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Controversial opinions - pure weather compensation, buffer tank, heat loss, oversized heat pumps

stuartornum · 2025-01-06T22:20:00Z

Reference: This is my heat pump. Just watched this video "Are buffer tanks required for Air Source Heat Pump systems?" - great video, very respectful conversation. My only point for potential improvement for next time... get someone with an different point-of-view involved. It made me think... Ok, moving on to the first (potentially) controversial opinion. Also, I'm very open to be convinced otherwise to any of the below!: 1. Heat loss calculations/surveys are a waste of time It's worth noting I still do it... just so I can laugh at it afterwards. Instead, where possible, get the customer to run their existing boiler like a HP and run this test:- Wait for a really cold day, ideally the "design temperature" for the area you are in- Lower the flow temp on the boiler to as low as you can and still maintain comfort (get a few cheap temp sensor displays and put around the house)- Set the room temp schedule to be 20c (or whatever you want) for the entire period of the test- Turn off the DHW schedule from the (system) boiler, use the immersion for the test period - we're testing heat loss not DHW production...- Make a note of the gas meter reading and time- Wait 24 hours and make a note of the gas meter reading again Heat loss = (gas consumption (in kWh) x 0.9 ) / 24 For example, if you consumed 120 kWh of gas in a 24 hour period then: (120 x 0.9) / 24 = 4.5kW heat lossAssumption:- Your boiler is a condensing boiler (installed after 2008 I believe are all condensing) - we're being generous and saying the boiler is 90% efficient- Your flow temp is below 55c (it's actually going to condense)- For reasonable results, you want the average outdoor temp to be the MCS design temp- Ideally, try to reduce solar thermal gain as well...- Do this for several days when it's cold, compare the results... I've asked people for their smart meter data before... makes life so much easier...!I used this method for my own house, it's looking spot on (4.5kW). The heat loss calculation (traditional way) came out at 6.76kW... 2. Get a bigger heat pump than required I reckon this might be the most controversial one...Ok, here goes... 1. Extending your house ever? Bigger heat pump...2. We're told to match the homes heat loss to the heat pump itself... what about DHW production..? What about DHW production at -3c? What about DHW production at -3c when the tank is 300L and the heat pump rating is ~4kW...?!? Some explanation may be required for homeowners on this one: Generally speaking, when in DHW mode, it means there is no heat for central heating. When it's -3c and you have a 300L cylinder, it'll take at least 2.5 hours to heat the tank to ~55c. That's 2.5 hours your house has no heating. If you size the HP for your heat loss, it will struggle to get the house back up to temperature. 3. Bigger heat pumps generally have better COP figures. The most efficient heat pump, is the heat pump that is NOT running. I chose a heat pump based on performance figures (and... the ability to tap into the Modbus to re-write the weather compensation algorithm...) - I try and keep my heat pump in it's optimal COP range, regardless if it's producing too much heat. I can just turn it off... (More explanation in the next controversial opinion) UFH helps a lot here, as it takes a while to react as it's a huge amount of thermal mass. 4. You can run a hot tub off it!! This was genuinely one of my drivers for going bigger... and the reason for controversial opinion No.4 However, if you do a standard heat loss calculation survey (not what I suggest above), you'll be fine... because it'll be ~33% bigger than the actual heat loss anyway ;) 3. Pure weather compensation is bad Disclaimer/Admission: I'm a software engineer... I'm not using the standard weather comp control from the manufacturer, I wrote my own. I appreciate this is not available to the general public. The point I'm making is, with better controls, you can achieve better efficiency. Manufacturers... DO BETTER! Room influence is really important, maybe it's just my lack of understanding here, but asking the customer to change the weather comp curve...??? Is this the best we can do...? I get the impression the industry has settled on pure weather comp being "the best" because it's easy way out, rather than actually trying to model the property and figuring out what works best. Hint... knowing what is happening inside and outside the property is really helpful for controlling it! As mentioned above, bigger heat pump dumps heat into the house at optimal COP (500%+), calculate how much heat has been provided over a given time, calculate what the heat loss was for that period... turn the heat pump off allowing actual heat loss to catchup... monitor indoor temp and adjust accordingly. 4. Not all hydraulic separation is badFirstly, traditional buffer tanks are dogshit and should never be used... However, the NRG Zone is a work of art! I have one, the flexibility it provides is just amazing. My original desig...

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Air Source Heat Pumps (ASHPs)

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JamesPa

(@jamespa)

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Joined: 3 years ago

Posts: 4800

14/02/2025 10:57 am

Posted by: @transparent

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That's it!

Emitters on a roll.

Now why didn't I think of that? 😜

Available (and indeed standard) in Italy I am told (by an Italian). Radiators are assembled onsite from small, standard, sections that plug together (and presumably seal!) into the length required.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

👌

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Heatgeek

(@heatgeek)

Estimable Member Member

Joined: 1 year ago

Posts: 117

14/02/2025 11:02 am

@jamespa It is all about design choices. In my view heating at extreme low end temperatures is a small minority of overall heating which might not even occur some years. I choice to match a more efficient, smaller heat pump to 15C OAT to eliminate cycling and provide a wood-burner in the lounge coupled with a thermostat. At low temperatures, the wood-burner would be activated and the large lounge load from HP would be switched off by the thermostat, allowing the HP to concentrate its heat on the rest of the house.

The major mistake I made is mixing radiators in bedrooms with UFH which complicates things enormously. In hindsight I should have implemented UFH throughout regardless, whereby WC would be a breeze. There is still the problem at low temperatures due to HP modulation range but I solved that one. I am a believer in designing from the 15C OAT end. I also believe in 2 heat pumps, one for UFH and one for DHW.

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.

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Transparent

(@transparent)

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Posts: 3178

14/02/2025 11:18 am

Posted by: @jamespa

↑

Im cant see why two compressors in the same heat pump falls foul of the rules.

Because there isn't one on the MCS Approved list.

This list can be bizarre and overly pedantic at times.
For example, installations of rooftop PV panels must use 65mm screws to attach the brackets to rafters.

I fell foul of that because there's 50mm of insulation and a 22mm counter-batten on top of my rafters!
When I handed the installer a box of 140mm stainless steel screws, he said that was 'non-compliant'.

We arrived at a compromise which allowed him to produce the MCS certification...
... and you'll appreciate why I won't explain more here on an open forum 😉

I like the idea of a double-compressor heat pump.

But as I'm based not far from the workshops where Newcomen and Trevithick pioneered Cornish steam-engine designs, I favour a more innovative approach.

There should be two impellers on the same shaft, whereby the energy still remaining from the first stage can be fed to the second.

Ie, this mirrors the compound engine, whereby dual pressure expansion cylinders greatly increased the efficiency of early steam engines.

Save energy... recycle electrons!

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JamesPa

(@jamespa)

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Joined: 3 years ago

Posts: 4800

14/02/2025 11:26 am

Posted by: @heatgeek

↑

@jamespa It is all about design choices. In my view heating at extreme low end temperatures is a small minority of overall heating which might not even occur some years. I choice to match a more efficient, smaller heat pump to 15C OAT to eliminate cycling and provide a wood-burner in the lounge coupled with a thermostat. At low temperatures, the wood-burner would be activated and the large lounge load from HP would be switched off by the thermostat, allowing the HP to concentrate its heat on the rest of the house.

As it happens I wouldn't disagree, but if it can in fact eliminate cycling then it wont meet the requirements of the BUS grant to be capable of heating the house 100%, at least not with a 3:1 modulation ratio.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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Heatgeek

(@heatgeek)

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Joined: 1 year ago

Posts: 117

14/02/2025 12:11 pm

@jamespa I agree. That is where the design choices come in. You have to design within the constraints imposed on you, realistic or not.

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.

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RobS

(@robs)

Estimable Member Member

Joined: 2 years ago

Posts: 128

14/02/2025 7:03 pm

Posted by: @transparent

↑

Posted by: @jamespa

↑

Im cant see why two compressors in the same heat pump falls foul of the rules.

Because there isn't one on the MCS Approved list.

There is, the Mitsubishi R290 8kW has a 6kW inverter and a 2kW fixed in the outside unit. The 2kW is only used when more output than the 6kW can produce is needed. As a result it can modulate to less that 25% of its badged max output.

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SUNandAIR

(@sunandair)

Honorable Member Member

Joined: 3 years ago

Posts: 569

14/02/2025 7:37 pm

As per Robs comment @robs

this is a R290 model which will operate down to 2kwh output….

ironically R290s are being hailed by some as the high temperature heat pump but here is one which modulates down to 2kw minimum output.

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 4800

14/02/2025 7:47 pm

@sunandair @robs

Makes a lot of sense to me. I wonder how much adding a second fixed output compressor adds to cost, I would guess very little.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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RobS

(@robs)

Estimable Member Member

Joined: 2 years ago

Posts: 128

14/02/2025 8:12 pm

@jamespa

The 6kW is £4666 and 8kW £5295 on midsummer's, so no not much.

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Heatgeek

(@heatgeek)

Estimable Member Member

Joined: 1 year ago

Posts: 117

14/02/2025 8:13 pm

@jamespa Quoted performance figures @35C flow, -7C OAT = 8.2kW and @25C flow, 15C OAT = 2.1kW. Quite something. Other output models as well.

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 4800

15/02/2025 9:47 am

Posted by: @heatgeek

↑

@jamespa Quoted performance figures @35C flow, -7C OAT = 8.2kW and @25C flow, 15C OAT = 2.1kW. Quite something. Other output models as well.

Nice!

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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Heatgeek

(@heatgeek)

Estimable Member Member

Joined: 1 year ago

Posts: 117

15/02/2025 10:59 am

@sunandair This new Ecodan validates your argument about following WC closely for maximum efficiency. You can now set your design flow and radiator size at 15C and turn down the top end accordingly. However, the WC facility is linear which is fine for UFH. It appears that there are no radiator compatible curves. There is a mid-point setting so you can produce a simple triangle. Nevertheless, it will do me fine as it meets the 25% criterion and all-UFH is the only way to go if you are rational. As @jamespa rightly said in my view, if you want air to air heating, use fan coils, not radiators.

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.

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