ash_scotland88

Vaillant Arotherm Plus

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3 minutes ago, PeterW said:


That isn’t correct. You can’t create energy, and a heat pump works by incremental heat increase to the flow and it is when the output temperature is above 45°C then the units are only able to add 1-2°C to the water circulating. The pump is a constant flow so the heat introduced to the tank - or the J/hr - is not linear as the deltaT reduces at higher temperatures. 
 

This is why heat pumps have temperature curves, not linear constants due to thermodynamics. 

 

Point is, if I set the tank to 65°C, heating it the last 10°C in particular takes ages, because the COP is very low for that part of the cycle (observed specifics, power drawn 5.4kW and cooling capacity <5kW). However, setting the tank lower means the flow temp is lower where the COP is higher. At 40°C it shows a total transfer of about 17kW. Which means the recharge is shorter even though more water is drawn from the DHW cylinder.

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8 minutes ago, PeterW said:


No, it’s not...!! There is no legal - in building regulations or water regulations (WRAS) - to provide a legionnaire cycle to a domestic installation. There are no recorded instances of Legionella in a domestic environment - none.  
 

It is a closed system with no way for air or bacteria to enter the system. 

 

Also interesting that the MCS numbers include provision for legionaires programme.

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1 hour ago, J1mbo said:

 

interesting, so the manual is erring on the side of caution then. I will adjust accordingly.


Its because the Heat Pump manufacturer doesn’t have control of the final installation, and regulations vary from country to country.
 

In some European countries the water systems are cistern based so a legionella cycle is required as the chance of contamination is higher; look at Italy though and the use of sealed and unvented electric heated tanks is commonplace and yet they have no G3 equivalent for over pressure ..! 
 

MIs these days are common across large regions which is what can cause the potential for problems. 

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My particular installation retains the header tank, and I’ve always been nervous about legionnaires after it was found in school showers (many years ago)

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So it’s not an unvented cylinder ..?? Not mains pressure ..?

 

Or is the header on the heating side at which point you’re back to a sealed system. 

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Ordinary Telford heat pump cylinder, just plumbed up with 50 gallon header in a vented config.

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1 hour ago, J1mbo said:

I’ve always been nervous about legionnaires after it was found in school showers

The bacteria is quite commonly found lurking in hot water taps. There is generally not enough atomisation if the right sized droplets to cause a problem.

But there is a whole industry based on scaring the shit out of people about it.

On another site, someone claimed that pneumonia was caused by contaminated showers, and she truly believed it, no evidence to the contrary would convince her otherwise. 

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1 hour ago, J1mbo said:

Ordinary Telford heat pump cylinder, just plumbed up with 50 gallon header in a vented config.


OK so you “should” consider how the header and tank are kept sanitary but that starts with the header cover and keeping that sealed. 

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On 07/03/2021 at 11:09, J1mbo said:

The SCOP numbers assume weather compensation. it does not deliver 55°C at a COP of 3.4. It does provide 44°C at 0°C at about 3.2. The max output tables give the actual COP at 55°C, which IIRC is about 2.4. The SCOP is higher as most the year it will be running lower, although I don't know the parameters against which it is calculated.

 

Which "max output tables" are you referring to?   Only value I've seen is A7/W55 COP is 2.9 (7kW version),

 

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7 hours ago, Dan F said:

 

Which "max output tables" are you referring to?   Only value I've seen is A7/W55 COP is 2.9 (7kW version),

 

 

Here: arotherm-plus-spec-sheet-1892564.pdf (vaillant.co.uk)

 

See page 6 for max outputs. e.g. the max compressor current listed on page 8 (e.g. 23.3A = 5.4kW for the 12kW HP), at 0°C, 55°C flow max output is listed as 12.3kW. 12.3/5.4 = COP 2.3 at max output in that scenario. By observation, the ratio does improve at lower loads, especially between approx. 35% and 60%.

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Posted (edited)
On 07/03/2021 at 17:34, PeterW said:


OK so you “should” consider how the header and tank are kept sanitary but that starts with the header cover and keeping that sealed. 

 

Thanks for the reply Peter. I did have the loft tank changed for a new one with a lid and the vent pipe passed in through a tight hole. Better than the large open bowl thats sat in the loft for the 40 preceeding years! It was disgusting actually. Will keep the weekly cycle, actually it's quite useful on a Monday morning when everyone is late trying to get through the shower back-to-back to have more energy stored anyway.

Edited by J1mbo
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Posted (edited)
2 hours ago, J1mbo said:

 

Here: arotherm-plus-spec-sheet-1892564.pdf (vaillant.co.uk)

 

See page 6 for max outputs. e.g. the max compressor current listed on page 8 (e.g. 23.3A = 5.4kW for the 12kW HP), at 0°C, 55°C flow max output is listed as 12.3kW. 12.3/5.4 = COP 2.3 at max output in that scenario. By observation, the ratio does improve at lower loads, especially between approx. 35% and 60%.

 

That calculation assumes the ASHP uses maximum power output across the full range of air/water temperatures and this isn't the case.  At A7W55 the 12kW unit only consumes 2.94kW delivering 9.1kW heating ouput (COP 3.1).   Your calculation would assume it uses 5.4kW and give a COP of 1.7 which seems low and neither does it match the datasheet.

 

UK datasheet only gives the COP for A7W55. but if you look elsewhere you can find A-2W55=2.7, so the reality is probably around 2.8 for A0W55,   (on paper of course).

 

Edited by Dan F

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On 07/03/2021 at 11:22, J1mbo said:

Point is, if I set the tank to 65°C, heating it the last 10°C in particular takes ages, because the COP is very low for that part of the cycle

 

Also, once you're at the maximum flow temp, the temperature difference between that and the water in the tank will approach each other, so energy transfer is slower.

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Posted (edited)
On 09/03/2021 at 12:23, Dan F said:

 

That calculation assumes the ASHP uses maximum power output across the full range of air/water temperatures and this isn't the case.  At A7W55 the 12kW unit only consumes 2.94kW delivering 9.1kW heating ouput (COP 3.1).   Your calculation would assume it uses 5.4kW and give a COP of 1.7 which seems low and neither does it match the datasheet.

 

UK datasheet only gives the COP for A7W55. but if you look elsewhere you can find A-2W55=2.7, so the reality is probably around 2.8 for A0W55,   (on paper of course).

 

 

It was 8°C here this morning so I took a couple of readings from the HP interface:

 

  • A8W55, cooling 8.1kW consumption 5.2kW, COP 2.6
  • A8W42, cooling 11.6kW consumption 5.3kW, COP 3.2

 

assuming zero losses from the compressor... so a bit less than that.

Edited by J1mbo

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9 hours ago, J1mbo said:

A8W55, cooling 8.1kW consumption 5.2kW, COP 2.6

 

So 20% less than spec.   Surely this is heating and not cooling?

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Posted (edited)

For completeness, here's a part-throttle reading:

 

A5.5/W42 - cooling 3.8kW, consumption 1.2kW, Compressor load 27.2%, COP 4.1. Building flow 1980 litres per hour.

 

The HP shows the evaporator cooling and compressor consumption metrics. COP I've calculated as (cooling+consumption)/consumption. Obviously there will be some losses in the compressor so maybe 80 or 90% of the compressor consumption ends up as heat in the water... I don't know.

Edited by J1mbo

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@J1mbo  How are you controlling you Arotherm Plus?

 

- Vailant controller + thermostats.

- Vailaint controller + external call for heat

- EEBus/ Modbus integration

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Am using Ambisense now. The VRC700 controller is however the only reference point for room temperature modulation so that needs to be cited in the typically coolest location.

 

There looks to be some work going on with Ambisense with home assistant so perhaps some bridge will become possible through that.

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Is anyone else having flashbacks to the Zoot story?

  • Haha 1

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I don’t know what that means.

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13 minutes ago, J1mbo said:

I don’t know what that means.

Poorly fitted / incorrectly spec’d ASHP job ;)  Long story. 

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5 hours ago, Nickfromwales said:

Poorly fitted / incorrectly spec’d ASHP job ;)  Long story. 

And how many pages did it take to arrive at that summary?

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Ok thanks, not sure I really follow the correlation.

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43 minutes ago, J1mbo said:

Ok thanks, not sure I really follow the correlation.

because the flow temperature seemed to change randomly and now the temperature modulation thing is discussed...

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