ground source heat pump vs VRF

[Superchief]

Developer/GC on a residential project where I have a choice between geothermal and VRF (possibly heat recovery) - no gas service, and we do not want propane, so any solution is electric.

Zone 4a, big shoulder months, a lot of south facing glass, the house is very tight (closed cell foam and ICF construction) which in my experience means the Manual J calculations are conservative.

Preliminary Manual J

152,910 BTU's Cooling
149,526 Sensible Load
3,384 Latent Load
147,582 BTU's Heating

I need to decide which way to go now because the system is intended to be independently engineered and then bids solicited.

I've done geothermal before but VRF looks attractive to consider though I have no experience with it. I believe I have a reasonably good understanding of it at this point and am only considering Mitsubishi or Daikin equipment based upon what I've learned so far.

Amongst the specific concerns I have with VRF is how it will handle the low load conditions, how often a pressure tested line set goes bad without having been damaged, and life safety issues if there was a refrigerant leak?

Comfort and reliability are my priority assuming there does not turn out to be a large price disparity, and my investigation so far suggests there will not be.

So, does anyone with experience with both solutions want to offer advice on whether they would strongly consider VRF if this were their home?

 

[MintJulep]

GSHP with a single air handler and ducted distribution vs multiple ductless mini-split units with vrf?

 

[Superchief]

GSHP would have to be 4 units with 4 air handlers because I do not believe you can put 2 air handlers on a single unit? Each air handler would handle a separate section of the building with a short trunk and because the house is tight I don't usually run ducts to the windows, just to the rooms, so I anticipate ductwork is relatively short, straight, and inexpensive.

Mini split I'd likely use ducted mini splits in some places (e.g. serve 4 kids bedrooms with one unit since they are clustered together) while some rooms might get a ceiling cassette but it occurs to me I want to do some kind of air exchange and filtration, how does this get handled with VRF? I've used smaller mini splits for additions but never really worried about air exchange in those situations.

There are 5 bedrooms, master is completely separate wing so probably 5 zones but 4 could be on the same air handler. 1st floor divides into 3 zones, basement into 2.

I should mention the house will have an automation system that can manage zoning with GSHP, so zoning GSHP via electric dampers costs me < $200 installed per zone.

 

[EnergyProfessional]

You can do geothermal VRF. You also can do geothermal AHU and geothermal hydronic heating. So it isn't a one or the other.

It also depends on your climate. Because if the winter design temperature is low, air source heatpump is out unless you want to burn electricity at a COP of 1.

 

[Superchief]

I could do geothermal VRF, had not considered that option.

My area DT

heat 99.6 12.6
99% 16.9

cool
DB 88.0
MCWB 88.0



Would you consider 16.9 low, I cannot find a COP table for the City Multi but it is about 4 for AHRI 1230 test method - that tells me nothing but perhaps useful to you?

For standard Mitsubishi non VRF at DT the COP should be in the 2.5 - 3 range so I'd expect the VRF was at least that good?

 

[EnergyProfessional]

For an air source heatpump, forget about the 99.6% data. Look at historic data and the actually lowest value. unlike a combusting device, an air source heatpump will actually stop working and you don't want that on the coldest day. My guess your lowest will be at 10°F? they have hyper-heat units etc. You should study their documents to see the heat output at 10°F. COP will be low, but 10°F is only for a few hours a year. An energy simulation will be useful to compare the different options.

 

[Superchief]

We do sometimes drop below 0F though it is rare and never for long. I just looked at historical data and the lowest mean temp on record was 12.3F 40+ years ago and the coldest ever was -11F in 1934 though in the last 50 years there were 2 days that reached -6 and -5.

I know the Mitsubishi says it works at those temperatures at 70% capacity (I'm sure the COP is not good) but I could have backup heat, though I'll leave that to the engineer to figure out - I just need to decide if I want to seriously consider VRF as an alternative to a standard GSHP.

... and if I do, do I want to consider GSHP + VRF which would allow it to function in those temperatures without derating ...

 

[EnergyProfessional]

If -11°F is a possibility, the system should at least be bale to work at that temperature. It would be acceptable if it doesn't keep it all at design temperature (like 70°F). a boiler system would also not be bale to keep up, but it wouldn't stop working. But an air source heatpump will literally stop working and what do you tell the owner then when all their pipes burst??

yes, at low ambient COP basically will be 1. So for some redundancy you also could add electric resistance heat. I don't have experience with the hyper-heat VRF, but if they run a lot in winter, I assume there is more wear on the compressor. they also may have some other shortcoming for the rest of the year compared to regular air source heat pump (more expensive, is one). I also would ant to see where an dhow they were used in your area.

We have an air source VRF system where the engineers allegedly designed it to heat in winter. But we had to modify the system because the whole outdoor unit was one block of ice and the building was cold. So just because a sales person says so, doesn't make it true. it now runs very inefficiently with gas-fired unit heaters, but it works.

You can do geothermal, but make sure the designer has experience. We have some geothermal systems that also use a boiler (and the boiler runs most of winter....), and even a horizontal system (engineer was an idiot!--- we get -20°F and that will cool the soil in 8' depth!), where we had to add a boiler after the fact to heat.

 

[Superchief]

There must be more wear with Mitsubishi "injection technology" (I assume other vendors use similar technology) as they run at much higher speeds ... the GSHP version would avoid that problem.

One of the major issues is that the independent engineers who do geothermal mainly do commercial - I just had one quote me $30k for design which is a non-starter, but it indicates he really is not interested in residential.

So that means I'm dependent on the supply house engineer for the HVAC contractor, but I want to specify the outline of the solution so I do not end up with several contractors quoting wildly different proposals that I cannot compare... which is why I turned to the forum because in the end I want to tell the contractor "propose a VRF solution" or "propose a GSHP solution".

One nice think about GSHP is I could do radiant in some places and the thermal mass would help smooth out those temperature low points ... now that I'm thinking about it that is attractive, but then no mini split system because they do not do water-to-water.

 

[EnergyProfessional]

I wouldn't use thermal mass as a safety factor. When it is cold, it is cold for hours and days. the heating system needs to have enough capacity without relying on sored energy.
If you have thermal mass, you also need excess heat to heat it up to begin with.