Recommendations/experiences with house heating from a Viessmann heatpump

Zenith74

Hey all, wondering if I could consult the hive mind for any thoughts/experiences/recommendations you might have on this topic.

I had an old boiler replaced with a 16kW Viessmann 150A air-to-water heatpump earlier in the year. For better or worse I decided to do some of the electrical/control side of things myself as I have some DIY experience in that area. At this point the DHW has been running well for 5-6 months but as the colder weather sets in I need to finalise the heating side of things. But I'm struggling to decide what is the best way to configure things and I'm hoping for some experiences and insights if I may.

So on the heating side of things, what I have is the heatpump connected to a 200L buffer cylinder with a temperature sensor in it. An outside temperature sensor is in-place for the weather compensation. On the secondary side of the buffer cylinder I have a circulation pump that would send water to the house (heatpump is in the garage). In the house there are 3 heating circuits, two with radiators and one with UFH. Because the UFH was installed while the oil boiler was in-place, it actually has its own mixer/temperature-limiter and circulation pump. All three circuits have electronic valves fitted (used with Nests and the oil boiler). I have four cables to various points in the house for potential room stats. All rads except bathrooms have TRVs. Essentially I have a blank sheet in-terms of heating control wiring now and it seems like a few ways to configure things.

Option 1: Do not connect the four electronic valves or room stats, just leave them open. Connect the P1 (heating pump output) output from the Viessmann to the heating circulation pump and UFH heating pumps (via a relay so the Viessmann control board is protected). I assume the Viessmann will provide power on P1 constantly for any time periods where the schedule is set to Active Heating (that's how this output works right?), and it will use a combination of heating curves, the room temperature set in the app and the external weather sensor to determine how much heat to produce. The house will not overheat because eventually the TRVs would shut, the buffer would start to fill with hot water and the temperature sensor would eventually tell the heatpump to turn off (unclear to me if P1 continues to run the heatinc circulation pump in this instance). This is what the Viessmann disti here was recommending to me I think. What I struggle with in my head is that I'm setting a room temperature in the Viessmann app but with no actual room sensors it's just guessing how warm the rooms really are...

Option 2: I could put in Nests or equivalent in the three zones and have them connected to the electronic valves for those zones. When any of the Nests 'call for heat' it would start the main heating circulation pump and draw hot water from the buffer cylinder. Once the temperature drops in the buffer cylinder the heatpump would start to heat it again. A few hours later the rooms would reach temperature, the electronic valves would shut and the heatpump would realise the buffer has filled with hotwater and turn off.

Option 3: Viessmann room stats instead of Nests? I don't think this is possible with my Viessmann, it seems to only support a single heating circuit, even though there are connections for 3-4.


I could just try Option 1 first and see how it goes, but I'd be interested to hear what others have to say.

Thanks!

HVACNUT

To be perfectly honest and forthcoming, I don't know much about this design, but checking the Viessmann brochure on the 150A, it seems the only thing you forgot was the kitchen sink.

According to Viessmann, the buffer tank and all the near boiler piping can go. It has the ability to internally mix numerous water temperatures.

Did you use a piping diagram out of the manual? The piping diagram in the brochure is pretty simple. Maybe I'm missing something. 

And nowhere in the whole brochure was Nest ever mentioned. They have their own app. Why not use it?

Zenith74

Hey thanks for the response!

HVACNUT said:

To be perfectly honest and forthcoming, I don't know much about this design, but checking the Viessmann brochure on the 150A, it seems the only thing you forgot was the kitchen sink.

LOL yes it does seem that way. Most of it was actually already in-place for the oil boiler setup (electronic valves, Nests, UFH setup, TRVs). Given that it can all be left disconnected I felt it was better to leave there when the heatpump was installed, in-case it gave me more options. I was also a bit suspicious that going the way the installer/disti suggested (no TRVs, no zones, no room stats) was going to lead to temperature management that would feel a little last decade, having been used to Nests . I listed everything so somebody responding might go 'Oh actually if you have X than really the best setup would be Y'.

HVACNUT said:

According to Viessmann, the buffer tank and all the near boiler piping can go. It has the ability to internally mix numerous water temperatures.

Did you use a piping diagram out of the manual? The piping diagram in the brochure is pretty simple. Maybe I'm missing something

Yes the plumbing is the setup Viessmann suggest in this example scheme - https://static.viessmann.com/resources/vitodesk/schemes/4804407_04/Documents_GBR_EN/4804407_2110_01.pdf. As you say, once you translate all the hieroglyphics it's not so complex. No room stats in this scheme and I'd love an explanation for when the P1 output is actually running the circulation pump.

HVACNUT said:

And nowhere in the whole brochure was Nest ever mentioned. They have their own app. Why not use it?

My preference would absolutely be to use a single app and that to be the Viessmann one, it's quite good in fariness. However the heatpump I have only seems to be able to control a single heating zone, so if you wanted to manage the room temperature in multiple zones you would need to use some other type of heating control is my understanding.

Hot_water_fan

Is this installed in the US? Would love to see some pictures and hear about your experience this winter. 

Zenith74

Installed in Ireland.

Hot_water_fan

There’s no real reason to use a nest here, any cheap thermostat will be capable of on/off function. Option 1 is good - I’m not sure if the heat pump has this function, but a “ramp” function might be needed. Basically, if after a set time period, the buffer hasn’t reached temperature, the heat pump increases the set point. The goal is to get the TRVs to close, then the buffer will hit the limit. 

Zenith74

Hot_water_fan said:

There’s no real reason to use a nest here, any cheap thermostat will be capable of on/off function.

Good point. I was thinking about the convenience of being able to increase the temperature before you arrive home or turn on the hot water heating, but sure that's all handled by the Viessmann app anyway. I have a couple of Honeywell thermostats that will do the job if I need them, but as you say I think Option 1 is probably the place to start.

I think the Viessmann will ramp up the power depending on the return temperature its seeing from the buffer cylinder, it has three instantaneous heaters internally that it can use to boost the temperature if the heatpump alone is not doing the job.

The last thing I have a question about is the behaviour of the P1 output from the Viessmann, which the manual says can be connected to a heating circulation pump. I've connected that (via a relay) to my heating pump and UFH pumps. The pumps come on when the Viessmann is set to Active Heating and the room temperature is set to something high (30 degrees C for example), but they were still running a few hours after I'd turned the room temperature back down (well below the outside air temperature). Does anybody know the expected behaviour of this output?