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ASHP commissioning woes - am I being overly critical?

Air Source Heat Pumps (ASHPs)

Last Post by Mars 2 years ago

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akius

(@akius)

Active Member Member

Joined: 2 years ago

Posts: 3

Topic starter 26/07/2023 11:49 am

Hi everyone, new to the forum and would appreciate some input.

We are renovating a 200 sqm terraced in south of England. We've done fabric first and upgraded insulation, roof, and windows. Plan is to install UFH throughout to be able to minimize the flow temp to push the heatpump efficiency. In line with that we are installing a thin self-level screed everywhere except the new extension that is getting normal thick screed. Thin linoleum (2.5 mm, low R) throughout except in bathrooms (tile) and reception rooms (engineered wood).

Have had a heating engineer who was heatgeek certified engaged but was a bit surprised about what he came back with. He has proposed a Nibe F2040 12 kW which seems to be both slightly oversized (preliminary heat loss calcs are around 9-10 kW) but more importantly, fairly inefficient at 4.43 SCOP at 7 / 35 (outside temp / flow temp). This seems like a foolish choice to me given it is 10 years old and all the new efficient heat pumps are around 5 in SCOP. We are spending a lot of money on maximizing our sustainability for this house so throwing away 13 percent (5/4.43) of all of the previous measures effectiveness seems quite large to me? Heating engineer also said we should be getting around 3.5 in SCOP for the whole system which seems awfully low for me given the UFH? He countered that it's difficult in old buildings but I don't understand why the SCOP would be influenced by the building, that it's an old building should just impact the baseline heat loss in my mind?

One caveat is that we are in a conservation area so we need to have a single fan unit heatpump, so many others are not feasible (Valiant Aerotherm plus, Weissmans etc) but Nibe themselves have one that would work, S2125.

My builder is pissed cause we are risking delaying the project at this point due to this because we will struggle to find a new heating engineer in time for when the floor is going in (4-5 weeks). We were told we were gonna get a really good system by going with NuHeat but this seems to not be the case. Am I overcooking this?

Appreciate any thoughts.

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Kev M

(@kev-m)

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Joined: 5 years ago

Posts: 1276

26/07/2023 7:04 pm

Hi and welcome to the forum.

It's hard to equate test conditions performance to real life. Having said that things have moved on, and modern ASHPs will tend to be more efficient than 10 year old ones. I know nothing at all about Nibe other than that their conventional ASHPs seem to have a good reputation. The important thing is to get a good design and I'd assume you'd get that with a HG trained engineer.

On the sizing question, you need to be careful of the 'badged' numbers on ASHPs. Some can't deliver their badged rating at typical UK winter temperatures. Have a look at the below on page 67 or thereabouts.

PARTNER.NIBE.EU "https://partner.nibe.eu/nibedocuments/25738/231844-8.pdf"

Nibe don't seem too bad reading that. They are also realistic about defrosts, hence the big dip around 0 degrees. At 0 degrees, the 12kW isn't giving out much more than 8kW. The same happens with all ASHPs btw. It's not necessarily a problem if it's not very cold very often but I'd make sure it can cope in the 0-5 degree range because that's quite common.

I agree about your heat loss/old building comment; as long as it's been calculated correctly, heat loss is heat loss. COP depends on the flow temperature of course. What is your design ambient/flow? It'll be something like 40@-3.

A COP of 3.5 is probably average for ufh; a well designed low temp system could achieve more.

This post was modified 2 years ago by Kev M

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Tim441

(@tim441)

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Joined: 3 years ago

Posts: 357

30/07/2023 7:47 am

we have 2 ASHPs (LG) in 2 different buildings. Installed 2020.

1. In very old building with modern extension. 16kw ashp. Total MCS calcs for energy usage around 28k kwh.
Actual TOTAL energy usage around 9-10k with ashp

2. In 25 year old building. 9kw ashp. Total MCS calcs for energy usage around 16k kwh.
Actual TOTAL energy usage around 6k with ashp

Understanding and using weather compensation has been vital to getting best efficiency

Listed Grade 2 building with large modern extension.
LG Therma V 16kw ASHP
Underfloor heating + Rads
8kw pv solar
3 x 8.2kw GivEnergy batteries
1 x GivEnergy Gen1 hybrid 5.0kw inverter
Manual changeover EPS
MG4 EV

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akius

(@akius)

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Joined: 2 years ago

Posts: 3

Topic starter 31/07/2023 1:37 pm

@kev-m thanks for your thorough reply!

Yep, good point about the badge rating not being super useful all the time. Have since also realized that Vaillant has the opposite issue, the 7 kW unit actually outputs 8.8 kW at -3. Have also, in part thanks to your comment and reading the specs more carefully, realized that SCOPs vary wildly between different models even within the same range. Vaillant arotherm plus 10kW as scop 5.03 but same model but in 7 kW around 4.5, fair bit lower.

What is your design ambient/flow?

We are explicitly putting screed and linoleum in the whole (well, most of the) house with the explicit target of being able to push down the flow temperatures <30. We'll see how low we'll get. Waiting for a new set of heat loss calcs and hope the system model and targets get clearer from there.

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akius

(@akius)

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Joined: 2 years ago

Posts: 3

Topic starter 31/07/2023 1:40 pm

Posted by: @tim441

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we have 2 ASHPs (LG) in 2 different buildings. Installed 2020.

1. In very old building with modern extension. 16kw ashp. Total MCS calcs for energy usage around 28k kwh.
Actual TOTAL energy usage around 9-10k with ashp

2. In 25 year old building. 9kw ashp. Total MCS calcs for energy usage around 16k kwh.
Actual TOTAL energy usage around 6k with ashp

Understanding and using weather compensation has been vital to getting best efficiency

This is useful data for me, thanks for sharing! It's wild that the MCS would be so inaccurate. Yepp, we are going to be trying to get the weather compensation right as well as work on minimizing the cycling.

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Kev M

(@kev-m)

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Joined: 5 years ago

Posts: 1276

31/07/2023 2:13 pm

Posted by: @akius

↑

Posted by: @tim441

↑

we have 2 ASHPs (LG) in 2 different buildings. Installed 2020.

1. In very old building with modern extension. 16kw ashp. Total MCS calcs for energy usage around 28k kwh.
Actual TOTAL energy usage around 9-10k with ashp

2. In 25 year old building. 9kw ashp. Total MCS calcs for energy usage around 16k kwh.
Actual TOTAL energy usage around 6k with ashp

Understanding and using weather compensation has been vital to getting best efficiency

This is useful data for me, thanks for sharing! It's wild that the MCS would be so inaccurate. Yepp, we are going to be trying to get the weather compensation right as well as work on minimizing the cycling.

@akius,

the total energy usage in the MCS sheet is the amount of delivered energy your house needs for space heating and DHW. If @Tim441 is consuming 9-10kWh (I think that's what he means) then if the total energy usage is 28kWh then the SCOP is around 3, which for heating and DHW is in the right ball park.

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Tim441

(@tim441)

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Joined: 3 years ago

Posts: 357

31/07/2023 2:59 pm

@kev-m yes, agreed. I think overall SCOP is ok

We run our DHW at just 43deg most of the time (with weekly legionnaires cycle). Boosting higher if a house full of people just to make sure. Never seem short of HW. 43deg still means adding cold for showers/bath so is hot enough.

Usage has been complicated a bit by charging batteries last autumn/winter and then exporting back to grid at higher prices. As well as hot tub usage last year (massively reduced this year!) and adding EV.

So I try to take readings from all the different meters and Givenergy app for PV production, grid import, grid export, ashp usage to get a feel of what is going on. Different weather impacting both ASHP usage and solar prod so makes it hard to make very precise comparisons. But overall the data builds up to give a decent idea.

Weather comp has taken us to flow temps for heating that are mostly well under 40deg and near 30deg on mild days. We have a woodburner but last winter hardly used it as the house was plenty warm enough - just used high days & holidays for ambiance!

We then use setback of 1 or 2 degrees overnight only. Result has been a very comfortable heat and lower bills.