Vaillant Arotherm in open loop, with buffer

[sharpener]

2 hours ago, Peter269 said:

Down to the nitty-gritty. Good, this has been one of my big headaches.

[it's the internal pump I'm talking about, not the house one]

 

So the ASHP pump (a) runs continuously, or (b) definitely doesn't.

Can anyone give a definitive answer, specifically for the Arotherm Plus?

Thanks

 

I don't recall there being any options for setting this. You could explore the simulator I linked to earlier.

[Peter269]

A week after my first post - thanks for all the useful suggestions, and here's an update. I've been researching, reading manuals, and playing with the sensoCOMFORT simulator. I can now answer some of my own questions and I'm posting the answers here in case anyone else has the same questions.

 

I've taken on board what others have said about the pros and cons of buffers, and I'm now thinking about some hydraulic alternaitves -
a) buffer, secondary pump, TRVs on all rads but mainly fully open or fully closed. (my original plan)
b) no buffer, TRVs on all rads, but with Automatic Bypass Valve and volumiser
c) no buffer, TRVs on all rads bar one (probably the hall) with volumiser
d) no buffer, no TRVs, no Evohome. All rads open, fed by ASHP pump open loop.
They all have advantages and disadvantages, and I will have to give some thought as to which is the least worst. I might do some modelling.

 

And here's my answers to the other questions -

• To run the Arotherm open loop, I will need at least a 'heat pump interface' maodule (VWZ AI). The outdoor temp sensor, DHW temp sensor, and (optional?) flow temp sensor connect here. This allows me to set most of the settings I need, including selecting a weather compensation curve (0.4 to 4.0). However it doesn't allow me to specify a heating schedule. I could instead use a 'hydraulic station' (VWZ MEH 97) which has all the same functions but includes integral expansion vessel, back-up heater and diverter valve. I don't have space for that though, so I'll be using discrete hydraulic components.
• To set a heating schedule, I need a 'system controller' as well, either a VRC 700 or the newer VRC 720 sensoCOMFORT. This allows up to 12 periods a day to be defined, each with its own set temperature. The 'set-back' temperature is used for all times outside of the defined periods. So potentially 13 different target temperatures per day, more than enough! Changing the target temperature modifies the WC curve up or down, so determining how warm the house gets even though there is no room temperature measurement. The sensoCOMFORT also gives a wider range of WC curve (0.1 to 4.0) than the VWZ AI.
• I haven't got to the bottom of exactly how the ASHP pump behaves. I know that the compressor (and fan?) can modulate and cycle based on degree-minutes, and I know from the spec that the pump can modulate between 9 lpm and 20 lpm for the 7kW model. The VWZ AI allows the pump speed to be set from 50% to 100%, or in Auto, but the only reference I could find to the pump actually switching off is a "summer switch-off temp" which can be set from 10 to 90 degrees. Why does this matter to me? If I retain the Evohome, I expect there will be times when the house doesn't need any heat so I will either have to accept the ASHP pump running 24x7x52 or find some way of switching it off from the Evohome when not required. I've not found the answer, but I'm prepared to suck it and see.

So I think I now have enough knowledge to go back to the installer and discuss options with him.
Thanks all

 

[sharpener]

Don't forget to let us know how you get on!

[markocosic]

The circulation pump will throttle back if the compressor isn't running. (i.e. it circulates thr water to sense the flow/return temperatures)

 

Example:

 

 

(this is a 7 kW unit running with a fraction the complement of radiators in operation as only north end of house needs top-up heat)

 

When summer arrives / you don't need any heat due to outside air temperature it stops entirely.

 

When the compressor is running you need an absolute minimum of 9L/minute to take the heat away fast enough for it to be happy. This are your minimum "open zone" flowrate. The nominal / design flowrate is 20L/minute.

 

When heating season actually begins in earnest it sits there running most of the time.

 

evohome you can probably bin. Else use it as a glorified TRV for the "additional zone" bit leave them main part of the house under full control of the heat pump IMO.

[markocosic]

In heating session that house runs all rads open downstairs; TRVs as temperature limiters upstairs.

 

Currently half of downstairs is not connected. (renovation)

 

Not efficient for the heat pump, and there's a sensible limit on how low you can go, but when just a bit of the house is calling you can indeed serve just that.

 

Type 22s sized for 45C at design condition. Direct. No volumiser. 

 

Work out the minimum heat pump output at say 12C and 35 degC flow. Work out your (partial) system output at 35 degC flow. Work out your (partial) system volume. Calculate the "run time" to raise the flow temp say 3degC. If that's more than 30 mins it's ok to run a zone that small. If shorter thats not ideal.

[HughF]

On 23/09/2023 at 22:02, markocosic said:

In heating session that house runs all rads open downstairs; TRVs as temperature limiters upstairs.

 

Currently half of downstairs is not connected. (renovation)

 

Not efficient for the heat pump, and there's a sensible limit on how low you can go, but when just a bit of the house is calling you can indeed serve just that.

 

Type 22s sized for 45C at design condition. Direct. No volumiser. 

 

Work out the minimum heat pump output at say 12C and 35 degC flow. Work out your (partial) system output at 35 degC flow. Work out your (partial) system volume. Calculate the "run time" to raise the flow temp say 3degC. If that's more than 30 mins it's ok to run a zone that small. If shorter thats not ideal.

Same as my setup, rads sized for 45deg, open loop, pump runs whenever there is a call-for-heat. Thermostat does a nighttime setback, rest of the time it’s a hi-limit stat which with the current weather comp settings, it never reaches.

 

I’m short cycling like mad, but it’s not costing me the earth and the house is warm. Shame the heatpump doesn’t have a wider modulation ratio.

[Dreadnaught]

2 hours ago, HughF said:

I’m short cycling like mad, but it’s not costing me the earth and the house is warm. Shame the heatpump doesn’t have a wider modulation ratio.

 

@HughF, that's interesting. Do you know the approximate volume of your system? Mine will be about 95 litres. It will be a similar open-loop system to the ones in this thread.

[HughF]

I'll try and calculate it once I've measured my pipe length inside the house. I know I've got 15m of twin 32mm underground, so that gives 25ltrs ish... Probably close to 50ltrs I expect?

[markocosic]

On 25/10/2023 at 10:24, HughF said:

Same as my setup, rads sized for 45deg, open loop, pump runs whenever there is a call-for-heat. Thermostat does a nighttime setback, rest of the time it’s a hi-limit stat which with the current weather comp settings, it never reaches.

 

I’m short cycling like mad, but it’s not costing me the earth and the house is warm. Shame the heatpump doesn’t have a wider modulation ratio.

 

Define "like mad" here?

 

If it's open loop and sized for 45C at design condition there ought to be a decent volume there.

[HughF]

Compressor and fan runs probably 20% duty cycle in a 10 minute period.

 

 

[markocosic]

2 hours ago, HughF said:

Compressor and fan runs probably 20% duty cycle in a 10 minute period.

 

Ouch. 

 

That doesn't feel like an open system sized for 45C at design condition.

 

What are the emitters / how is there so little water volume in the system that the temperature can change that quickly even at minimum input?

[HughF]

6 minutes ago, markocosic said:

 

Ouch. 

 

That doesn't feel like an open system sized for 45C at design condition.

 

What are the emitters / how is there so little water volume in the system that the temperature can change that quickly even at minimum input?

At the moment we’ve got 5 rads in circuit, and close to a 40m loop of primary pipework (32&28) before the three port.

 

There are two fan coils in circuit at the moment but they’re not emitting as the electrician is messing about with the wiring. One more fan coil to go back on the wall.

 

All the rads are wide open, trvs upstairs only. I’ve yet to connect the ufh mixer but that’s running into 2x 50m loops.

 

I designed for 21 at -4, but the wife isn’t happy with 21 so I’ve bumped the weather comp up 2 degrees on each setpoint. It’s running 40@15, 47@0 for now, as we’ve got reduced emitter output into the house.

 

I am having an issue getting any sort of delta-t across the rads when I try and balance them, they’re all close to 1 degree between flow and return. I’ve got the pump pwm set to auto and the target delta-t is 5. It works sweetly on dhw.

Edited October 28, 2023 by HughF

[markocosic]

Short circuiting then; rather than insufficient volume in the system?

 

(whizzing through fancoils, or a rad that's wide open, rather than all of it going through emitter that equally take time to heat and equally drop the temperature) 

 

 

[HughF]

6 minutes ago, markocosic said:

Short circuiting then; rather than insufficient volume in the system?

 

(whizzing through fancoils, or a rad that's wide open, rather than all of it going through emitter that equally take time to heat and equally drop the temperature) 

 

 

Yep, short circuiting….. I need to get the lockshields set I guess.

 

Rads are plumbed tboe but some of them might have the flow to the top and others to the bottom….. plumbers….

 

All the rads are currently wide open, balancing is needed.

Edited October 28, 2023 by HughF

[markocosic]

3 minutes ago, HughF said:

plumbers….

 

Always the (expletive deleted)ing plumbers. 🙂

 

 

[HughF]

Moved my rad balancing/delta-t issue to another thread…

[BobBiggar]

On 21/09/2023 at 13:18, Peter269 said:

A week after my first post - thanks for all the useful suggestions, and here's an update. I've been researching, reading manuals, and playing with the sensoCOMFORT simulator. I can now answer some of my own questions and I'm posting the answers here in case anyone else has the same questions.

 

I've taken on board what others have said about the pros and cons of buffers, and I'm now thinking about some hydraulic alternaitves -
a) buffer, secondary pump, TRVs on all rads but mainly fully open or fully closed. (my original plan)
b) no buffer, TRVs on all rads, but with Automatic Bypass Valve and volumiser
c) no buffer, TRVs on all rads bar one (probably the hall) with volumiser
d) no buffer, no TRVs, no Evohome. All rads open, fed by ASHP pump open loop.
They all have advantages and disadvantages, and I will have to give some thought as to which is the least worst. I might do some modelling.

 

And here's my answers to the other questions -

• To run the Arotherm open loop, I will need at least a 'heat pump interface' maodule (VWZ AI). The outdoor temp sensor, DHW temp sensor, and (optional?) flow temp sensor connect here. This allows me to set most of the settings I need, including selecting a weather compensation curve (0.4 to 4.0). However it doesn't allow me to specify a heating schedule. I could instead use a 'hydraulic station' (VWZ MEH 97) which has all the same functions but includes integral expansion vessel, back-up heater and diverter valve. I don't have space for that though, so I'll be using discrete hydraulic components.
• To set a heating schedule, I need a 'system controller' as well, either a VRC 700 or the newer VRC 720 sensoCOMFORT. This allows up to 12 periods a day to be defined, each with its own set temperature. The 'set-back' temperature is used for all times outside of the defined periods. So potentially 13 different target temperatures per day, more than enough! Changing the target temperature modifies the WC curve up or down, so determining how warm the house gets even though there is no room temperature measurement. The sensoCOMFORT also gives a wider range of WC curve (0.1 to 4.0) than the VWZ AI.
• I haven't got to the bottom of exactly how the ASHP pump behaves. I know that the compressor (and fan?) can modulate and cycle based on degree-minutes, and I know from the spec that the pump can modulate between 9 lpm and 20 lpm for the 7kW model. The VWZ AI allows the pump speed to be set from 50% to 100%, or in Auto, but the only reference I could find to the pump actually switching off is a "summer switch-off temp" which can be set from 10 to 90 degrees. Why does this matter to me? If I retain the Evohome, I expect there will be times when the house doesn't need any heat so I will either have to accept the ASHP pump running 24x7x52 or find some way of switching it off from the Evohome when not required. I've not found the answer, but I'm prepared to suck it and see.

So I think I now have enough knowledge to go back to the installer and discuss options with him.
Thanks all

 

It would be great to get an update on this and how you have been getting along with the system.

Specifically, did you ever get a satisfactory answer to the question about the Arotherm ASHP internal pump. More specifically, does it keep running 24/7/52 at the minimum 9l/min even when the compressor is not operating? or is it doing something a little more energy efficient and turning off, then occasionally on to sense return temperature?

yours is the only online post I have seen asking about this behaviour and it seems relatively important when operating systems completely open loop as is becoming more common.

[JohnMo]

35 minutes ago, BobBiggar said:

More specifically, does it keep running 24/7/52 at the minimum 9l/min even when the compressor is not operating? or is it doing something a little more energy efficient and turning off, then occasionally on to sense return temperature?

You can normally program any heat pump to have either of these properties. But it depends on your system, if one method better than another. I found I got unnecessary frequent starting and short runs with the circulation pump off or in sniffing mode. But makes a big difference if UFH or radiators also. I have UFH and the continued circulation helps the system determine based on return temp when the floor needs more energy adding.

[marshian]

On 25/10/2023 at 11:37, HughF said:

I'll try and calculate it once I've measured my pipe length inside the house. I know I've got 15m of twin 32mm underground, so that gives 25ltrs ish... Probably close to 50ltrs I expect?

 

I know it's an old thread - but I got a very good measurement of my heating circuit by reading the water meter before a refill after a drain down (I subtracted the volume of the header tank

[sharpener]

9 hours ago, BobBiggar said:

Specifically, did you ever get a satisfactory answer to the question about the Arotherm ASHP internal pump. More specifically, does it keep running 24/7/52 at the minimum 9l/min even when the compressor is not operating? or is it doing something a little more energy efficient and turning off, then occasionally on to sense return temperature?

 

If there is no call for heat the internal pump in the outdoor unit turns off when the compressor shuts down, you can see this as the flow rate on the VWZ AI falls to zero when the HP goes to Standby.

 

If you have a VR71 Wiring Centre the way you can control this behavious is to wire an external enable contact closure acorss S6/S7/S8 for the 3 individual circuits. If you only have one circuit and hence no VR71 the same effect can be achieved with the EVU terminals in the VWZ AI or ?S20 on the outdoor unit itself.

 

Like the OP I have Honeywell Evohome. In my case the BRD-91 Boiler Relay provides a mains voltage signal over pre-existing wiring, so this is interfaced by using a relay to provide a voltage-free signal. But with enough cores in the cable you don't need to do that.

[BobBiggar]

21 hours ago, JohnMo said:

You can normally program any heat pump to have either of these properties. But it depends on your system, if one method better than another. I found I got unnecessary frequent starting and short runs with the circulation pump off or in sniffing mode. But makes a big difference if UFH or radiators also. I have UFH and the continued circulation helps the system determine based on return temp when the floor needs more energy adding.

thanks @JohnMo 

First of all, apologies if some of my terminology/jargon isn't quite right. I'm only just getting to understand our system after 6 months living with it and am trying to learn how to optimise it's operation. My observations are all a bit subjective but I haven't yet worked out how to pull data off the system and analyse this objectively.

My system has GF UFH and 1st Fl rads, but all designed at 35 deg WC flow temp and configured as a single zone. There is a buffer between HP and emitters which I'm not sure is necessary. It's a modern highly insulated building including a MVHR system. Essentially everything on all the time trying to maintain 21 deg (playing around a little with 20deg or 19deg setback). It doesn't seem to cycle too often. The installer fitted a 12kW Arotherm which I think is overkill. I haven't noticed it running at more than 30-40%. Heating curve is 0.55 and temperature throughout the house is consistent and doesn't decrease too quickly. Sensing temperature every 30 mins or even 60 probably wouldn't see much difference.

We have just been through our first summer and the internal circulation pump ran continuously (at 1100 lph according to VWZ AI) when the HP was only active a couple of hours each night for DHW. 6 months of circulating with no heat demand seems like a real waste of energy. I know it's possible to alter this pump behaviour but I have only seen either fixing it to between 50%-100% or "Auto". Its set to Auto but I don't know what that means since it only seems to switch between 1100 lph (compressor off) and 2000 lph (compressor on).

 

[BobBiggar]

13 hours ago, sharpener said:

 

If there is no call for heat the internal pump in the outdoor unit turns off when the compressor shuts down, you can see this as the flow rate on the VWZ AI falls to zero when the HP goes to Standby.

 

If you have a VR71 Wiring Centre the way you can control this behavious is to wire an external enable contact closure acorss S6/S7/S8 for the 3 individual circuits. If you only have one circuit and hence no VR71 the same effect can be achieved with the EVU terminals in the VWZ AI or ?S20 on the outdoor unit itself.

 

Like the OP I have Honeywell Evohome. In my case the BRD-91 Boiler Relay provides a mains voltage signal over pre-existing wiring, so this is interfaced by using a relay to provide a voltage-free signal. But with enough cores in the cable you don't need to do that.

thanks @sharpener, this is very helpful.

As mentioned in my previous comment, I don't see the internal circulation pump turning off when the compressor shuts down. However, this is perhaps because the wiring configuration is not as you outline. I don't have a VR71 so using the EVU terminals in the VWZ AI looks like an option to try for me.

[sharpener]

14 hours ago, sharpener said:

If there is no call for heat the internal pump in the outdoor unit turns off when the compressor shuts down

 

Was referring to the external call for heat from the 3rd party controls like the Evohome the OP and I have got.

 

Without that I think the Arotherm Plus internal pump will run all the time it is on a timed heating or HW schedule. So if you want it to shut off for more of the time you need to tighten the schedules.

 

1 hour ago, BobBiggar said:

We have just been through our first summer and the internal circulation pump ran continuously (at 1100 lph according to VWZ AI) when the HP was only active a couple of hours each night for DHW.

 

For instance assuming the heating circuit was set to OFF in the summer it sounds as if you might have the HW heatup enabled for an unnecessarily long time, many ppl find a defined 2 hr window during the night-time cheap rate is enough for their HW needs, the pump shouldn't run outside that.

 

1 hour ago, BobBiggar said:

I don't have a VR71 so using the EVU terminals in the VWZ AI looks like an option to try for me.

 

Not sure what you would connect to the EVU terminals, have you got any third party controls at all? A room thermostat or timer could be used but the effect would be pretty much the same as judicious setting of the time schedules on the SensoComfort and using its internal thermostat in Room Temp Mod = Expanded mode, this shuts off the whole system when the index room is up to temp.

[BobBiggar]

3 hours ago, sharpener said:

Without that I think the Arotherm Plus internal pump will run all the time it is on a timed heating or HW schedule. So if you want it to shut off for more of the time you need to tighten the schedules.

 

For instance assuming the heating circuit was set to OFF in the summer it sounds as if you might have the HW heatup enabled for an unnecessarily long time, many ppl find a defined 2 hr window during the night-time cheap rate is enough for their HW needs, the pump shouldn't run outside that.

 

thanks again, this gives me a focus to look for what is keeping the pump on. I think you might be right about the DHW schedule as the source of this.

Assuming this resolves the constantly running pump I still don't know how the HP will know when to start heating again in open loop configuration (no external demand signal). If it works, I suppose I don't have to and can just let Valliant do it's WC fully open loop thing.

[sharpener]

59 minutes ago, BobBiggar said:

I still don't know how the HP will know when to start heating again

 

See the Energy Integral thread.