose the 6kW ASHP, as a hot tub heater (via a plate exchanger), or sell it on to someone with a suitable heat loss.

[jimseng]

Just to resurrect this thread. I am going to pay more attention before I go any further with the install. I am going to go back to the suppliers for more information but to get me started, I need to make sure I understand what I have to establish.
To take the figures from the UFH supplier:

 

If I add the output column for this floor I get a total of 4026. Is this 4kw of heat output? With similar for the second floor that is close to 8kw. A house with a supposed heat loss of 3.2kw this seems too much. But I presume this is with a flow temp of 45 deg (45/40, is this flow temp and DT?)

I guess I am going to have to calculate the output with a flow temp that matches my heat loss? This would presumably determine the flow temp of the heat pump and therefore the required size of the heat pump.
Am I on the right track or wildly mis-understanding things. Let's start with this given that everybody is so helpful.
I need to look closer at how the heat loss was established, the SAPS printout doesn't help me because it is hard to decipher but I am working on a 3.2kw heat loss for the house at the moment which I guess is about right. 

[SimonD]

1 hour ago, jimseng said:

I guess I am going to have to calculate the output with a flow temp that matches my heat loss? This would presumably determine the flow temp of the heat pump and therefore the required size of the heat pump.
Am I on the right track or wildly mis-understanding things. Let's start with this given that everybody is so helpful.

 

Yes, you're almost there.

 

1. You must confirm your calculated room-by room heat loss - this is the foundation of everything.

2. You then take each room's floor area and divide the room's calculated heat loss by the area. This gives you a W/m2 value.

3. You would then normally use this W/m2 value to determine the pipe spacing and output for each room, getting the flow temperature as low as possible

 

Unfortunately, the UFH has already been installed in your ground floor, but you can work around this using both mean water temperature and flow rates through the system.

 

At present, assuming the 3.3kW heat loss is the correct value the UFH system you have had designed and partially installed would output this at a mean water temperature (flow temp+return temp/2) of around 27C - obviously this is just an averaged value as it just divides your heat loss by your total floor area of 158m2. This gives about 21W/m2 that the underfloor heating needs to produce.

 

The advantage you have here is that the spacings are pretty close and this means you have the opportunity to run your heat pump at low flow temperatures.

[jimseng]

Quote

You must confirm your calculated room-by room heat loss - this is the foundation of everything.

Thanks @SimonD Yes indeed. OK so I have been relying on figures from the heat pump quote. Perhaps you could indulge me so I can go through and try and calculate each room myself to check the figures. Following Heat Geek's guide I have done a calculation for my living room. I have probably got this wrong so forgive my ignorance.
Also, since the house is still under construction I have to go with the architects wall build up U values, proposed window U values and hope the builder achieves the figures.
"External wall to achieve: 0.13 W/m²K"

"Windows to achieve: 1.4W/m²k"

I don't know how we consider the internal walls, or the fact I have an MVHR but perhaps we could start slowly so my head doesn't melt.

2 external walls with 2 windows, DT of 24 deg:

    Walls 22.7 m2 (22.7*0.13*24) = 70.83w

    Windows 6.05 m2 = (6.05*0.13*24) = 203.3w

Total 274w for the two external walls.
Where do I go from here?

 

[SimonD]

48 minutes ago, jimseng said:

Where do I go from here?

 Here's a single sheet spreadsheet you can use that covers all the basic heat loss calcs. Should be fairly self-explanatory. I built it using open document format so it runs best in LibreOffice or similar, but excel will be fine even if it then throws up compatibility errors. For the whole room losses you need to calculate all walls, ceilings/roof and floor and then air change rates. If you have a solid ground floor, room temp - 7C for design temp difference should be fine although some regions say 10 is fine. For any part walls, which I assume you may not have, use your target room temp - 10C.

 

For your air change rates, with MVHR, 0.5 is the max you want to go for.

 

Internally, with a well insulated home with MVHR, you can assume no difference in temperture between rooms so those internal walls/ceilings/floors can be set to 0 losses, unless you want to have different temperatures, then you need to input U-Values and temperature differences between those rooms.

 

Have a crack at it and let us know.

Heat loss single sheet.ods

[jimseng]

Thanks for this.

Not quite sure what to put in place for ceiling, floor or ventilation loss. The floor is UFH sat on top of a layer 150mm of celotex insulation, then screed. For the room I am calculating it is a living room with a heated room above.

Can you check my inputs and correct accordingly. If I can get this one right I can go through the rest of the rooms.

I really appreciate your efforts.HeatLoss1.ods

[marshian]

5 hours ago, SimonD said:

 Here's a single sheet spreadsheet you can use that covers all the basic heat loss calcs. Should be fairly self-explanatory. I built it using open document format so it runs best in LibreOffice or similar, but excel will be fine even if it then throws up compatibility errors. For the whole room losses you need to calculate all walls, ceilings/roof and floor and then air change rates. If you have a solid ground floor, room temp - 7C for design temp difference should be fine although some regions say 10 is fine. For any part walls, which I assume you may not have, use your target room temp - 10C.

 

For your air change rates, with MVHR, 0.5 is the max you want to go for.

 

Internally, with a well insulated home with MVHR, you can assume no difference in temperture between rooms so those internal walls/ceilings/floors can be set to 0 losses, unless you want to have different temperatures, then you need to input U-Values and temperature differences between those rooms.

 

Have a crack at it and let us know.

Heat loss single sheet.ods 36.2 kB · 3 downloads

 

Oh that's rather neat but I have a question, related to it 

 

When I did my heat loss with an on-line process (heat engineer software) I used the internal wall measurements but your sheet seems to require external wall dimensions - is that normal practice?

[SimonD]

19 hours ago, jimseng said:

Thanks for this.

Not quite sure what to put in place for ceiling, floor or ventilation loss. The floor is UFH sat on top of a layer 150mm of celotex insulation, then screed. For the room I am calculating it is a living room with a heated room above.

Can you check my inputs and correct accordingly. If I can get this one right I can go through the rest of the rooms.

I really appreciate your efforts.HeatLoss1.ods

 

You should have a ground floor u-value in the designs somewhere?

 

On ventilation loss, you're fine as you've put it in.

 

Regarding ceilings, what is the build up you have? I assume there's 100mm insulation in there?

 

 

 

[SimonD]

15 hours ago, marshian said:

 

Oh that's rather neat but I have a question, related to it 

 

When I did my heat loss with an on-line process (heat engineer software) I used the internal wall measurements but your sheet seems to require external wall dimensions - is that normal practice?

 

Thanks. Well spotted. The measurement procedure was changed in the latest CIBSE revision. It used to be full storey height, which was when I created the spreadsheet, now it's just internal measurements, ignoring height of floors. So you're fine. The old process also used to call for adding 1/2 thickness of all partition walls.

Edited 4 hours ago by SimonD

[jimseng]

Quote

Regarding ceilings, what is the build up you have? I assume there's 100mm insulation in there?

If we are talking about between GF and FF then just a layer of Rockwool. The roof space is 350mm warmcell insualtion as it is considered a warm room. I'll see if I can find a u value for the floor.

The heat loss from your spreadsheet is coming out at about 80% more than what I had back from the original UFH emitter schedule for the one room I have started with. I'm trying to find out why.