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…