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 Monday at 09:44 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.

[jimseng]

So I did have a fairly robust conversation with the ASHP supplier. I have opted to have no zoning and insisted on there being no buffer or secondary pumps. They are now installing a 30L volumiser (which is in fact a buffer tank but with only two ports connected apparently). 
He maintains that by not at least zoning each floor I might get an increased COP but it will cost more in the long run. I maintained that adding actuators to the manifold at a later date is something I can easily do if I need to, but I would prefer to monitor each room temperature, gather data and balance the system with the flow meters over time as I learn how everything performs.
As for the heat loss calculations, which are all based on drawings because the house is still in build the ASHP company said:

"Our heat losses are based on MIS 3005-D we have to use this standard when sizing and designing a heat pump."

 

His view was that recently (in the last 3 months) the ventilation heat loss calculations have changed and been downgraded as they were considered too high. Certainly, for my living room they comprise nearly 50% of the heat loss calculation. It is all bewildering to me. 

 

Quote

Pretty old view of how to install an efficient modulating ASHP.

I concur, but what do I know. I have never owned a heat pump, an efficient house or lived with low temperature heating before.

[JohnMo]

Are you actually having to pay any thing for this ASHP install?

[jimseng]

Yes. It is going to cost me 6.5k materials and 3.5k install and commissioning minus the 7.5k BUS. Although I haven't had the cost of the secondary pumps taken off yet. And I opted for a 300l UVC. They are funding the BUS. This was significantly cheaper than another quote I had for a Stiebel ASHP. The plumber said he could do it cheaper but it required a big bill for the MCS inspection and sign off, and I would have had to fund the whole thing up front so I went with the simpler option.

[JohnMo]

6 minutes ago, jimseng said:

They are funding the BUS

Not really the tax payer is, they shouldn't be charging you that. They get approval for the grant before they start.

 

7 minutes ago, jimseng said:

Stiebel ASHP

Maybe because they are super expensive, before you add other stuff. Plus from the sounds of it, was doing the install via an umbrella scheme. Not a bad thing, but he's taking the piss charging up front, as the MCS umbrella company wouldn't be charging him.

[SimonD]

2 hours ago, jimseng said:

"Our heat losses are based on MIS 3005-D we have to use this standard when sizing and designing a heat pump."

 

Here we go again. In some ways he's correct. In others he isn't. MIS 3005-D provides a basic procedure that must be followed. The Standard for heat load calculations and the correct method is defined by BS EN1283-1 2017 which is also covered in the CIBSE Domestic Heating Design Guide. Lots of designers and installers falsely believe it's prescriptive, but it isn't, especially where building details are known. The outputs aren't defined by the procedure/standard/methods, but fundamentally by the inputs.

 

2 hours ago, jimseng said:

His view was that recently (in the last 3 months) the ventilation heat loss calculations have changed and been downgraded as they were considered too high. Certainly, for my living room they comprise nearly 50% of the heat loss calculation. It is all bewildering to me. 

 

Again, this is kind of correct. The changes were implemented in June this year according to the standard  BS EN1283-1 2017. This provides two methods of ventilation calculations. one based on actual door blower tests and the other based on a simplified method, which isn't too many miles off the old one. It's also based upon the designer's perspective on the building. The standard also includes infiltration losses based on the building height and exposure.

 

2 hours ago, jimseng said:

He maintains that by not at least zoning each floor I might get an increased COP but it will cost more in the long run.

 

What was his basis for telling you this?

 

Is this installer MCS certified or using an MCS umbrella? What heating qualifications does the installer hold above the basics? I'd really be wanting to use someone who has at the very least done extra training by some recognised provider in hydronics.

 

So in terms of calculated heat loss what are the figures being used for reference for all their design?

[SimonD]

1 hour ago, JohnMo said:

MCS umbrella company wouldn't be charging him.

 

A lot of them are nowadays - even ones set up by merchants charge a hefty 1000-1500 for the privilege. Some will also send someone to site to complete the commissioning costing another 400-500

[jimseng]

 

Quote

What was his basis for telling you this?

I think the point he was making was that with solar gain, being able to shut off a zone will reduce the amount of heat required and therefore reduce the amount of energy consumed. Or something.

 

Quote

So in terms of calculated heat loss what are the figures being used for reference for all their design?

I'm not sure what you mean by this? I gave them the plans and they returned a quotation which included an emitter schedule with the heat loss for each room. I added up the totals and that is where I got the 3.3kw heat loss for the building. Although my heat loss figures come out higher based on your spread sheet. The architect is coming back to me with their figures.
The installer is MCS accredited. 

[joth]

12 hours ago, jimseng said:

think the point he was making was that with solar gain, being able to shut off a zone will reduce the amount of heat required and therefore reduce the amount of energy consumed. Or something

Sorry i lost track but is he still assuming a flow temperature of 40°C or something?

If you're running low and slow, any rooms that are already over 24°C simply won't take any more heat from an emitter that is barely any hotter than target temperature. Whereas rooms that are (significantly) colder will have a steeper temperature gradient and hence transfer more heat.

This is one of the great advantages of low and slow in a low energy home: it's largely self balancing and so you can jettison all the complicated controls as @JohnMoand others evangelise so well here 

[SimonD]

13 hours ago, jimseng said:

I think the point he was making was that with solar gain, being able to shut off a zone will reduce the amount of heat required and therefore reduce the amount of energy consumed. Or something.

 As @joth says, there are other factors to consider in how the system will work with solar gain with plenty of ways to mitigate the problem without zoning that shuts down flow to a specific area. The zoning proposal is more likely to increase your costs, not the other way round.

 

13 hours ago, jimseng said:

I'm not sure what you mean by this? I gave them the plans and they returned a quotation which included an emitter schedule with the heat loss for each room. I added up the totals and that is where I got the 3.3kw heat loss for the building. Although my heat loss figures come out higher based on your spread sheet. The architect is coming back to me with their figures.
The installer is MCS accredited. 

 

Okay, so you will have received a design from the installer. So what are the individual room loss the installer calculated? And how do these each match up with the UFH design? And what flow temperature is being proposed. What has the installer calculated the existing UFH output to be at design temperatures?

 

Additionally, when doing a heat loss calculation without blower test results, a lot of assumptions have to be made, including with ACH and MVHR. Also, because heat loss calculations are fundamentally based on SAP too, there are further adustments that can be made for thermal bridging, which can have a big impact on the overall heat loos calculation. In addition to this, and something that is often overlooked is that loss through the floor and therefore the floor's U-value, varies depending on the floor dimensions and the specific arrangement of the room and external walls - a lot of designers don't know this and put a single figure in because it isn't specifically covered in the design training, not even in Heat Geek training.

 

Here's a specific example regarding my house:

 

MCS Umbrella does a heat loss calcuation using my given U-values and floor plans - heat loss about 7.5kW

My heat loss calculation - 4.83kW

 

This is the difference between a 9kW heat pump and a 6.5kW heat pump by the chosen manufacturer.

 

Now, the other night, at -6 to retain a steady state of about 21.5C indoors my heat pump had to produce 3.8kW for 2 hours and for the rest of the time knocked itself back to 3.1kW. So even my heat loss calcuation taking into consideration thermal bridging and careful calculation of ventilation rates is slightly high, but I dread to think what things would be like if I'd put in a 9kW unit.

 

Now, your installer may have covered all this, but I'm not so sure given the first design using buffers etc.

 

My other question has to be why no one, including your architect, has suggested waiting until you've completed your blower tests before doing the heat pump design? 

Edited 10 hours ago by SimonD