Can you have a primary and a secondary heat pump installation?

[Archer]

Sorry, this is another annoying, niche question that I have on our install... I'll ask our engineer as well but using this as a safe space first in case it's a really daft question... 

 

Current plan for our refurb is 2x outdoor units and 2x ducted indoor A2A units. The total capacity seems to be well over what's needed based on heat loss (but the pumps have a 1:6 modulation ratio so less of an issue than with an A2W system). The smaller of the two outside units has a significantly worse COP @ 7degrees than the larger - something like 15% worse. 

 

Does anyone know if it's possible to set a two pump system up so that one is the primary unit and the second pump acts as a backup/ only kicks in if the heat demand exceeds a certain threshold? There would be several advantages to this if possible - the minimum output would be reduced to reduce cycling; COP increased; noise reduced by sharing the load in cold weather; backup capacity for extreme cold snap etc. 

 

From what I can work out with our proposed system, it should be technically fine to run 2x indoor air handler units off a single outdoor. It's just whether there's a way to set them up in series? Obviously you'd need twice the refrigerant runs which would add cost and maybe be a bit ugly... But anything else, or stupid idea?!

[SteamyTea]

Can you do a sketch of what you have in mind.

Like most 'power' systems they can be run serially or in parallel.

One may be better than the other.

[Archer]

50 minutes ago, SteamyTea said:

Can you do a sketch of what you have in mind.

Like most 'power' systems they can be run serially or in parallel.

One may be better than the other.

 

My powers of sketch are worse than my technical knowledge but here we go... The below shows a very simplified sketch of the proposed system. Pump A serves the top floor and fancoil unit C; Pump B serves the ground floor and fan coil unit D also in the loft). 

 

My OP was asking whether it's potentially possible to have 2x refrigerant lines into each fan coil, so C+D are served jointly by A+B with a control to make B the dominant one and A to only kick in past a certain output (ie. in very cold weather if needed). Sorry I don't have circuit sketches or anything like that, just querying whether this is ever done?

 

I think I may have answered my only question and the fan coil units might not be set up for multiple refrigerant lines

 

 

 

[sharpener]

You would not want the complication of changeover valves in the refrigerant lines or the possibility of refrigerant being transferred from one circuit to the other.

 

So I think you would need two separate coils in fancoil unit C, then pump B (which I assume is the more efficient one) could supply them both when demand is low.

 

Don't know how you would control it. Commercial refrigeration specialist might know, having many units in tandem operation is fairly common, usually they are the same rating and are operated in a  cyclic sequence to equalise usage (like the mulitpley redundant pumps in a water treatment plant for example).

[JohnMo]

I would keep it simple, one outdoor unit for the room xyz, the other outdoor unit for the balance of the house. You could split bedrooms and house, then run the different units at different times etc.

[Archer]

Thanks everyone, yes I'm coming around to the keep it simple view as well, definitely

[saveasteading]

1 hour ago, Archer said:

keep it simple view as well, definitely

If its too cold for a few days per year, you just need to have it running for longer, run an old fashioned electric heater or woodburner briefly or put on a jumper.

The cost will be a fraction of a secondary system and more reliable.