Buffers on Air to Water HP

Keith W.

Hi there: I'm sorting out the options of an air-source hydronic heat pump supplying radiant floor heat and hi-wall fan coil cooling (choices are Daikin Altherma & Aermec, feel free to opine). We will likely use Aermec hi-wall fan coils either way, though I am checking into Magic Aire or some such. I have a fair bit of experience with Fujitsu mini-splits and hydronic heating, but have never done a hydronic heat pump.



I am curious about people's opinions about buffer tanks: the piping diagrams provided by the manufacturers are starkly different, with the Aermec plumbed directly into a buffer tank (min. 30g), with the loads pulled out the other side, generally with a primary-secondary arrangement with priority DHW pulled aside with a 3-way valve. Altherma provides much less explicit diagrams (that I've found so far anyway) that don't mention buffer thanks, though I've heard it suggested that at least a few gallons would be required to assure a successful defrost cycle.



I don't love the buffer tank in the middle of everything approach, it feels like it's a lowest-common-denominator approach, though I can see how it might provide the most level demand from the heat pump, and perhaps simplify controls too (not inconsequential with 4 radiant zones and 7 air zones, overlapping). My natural inclination would be to put a buffer in-line after all the secondarys (and not in the DHW loop). I'd be very interested to hear others thoughts.



Quick system details: very low-demand house, around 2 tons cooling, roughly equivalent heat. 4 underfloor radiant floor zones, and 7 hi-wall fan coils, 2000sf. Heating is primarily via the radiant, though the fan coils will be available to speed up heating. I'm thinking I can avoid tempering for the radiant: I can run the radiant as high as 100-110F, and even at those temps there's actually a fair bit of heat available from the fan coils, though the DHW will want hotter IMO (the heat pumps can deliver somewhere around 130F). There will be resistive backup on the DHW, and probably within the unit itself for space heating, though we are hoping to never use it (our design heating temp here is around 11F, and both these units can meet the demand at that temp).



Anyway, thanks for any helpful thoughts/feedback/criticism.



Keith

SWEI

Buffer tanks

were pretty much mandatory on earlier generations of heat pumps.  With VRF, assuming the minimum modulation rate is low enough you should be able to avoid one.

Keith W.

Need for buffer tanks with high-performance heat pumps

Hm: I think there are a few reasons one might need a buffer tank: it levels out the return temperatures to the heat pump, which in principle operates better when it doesn't have to accommodate big changes in demand... We are using the heat pump as sole heat source, so we will be demanding a lot of it in 20F weather. Another reason I've heard for requiring a buffer is to assure capacity for defrost cycles. In the former case, the central-mounted buffer approach a much better job of leveling demand, whereas I think the second issue is roughly equivalent given equivalent volume.



That said, I'm really just parroting the manufacturer's literature... which can be fairly brainless. For example, both systems "require" a 1.25" supply/return off the unit. However, on the smaller systems, the flow rate is only about 7 gpm. And in this case the primary loop is quite short, no more than 25' I think. So there's really no need for more than 1", imo. You can still pump it with a small pump, like an alpha or a bumblebee.