MCS heat pump sizing calc is 7589W but 12kW pump is being recommended...

[SBMS]

11 minutes ago, mk1_man said:

Hi Chris,  we are in final stages of build / retrofit of 250m2 house, aiming for as near passive house as we can with attention to air tightness, triple glazing throughout, removed chimney and have mvhr.  Also underfloor ground and first floors with 28mm primaries etc. I approached a few MCS suppliers and was being pushed towards 11 - 12kw units.  This didn't match my own calcs or how the house behaved when on gas boiler.  The route I have taken is DIY install via an MCS Umbrella company. I am doing all my own plumbing and electrics. Headline costs have been £4k for 7Kw Vaillant Arotherm, £1k for Vaillant 250l cylinder and approx £1600 for Umbrella costs.  ( All inclusive of vat )  This way you can still install via BUS grant but are not tied to crazy prices or forced into a heat pump that you don't want or need.

What’s the umbrella company called?

[AliG]

13 hours ago, JohnMo said:

14 hours ago, AliG said:

That seems to be what they are getting at

Heat loss isn't affected by height, just the external areas. Halls aren't normally designed for 21, and normally rob or borrow heat from every room attached to it.

 

Yes, but a double height hall has twice the wall area and one of the walls is entirely glass, so it has much more heat loss than a normal room of similar size.

 

13 hours ago, JohnMo said:

14 hours ago, AliG said:

Even with the UFH flow at 50C it struggles to heat the hall to 21C when it is below 0 outside

Do they have thick carpet? Have increased the flow rate in those loops to give more energy out. Run those loop 50% higher, so you get a lower dT (differential temperature).

 

14 hours ago, AliG said:

turned it up on the weather compensation curve for this reason

Then move the curve back down to lower flow temp.

 

I have never been above 35 and we have UFH at 300mm centres.

 

Various issues.

 

Late 70s parents like to keep the house ridiculously warm. It is uncomfortably hot for me. They don't like the hall being colder than other rooms.

 

The weather compensation curve was set to run the flow between 35 and 45C ramping up from 35C at 5C and above outside to 45C at -5C outside. I can see from the Heatmiser thermostats that the UFH rarely runs for more than 15 minutes in all the other rooms in the house. So for all the other rooms the flow could be 35C or indeed probably less.

 

It is a tiled screed floor.

 

The hall is an issue for multiple reasons. Many of these reasons will apply to any hall.

 

1. Double height means that there is a greater wall area to lose heat.

2. It contains about 30% of the glass in the whole house.

3. There are no UFH pipes under the stairs or in the understair cupboard

4. The pipework running to the other rooms is insulated, further reducing the floor area available to heat the hall (TBH I hadn't realised ow much area would be lost to this).

5. The only heating upstairs is electric bathroom radiators, so some heat is also lost to the upstairs rooms.

 

I guessed it might be an issue and had the installers reduce the pipework to 150mm centres. The problem is that because of the lost floor area mainly due to the stairs and UFH runs to other rooms, you are trying to heat the whole volume from only around 8sq metres of floor, the total floor area of the space is 18sq metres and it is 5.5m high. At 50C flow, this has an output of around 86W/m2, so only around 700W. I calculated that the space needs around 1.2kW when the temperature is -5C (The whole house only needs 3.5kW at this temp). So we need to add around a 2500BTU radiator to the space (Maybe 3000 to allow a bit of overhead). This would produce 500W of output with a flow temp of 45C.

 

@ChrisInKent likely has some of the same issues as regards to heat requirements for a double height hall versus the amount of floorspace available to produce heat from UFH. Although as I have said this issue would require an extra radiator, or closer pipe spacing, not a larger ASHP.

 

13 hours ago, JohnMo said:

14 hours ago, AliG said:

heat the hot eater faster,

What does that mean?

Sorry typo - Heat the hot water faster.

[ProDave]

What I learned in my first self build (ordinary insulation levels no mvhr) is heating in the hall, and definitely on the landing was not needed.  As stated up the thread they are "rooms" with little external wall and borrowed heat from other rooms, and in short the UFH never came on in either the hall or landing.

 

Second self build I did not waste the time and effort laying UFH pipe in either the hall or the landing.

[AliG]

It's the glass.

 

My hall and my parents' hall both have enormous amounts of glass and are double height.

 

I did, however, tell my mum to leave the kitchen door open and it made a noticeable difference tbf. Although of course she doesn't like it.

[JohnMo]

1 hour ago, AliG said:

The pipework running to the other rooms is insulated

Someone dropped a ball there.

 

1 hour ago, AliG said:

I calculated that the space needs around 1.2kW when the temperature is -5C

I would

1 Add a small supplement electrical panel heater. Get one with a thermostat so it only heats when needed, £100 and will take longer to collect than install

2 Reduce WC to suit all other rooms, so all rooms run below the thermostat set point (up setting on thermostat to just above target room temp). This will leave the heat pump to run itself.

 

Comment: Would suspect with most the house bouncing off thermostats the heat pump short cycles or has a buffer, either will affect efficiency

The uplift in efficiency will probably pay for the electric panel heater. Dropping flow temp from 45 to 35 will save 0.5kW of input electric on its own. Less ASHP cycling will save also, how much?

Using a flow temp of 35 would reduce the output of the UFH by 50% (ish) leaving a short fall of about 8-900W, so a 1 to 2kW heater, depending if they switch off during night and it has to recover the heat in the morning

 

 

[MikeSharp01]

1 hour ago, AliG said:

Yes, but a double height hall has twice the wall area and one of the walls is entirely glass, so it has much more heat loss than a normal room of similar size.

The house has MVHR so it looks like the overall outer envelope is what the calc should based upon that not individual spaces I would have thought. The whole place at a common temperature is the goal really although I suppose many would want the bedrooms cooler than the living area so that might be a problem. Life is full of compromises.

[AliG]

1 hour ago, JohnMo said:

Someone dropped a ball there.

 

I would

1 Add a small supplement electrical panel heater. Get one with a thermostat so it only heats when needed, £100 and will take longer to collect than install

2 Reduce WC to suit all other rooms, so all rooms run below the thermostat set point (up setting on thermostat to just above target room temp). This will leave the heat pump to run itself.

 

Comment: Would suspect with most the house bouncing off thermostats the heat pump short cycles or has a buffer, either will affect efficiency

The uplift in efficiency will probably pay for the electric panel heater. Dropping flow temp from 45 to 35 will save 0.5kW of input electric on its own. Less ASHP cycling will save also, how much?

Using a flow temp of 35 would reduce the output of the UFH by 50% (ish) leaving a short fall of about 8-900W, so a 1 to 2kW heater, depending if they switch off during night and it has to recover the heat in the morning

 

 

You’re right I have looked at putting in an electric panel heater and indeed have tried one which did the job. If it was me I might have done this.

 

But all the panel heaters I have looked at have horrible timer systems. They purport to have various apps etc but seem to be extremely fiddly to program. Unless you spend close to the price of adding a new radiator. It is a lot easier for my parents to have a radiator connected to the hall loop so it works off the same thermostat.

 

Indeed running the flow lower would improve efficiency and save money. That’s the plan once the radiator is in place.

 

Assuming something like 500kwh a year of output from this radiator, it saves around £75 a year using the ASHP vs a panel heater so it will soon pay for the modest extra cost on installation. I have been quoted around £500. It will be within a couple of feet of the manifold so is a very easy job.

[ChrisInKent]

19 hours ago, JohnMo said:

Just had a look at my SAP report, "as built" and looked at the "target emission rate", which is bog standard building regs, no MVHR, double glazed, min insulation, air tightness of 7. The target build adds around 35% to the as built house. That had a heat loss rate of about 4kW. We are 195m² single storey long thin building and lots of glass. Worst design for heat efficiency.

 

So your 12kW is about 4x oversized for well insulated 3g glass MVHR etc - walk away from that quote.

 

 

If I'm looking at the right area of the SAP the Heat loss calculations for Winter is between 3270 and 3677.  The UFH and ASHP supplier have it as 7336 W,

 

 

 

 

Edited August 7 by ChrisInKent

[ChrisInKent]

On 06/08/2024 at 08:57, ProDave said:

Definitely something odd here.  But we don't know the details of your "well insulated" house.

 

 

It's timber frame with 120mm PIR rigid insulation factory fitted and 50mm rigid PIR site-fixed, Glidevale VC reflective foil ultra air tight barrier, Wallboard TEN plasterboard, aiming for 0.11W/m2K.

 

Roof insulation 140mm factory + 40mm site-fixed PIR.

 

Tiple glazed windows 0.8-1.1, solid door 0.7, triple glazed doors 0.85. 

 

Edited August 7 by ChrisInKent

[ProDave]

26 minutes ago, ChrisInKent said:

 

I read that as 7336 total maximum heat loss, which seems high to me but is in line with what the MCS estimate says.

 

I found similar calculations in my SAP were 2 to 3 times over the actual real world heat loss.

[JohnMo]

8 minutes ago, ProDave said:

found similar calculations in my SAP were 2 to 3 times over the actual real world heat loss.

Must say look quite close, but I did correct all assumptions made and all standard details and it dropped considerably at revision 2.

 

But take from this is a ballpark of closer to 3kW, not 8kW and nowhere near 12kW.

 

Looking at the spreadsheet the fabric loss isn't too bad, the ventilation losses are stupid high for MVHR.

[ChrisInKent]

1 hour ago, ProDave said:

I read that as 7336 total maximum heat loss, which seems high to me but is in line with what the MCS estimate says.

 

 

Sorry probably have been clearer that the green one is the MCS calculation at 7336. The white one below is the SAP at a max 3677.

Edited August 7 by ChrisInKent

[ChrisInKent]

Do the predicted energy requirements for the house look right? It looks like the ventilation loss is skewing them -  the kitchen/diner, hallway and landing seem huge to me. Overall per year of the 4-bed house (just over 200sqm) is calculated to be almost 20,000 kWh/Yr. 

[JohnMo]

1 hour ago, ChrisInKent said:

Do the predicted energy requirements for the house look right?

No - do your own calcs, then you know what you are looking at

[haythorn_1]

On 06/08/2024 at 19:56, mk1_man said:

Hi Chris,  we are in final stages of build / retrofit of 250m2 house, aiming for as near passive house as we can with attention to air tightness, triple glazing throughout, removed chimney and have mvhr.  Also underfloor ground and first floors with 28mm primaries etc. I approached a few MCS suppliers and was being pushed towards 11 - 12kw units.  This didn't match my own calcs or how the house behaved when on gas boiler.  The route I have taken is DIY install via an MCS Umbrella company. I am doing all my own plumbing and electrics. Headline costs have been £4k for 7Kw Vaillant Arotherm, £1k for Vaillant 250l cylinder and approx £1600 for Umbrella costs.  ( All inclusive of vat )  This way you can still install via BUS grant but are not tied to crazy prices or forced into a heat pump that you don't want or need.

I presume this was Alto? How was your experience with their heat loss assessment as the initial indication I have had is quite high. The heat loss is double what I have calculated using the forum spreadsheet.