Vaillant Arotherm in open loop, with buffer

[Peter269]

(new member, first post)

I’m thinking of having a 7kW Vaillant Arotherm Plus heat pump installed and have had a heat loss report and quote. I’ve seen that there are several experts on the Arotherm on this site – I would appreciate it if you could have a look over this proposal and tell me what you think

 

Some years ago, I fitted an Evohome controller giving me individual time and temperature control over every room, and I estimate this reduced my gas bill by about 15%. The installer wants to get rid of the Evohome but I would like to keep the flexibility it gives, mainly because (a) there are some rooms in the house we don’t use except for when we have guests, and (b) the lounge has large SW-facing patio doors giving a lot of solar gain. I know I might get a small reduction in COP by turning unused rooms off, but (hopefully) it will be more than compensated by a lower total house heat loss – I’m more interested in running cost than achieving best efficiency.

 

The way I would like to set this up is to feed the heat pump flow into a 40-litre buffer (Vaillant “decoupler”), and set up the Arotherm to run in weather compensation mode only, with no room feedback (ie. “open loop”). A separate recirculating pump will feed the radiators, with the pump and TRVs being controlled by the Evohome, as at present. The CH pump will run in constant pressure mode, so that each radiator should operate at design flow without being affected if other rads in the house open or close.

 

If I didn’t have a buffer, I would be reliant on the internal pump, which I guess modulates to maintain delta-t rather than constant pressure, is that right? In any case, I couldn’t guarantee minimum water volume if some or all the TRVs close down.

 

To set it up, I plan to start with a weather comp curve of say 0.8 and a target temperature of 21’, giving a 45’ flow at -3’ which is what the radiator sizes will be based on. I will use the Evohome to fully open all the TRVs, then balance each radiator to get a 5’ delta-t. This assumes that all the radiators are correctly sized for the individual rooms – in practice some may need turning down a bit to achieve the design room temps, but at least one should be fully open on the check valve. If I set the Arotherm for a 5’ delta-t as well, I think the flows in and out of the buffer should match.

Once that is set up, I can use the Evohome to turn individual rooms on or off, and to stop rooms overheating when the sun shines. I might also turn some rooms down during the Octopus Cosy peak time of 16:00 – 19:00. And if we go away in the winter, I can use the Evohome to turn all the rooms down low and then turn them back up a day or two before we return. The Arotherm should just keep the water in the buffer at the correct temperature in all circumstances.

 

I guess I will have to experiment to find the lowest heat curve I can get away with, and it might be different between the autumn when the house and ground has some retained heat, and the spring.

 

So – some questions :
    • To run the Arotherm open loop, do I just need a ‘heat pump control module’ VWZ AI with an outdoor sensor, or do I need a Sensocomfort or VRC 700 as well?
    • In this configuration, can I still modify the heat curve up and down to a daytime target temperature with night setback, without room feedback? What settings do I need to do that, and is the night set-back just a single period, or can I set a more detailed set-back schedule to line up with the Cosy tariff?
    • If I shut off some of the rooms, initially some of the flow from the heat pump will flow through the buffer directly into the return, reducing the delta-t that the Arotherm sees. I assume that the internal pump will then modulate down to maintain 5’ delta-t, so the flows in and out of the buffer should equalise again, and there is no loss of efficiency. Is that true?
    • If there is no demand from the house – for instance if we go away – the heat pump won’t know about it, but I assume it will modulate down and eventually turn itself off. If the house then starts demanding heat from the buffer, how will the heat pump know to turn itself back on if the internal pump isn’t running? Will I need a sensor in the buffer, and how will this connect?

 

Sorry – lots of detail and lots of questions. Am I on the right track? Any snags?
Any advice will be appreciated.

Thanks for reading

 

[JohnMo]

Bit of a long post, but I would say there are two things that don't hang together. One weather compensation and having radiators opening and shutting. When running WC the flow temp is matched to the heat loss, so what energy flows into the radiator is equal to that lost by the room. So a room takes an age to change temp.

 

You seem to want to hang on to a gas boiler operating regime, which is high flow temps and very immediate, but install a heat pump which is the polar opposite.

 

Embrace simple, limited controls, very long run times.

 

Really don't understand the Cosy tariff and how it helps a heat pump.

[JamesPa]

31 minutes ago, JohnMo said:

Bit of a long post, but I would say there are two things that don't hang together. One weather compensation and having radiators opening and shutting. When running WC the flow temp is matched to the heat loss, so what energy flows into the radiator is equal to that lost by the room. So a room takes an age to change temp.

 

You seem to want to hang on to a gas boiler operating regime, which is high flow temps and very immediate, but install a heat pump which is the polar opposite.

 

Embrace simple, limited controls, very long run times.

 

Really don't understand the Cosy tariff and how it helps a heat pump.

+1 for that response, KISS.

 

One idea which has been advocated by an installer that has impressed me is this:

 

Identify your main group of adjacent living rooms, ideally mostly downstairs (because heat rises).  Operate these fully open loop thus guaranteeing sufficient engaged volume at all times = no need for buffer or separation or indeed any other 'add ons'.

 

Then use trvs/timers on the remaining rooms only,those rarely used/those that you need at a lower temperature.

 

I think the validity of this depends on the layout of the house and it's use patterns, but it is an appealing middle ground if the layout/use pattern fits.  In the coldest weather it might be necessary to turn on the rarely used rooms.

[sharpener]

I am planning something similar as I too have an Evohome setup, my house is long and thin and like you there are rooms we do not use much. However my plan is to wire the existing BRD91 to the Vaillant VR71 to act as an overriding call for heat. Doing this would allow you to shut the system down properly when you are away and re-start it remotely before you return.

 

I think balancing the flow through the two pumps might work as you describe but it is notoriously difficult and relies on quite a number of factors to get right. Using an automatic bypass valve with a volumiser on the return flow might be a lot simpler.

 

I am not sure you will get away without a sensocomfort control, much of the functionaility seems to be in that not the HP itself. There is a simulator here and a configuration tool here, which may help.

 

There is also an Arotherm Plus fb group but I have found it not very tolerant of this kind of query, the general Heating Design group is better and there is much discussion there of buffers. Heatpunk website has also got useful info on what can go wrong when you use them, generally they are to be avoided on thermodynamic grounds because the mixing causes an increase in entropy which results in irreversible losses. There is also a Caleffi site discussing different buffer topologies.

 

HTH

 

 

[JohnMo]

10 minutes ago, sharpener said:

would allow you to shut the system down properly when you are away and re-start it remotely before you return.

Don't you worry about a pipe freezing up while you are away? Not sure I have or ever would turn my heating off, in the heating season, even when I worked overseas, the heating stayed on low.

[sharpener]

Yes I worry but after installing the Evohome BRD91 in parallel with the original thermostat on the landing I have kept it as a frost stat set to 5C. The system is programmed to come on 0200 - 0500 if this calls for heat. And I can control the system remotely as well. In other houses I have left the system running on deep setback just like you @JohnMo.

 

Vaillant recommend using glycol rather than antifreeze valves, this gives total peace of mind and you will not come home to find all the coolant on your driveway. Unlike e.g. Daikin they do not waste a lot of electricity keeping the compressor warm either.

[ReedRichards]

6 hours ago, JohnMo said:

Bit of a long post, but I would say there are two things that don't hang together. One weather compensation and having radiators opening and shutting. When running WC the flow temp is matched to the heat loss, so what energy flows into the radiator is equal to that lost by the room. So a room takes an age to change temp.

 

You seem to want to hang on to a gas boiler operating regime, which is high flow temps and very immediate, but install a heat pump which is the polar opposite.

 

Embrace simple, limited controls, very long run times.

 

I disagree. 

 

There are different ways you can do Weather Compensation.  The most radical way is to try to balance the heat loss from the house with the input from the heat pump.  So you don't do anything internally that could cause this heat loss to vary.  You don't use a room thermostat or radiator thermostats.  You probably don't leave the house either as opening an external door would lose you a chunk of heat that this regime does not allow for.  In principle this is the most energy efficient way you can run a heat pump but is has disadvantages if you don't want the same temperature in each room all the time because "a room takes ages to change temp".  You'll find plenty of true believers here who seem to think this is the only way you can do Weather Compensation - but it isn't!

 

Another less radical way to do Weather Compensation is to cut yourself a bit of slack, make the leaving water temperature a bit higher than it would be in the purist scheme but still varied according to the external temperature.  This will cause your heat pump to cycle a bit more but get it right and these will be long rather than short cycles.  In principle your heat pump will perform a bit less efficiently but it will be able to warm up a room faster if necessary.  Now you can use room and radiator thermostats.  So you'll probably pay a bit more for your running cost but what this money buys you is more flexibility.  This is what I do and it works fine for me.   

Edited September 14, 2023 by ReedRichards

[JohnMo]

6 minutes ago, ReedRichards said:

This is what I do and it works fine for me.

Well done you.

 

But you do add complications, you possibly need or have a buffer

13 minutes ago, ReedRichards said:

You probably don't leave the house either as opening an external door would lose you a chunk of heat that this regime does not allow for. 

In and out quite a lot, good thing about airtight houses, opening a single door doesn't cause a draft.

 

I actually operate two WC regimes, shoulder months use floor as a storage heater, charge up at a higher temp for a out 6 to 7 hours instead of running all day with very little load. Then when it's really cold outside that run time extends across the day. But so the house is the right temp at bedtime, the heating is generally off about 6pm anyway. Still all done single zone. One thermostat/timer in the hall. 

[Big Jimbo]

@JohnMo The idea of having simple controls for heating really appeals to me. I've been in houses with thermostats in every room. My idea is to have One zone downstairs underfloor heating. One zone upstairs with thermo valves on the rads.

[Peter269]

Thanks for all your comments, and your patience.

 

7 hours ago, JohnMo said:

Bit of a long post

Sorry, yes a brain dump, I know. Been on my mind for a while!

 

7 hours ago, JohnMo said:

You seem to want to hang on to a gas boiler operating regime, which is high flow temps and very immediate, but install a heat pump which is the polar opposite.

Not really, I understand the difference, and the Evohome already gives me fairly low radiator temperatures, with a slow warm up time. Very different to my old on/off thermostat.

 

I think the shoulder months might be tricky with just weather comp. For instance, it's turned colder today and outside temps have been 13 rising to 18. My Evohome gave me just a bit of heat in my lounge with a radiator surface temp of around 28-30 (old rads, not yet upgraded). All the other rooms are at or above target temp so all other rads are off. If I had a heat pump with WC and no TRVs, the flow temperature would be about 28 flowing through all the other 8 rads even though those rooms are up to temperature. It might give a good COP but I can't see that it's an economical way of running.

 

7 hours ago, JamesPa said:

Identify your main group of adjacent living rooms, ideally mostly downstairs (because heat rises).  Operate these fully open loop thus guaranteeing sufficient engaged volume at all times = no need for buffer or separation or indeed any other 'add ons'.

 

Then use trvs/timers on the remaining rooms only,those rarely used/those that you need at a lower temperature.

I think that's about where I would like to end up. But I couldn't do without a TRV in the lounge. The sun has come out now, the Evohome has shut down, and the room has got up to 24. Might have to open the patio door! With pure open loop, the radiator would still be trying to heat the room.

 

6 hours ago, sharpener said:

I am planning something similar as I too have an Evohome setup, my house is long and thin and like you there are rooms we do not use much.

 

I think balancing the flow through the two pumps might work as you describe but it is notoriously difficult and relies on quite a number of factors to get right. Using an automatic bypass valve with a volumiser on the return flow might be a lot simpler.

 

I am not sure you will get away without a sensocomfort control, much of the functionaility seems to be in that not the HP itself.

Nice to hear from an Evohome user! Do you intend to use a buffer?

I need to think how I would do the balancing, but I guess that would be difficult even without a buffer. You can't just open all the rads, as they will have been sized for the next available increment above the design heat loss and the margin will differ from rad to rad. At least it should be easier with a constant pressure pump.

Thanks for the advice on the controls. I haven't yet got my head round all the modules and how they interact. I'll look again at the Sensocomfort.

 

2 hours ago, ReedRichards said:

Another less radical way to do Weather Compensation is to cut yourself a bit of slack, make the leaving water temperature a bit higher than it would be in the purist scheme but still varied according to the external temperature.  This will cause your heat pump to cycle a bit more but get it right and these will be long rather than short cycles.  In principle your heat pump will perform a bit less efficiently but it will be able to warm up a room faster if necessary.  Now you can use room and radiator thermostats.  So you'll probably pay a bit more for your running cost but what this money buys you is more flexibility.  This is what I do and it works fine for me.   

Thanks, that's exactly how I plan to work it.

[sharpener]

40 minutes ago, Peter269 said:

Nice to hear from an Evohome user! Do you intend to use a buffer?

 

Not as such. I plan on having a 200 litre thermal store so that the bedroom rads will run on off-peak heat. All the returns will go back to the HP via its bottom tappings, to give more engaged volume for defrosting. Installer seems happy with this and is considering my other suggestion of having a perforated baffle above that layer to improve stratification when the store is not in active use.

 

We are currently awaiting Vaillant's final say on the schematics. They will do one-offs for individual systems but are not very responsive, I would say first try and find an installer who believes in your ideas. V. will give you a few suggestions if you use their find-an-installer scheme.

[ReedRichards]

6 hours ago, JohnMo said:

 

In and out quite a lot, good thing about airtight houses, opening a single door doesn't cause a draft.

 

 

So maybe if you don't live in an airtight house (and sadly I don't) then you can't do the type of thermostat-free Weather Compensation that you favour?  At least not in the depths of winter when opening an exterior door can reduce the temperature of the room it leads into?

[Beelbeebub]

Isn't part of the idea of WC that, because the flow temps are low, the change in output from any temp deviation (solar gain or opening a door) has a disproportionately large effect on the power delivery? 

 

Eg when your radiator temps are 65ish, then the change in power output to the room at 17C vs 20C is negligible, Delta T goes from 50 to 53, 6% difference. 

 

But if your rads were 35C then the DT goes from 15 to 18, a 20% increase in power delivery. 

 

Likewise if the sun comes out and your room heats up to 25C, the DT drops to 10 and the power delivered drops by 30%

 

Basically the lower the design DT, the more self regulating the system is? 

Edited September 14, 2023 by Beelbeebub

[Lofty718]

Vaillant controls allow a mixture of pure weather compensation and also room influence. I have sensocomfort with WC on my gas boiler

[JohnMo]

1 hour ago, Lofty718 said:

and also room influence

Don't try it on UFH as it sends the room temp in to slow yoyo of undershoot and overshoot. The algorithm cannot get its head around why room temp isn't changing as predicted.

 

1 hour ago, Beelbeebub said:

Isn't part of the idea of WC that, because the flow temps are low, the change in output from any temp deviation (solar gain or opening a door) has a disproportionately large effect on the power delivery? 

 

Eg when your radiator temps are 65ish, then the change in power output to the room at 17C vs 20C is negligible, Delta T goes from 50 to 53, 6% difference. 

 

But if your rads were 35C then the DT goes from 15 to 18, a 20% increase in power delivery. 

 

Likewise if the sun comes out and your room heats up to 25C, the DT drops to 10 and the power delivered drops by 30%

 

Basically the lower the design DT, the more self regulating the system is? 

Exactly, this is especially true of UFH as the floor temperature may only be a degree or two higher than the room. So if floor temp is 22 and room temp 24, heat flow is either stopped or it is actually being removed from the room into the floor top surface. So there is no advantage having anything smart to compensate it does itself.

 

Also if the room isn't taking the heat away (or less of it) from the radiator, the boiler sees a higher return temp and automatically modulates it's output downwards.

 

 

[Lofty718]

2 hours ago, JohnMo said:

Don't try it on UFH as it sends the room temp in to slow yoyo of undershoot and overshoot. The algorithm cannot get its head around why room temp isn't changing as predicted.

 

Exactly, this is especially true of UFH as the floor temperature may only be a degree or two higher than the room. So if floor temp is 22 and room temp 24, heat flow is either stopped or it is actually being removed from the room into the floor top surface. So there is no advantage having anything smart to compensate it does itself.

 

Also if the room isn't taking the heat away (or less of it) from the radiator, the boiler sees a higher return temp and automatically modulates it's output downwards.

 

 

I think this would happen on a standard manifold mixing valve but I have my UFH controlled by an electronic esbe mixing valve wired directly into the Vaillant wiring centre so it offers much better control.

 

I've only run the system on pure weather compensation so far but I'm going to try it with room influence this winter and see how it goes.

[Peter269]

Perhaps not surprisingly, this thread has focused on the merits or otherwise of buffers. I'd like to bring it back if I may to one of my original questions which was a "how" rather than a "why".

 

Forget about the buffer. Imagine a house with weather compensation but no room compensation. On the Vaillant controller, 'Room temp. mod.' is set to 'None'.

 

Under low load conditions - say the outside temperature is in the teens, perhaps the HP is a bit oversized or there is some extra heating on somewhere - the HP will modulate down to its minimum and then start cycling on and off. What determines the start of each off cycle and on cycle?

 

I guess the off cycle starts when the HP sees the return flow too high (delta-t too low) so it switches the internal pump off. What then causes it to switch the pump back on? Does anyone know please?

[JoeBano]

The water pump is always running. The compressor will turn off when Flow and return temperatures are equal. The compressor will turn back on when the flow temperature is 5c lower than the target flow temperature. That’s the scenario on my heat pump (grant aerona3)  

[IanR]

3 minutes ago, Peter269 said:

What then causes it to switch the pump back on? Does anyone know please?

 

That is a good question, and one I've made assumptions on.

I have a 4P Buffer and and external ASHP pump. The buffer doesn't have any temp sensors that are part of the control system, and I assume the pump doesn't either, so to measure the return temp I'm assuming the water needs to be pushed back from the buffer to the ASHP. I've assumed the pump periodically runs, without starting the compressor, to circulate the buffer water through the ASHP and confirm the return temp. Then once the return temp is delta T + hysteresis below target flow temp, the compressor comes back on. But, I've not witnessed the periodic circulating of the flow and return, without the compressor active, to know this is actually happening.

 

My pump ASHP pump definitely does not run continuously, and of course for some of the time is pushing water to the DHW circuit, not the Buffer.

[JohnMo]

1 hour ago, Peter269 said:

Under low load conditions - say the outside temperature is in the teens, perhaps the HP is a bit oversized or there is some extra heating on somewhere - the HP will modulate down to its minimum and then start cycling on and off. What determines the start of each off cycle and on cycle?

 

I guess the off cycle starts when the HP sees the return flow too high (delta-t too low) so it switches the internal pump off. What then causes it to switch the pump back on? Does anyone know please?

Basics

Heat pump will have a target flow temp and dT between flow and return. First the circulation pump starts, the heat pump reads the return temperature. If the return temp plus dT is below the set point, the ASHP will start adding heat. Heat is only added to maintain dT. Through the heat cycle the return temp will increase as the emitter and room temp dT closes. Once the supply temp exceeds set point by a predefined margin say 0.1 deg, the heating cycle stops.

 

The circulation pump continues, and monitors return temp....

 

 

 

 

[JohnMo]

Forgot to mention there are two sides if you have a 4 port buffer. The side I explained is the heat pump side or if no buffer. With a buffer, the heating side pump is controlled by room thermostats only.

 

The two sides of the buffer work independently of each other.

[JohnMo]

8 minutes ago, IanR said:

pump ASHP pump definitely does not run continuously, and of course for some of the time is pushing water to the DHW circuit, not the Buffer.

I know my heat pump has a couple of circulation pump modes, the default to run continually when there is a heat demand, but I don't have a buffer. The other mode is a sniffer mode, where the circulation pump runs for a couple of minutes then off.

[Peter269]

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

[IanR]

41 minutes ago, Peter269 said:

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

 

Just to say, not all ASHP pumps are internal. My Nibe has no pump within the ASHP housing, the pump is placed in the house, by the 3-way valve. I also have a sperate pump, in the house, for the emitter circuits. I'm not discussing this one.

Edited September 15, 2023 by IanR

[JohnMo]

Not definitive, but the choice of how ASHP circulation pump runs would be an installer selection option. So could be either or. But to add confusion some control schemes work by having a thermostat the buffer, the thermostat being a run permission for the heat pump, in this case the circulation pump would almost definitely be off, while the buffer is not calling for heat.

 

Unless you know you are connecting everything up, having a buffer not having a buffer it's all very academic, and doesn't make a jot of difference