[Greenbuilding] mini-split system

[nick pine]

Sacie Lambertson <sacie.lambertson at gmail.com> wrote:

>Marc Rosenbaum... [says his heating costs came to $250/year.]

A 12'x32'x28' tall sunspace heats my 1820 stone farmhouse well on sunny 
days. After lots of rumination and a $6K quote for a 1500 gallon hot water 
tank for cloudy day heat storage, I got a SEER 26 Mitsubishi MUZFE12NA 
mini-split ($1750, including the indoor unit and 25' preflared line set) and 
a few PVs with yearly net metering for cloudy days. Marc has espoused this 
solution for many years with no solar air heaters, and it's finally starting 
to make economic sense with air heaters, IMO, now that mini-splits work down 
to -13 F and PVs are cheaper (67 cents/watt for the modules and 40 cents for 
a string inverter, before the 30% fed tax credit.) This heat pump costs 
about 25% more to run than natural gas at our local electric and gas rates. 
With a frugal PV ground mount installation, it could cost 4 times less than 
natural gas heat.

Page 3 of 
http://www.mitsubishipro.com/media/214712/muz-fe09-18na_service_obh543a_9-10.pdf 
says Mitsubishi's MUZFE12NA heat pump can supply 13.6K Btu/h with a 10.6 
HSPF (a
3.1 COP) at a 17 F outdoor temp. Page 4 lists the -13 F minimum outdoor 
temp. Page 11 shows a 55% heating capacity (7500 Btu/h) at 10 F, with a 600 
watt total energy consumption, ie a 7500/3.41/600 = 3.7 COP. It makes 13.6K 
Btu/h with 1 kW at 45 F, for a 13.6/3.41/1000 = 4.0 COP. How did they 
calculate a 10.6 HSPF?

The inverter versions seem to be super-efficient with light loads because 
the heat exchangers are a lot more efficient with small flows.

See also NREL's 2011 study of Fujitsu and Mitsubishi ductless minisplits. 
http://www.nrel.gov/docs/fy11osti/52175.pdf

>All sorts of caveats apply to his situation, including the quite high cost 
>of installation.

The mechanical and electrical installation was not expensive, with a wooden 
shelf on the wall and UF wire stapled to the house wall instead of conduit. 
It was hard to find an HVAC artist to purge it. Some said they would not 
work on equipment they had not installed. Others wanted $300 to do it, 
although it takes less than 1 hour with about $400 worth of equipment, eg a 
$212 BluVac micron vacuum gauge https://trutechtools.com/BluVac and a $155 
2-stage vacuum pump 
http://www.amazon.com/gp/product/B000O1C47M/ref=pd_lpo_k2_dp_sr_2?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B003DKT2J4&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1WG77BTZS2ZKPRRHFV9T I 
finally got someone to purge it for $112.

The installation instructions say:

Section 4.1 Purging procedures and leak test

1. Remove service port cap of stop valve on the side of the outdoor unit gas 
pipe. [Similar to a Schrader automotive tire valve 
http://en.wikipedia.org/wiki/Schrader_valve]
2. Connect gauge manifold valve and vacuum pump to service port of stop 
valve. [We only need a manifold gauge to add refrigerant.]
3. Run the vacuum pump 15 minutes or more.
4. Check the vacuum with the gauge manifold valve, then close it and shut 
off the vacuum pump.
5. Leave as it is for 1 or 2 minutes. Make sure pointer gauge manifold valve 
remains in the same position. Confirm that pressure gauge shows -30 "Hg. [Or 
use the micron vacuum gauge to make sure the pressure is 500 microns max and 
doesn't rise fast when it is isolated from the vacuum pump.]
6. Quickly remove gauge manifold valve from service port of stop valve.
7. After refrigerant pipes are connected and evacuated, fully open all stop 
valves on both sides of gas pipe and liquid pipe.
8. Charge the prescribed amount of refrigerant if needed. [No additional 
refrigerant needed, with a 25' line set.]
9. Tighten cap of service port.
10. Conduct a leak test.

Mitsubishi tech support says the leak test (eg soap solution on 4 flare 
connections) is not necessary when using the micron gauge. Any condensation 
(water) in the line set will evaporate at 500 microns, and the pressure 
won't continue to rise fast after the water evaporates unless there is a 
leak.

The outdoor unit is precharged with 41 ounces of R410A refrigerant which 
flows through the lines and the indoor unit when the valves are opened in 
step 7. The tech support person suggested doing step 6 after step 7 (with 
the stop valves partially open), which would allow a tiny amount of 
refrigerant to leak out but would avoid allowing air to leak into the vacuum 
when the hose is disconnected from the Schrader valve.

Most HVAC techs fill systems with nitrogen at 150-400 psig and check to see 
if the pressure falls much over an hour or overnight, using an analog 
manifold gauge meter, which has a very low resolution compared to a micron 
gauge, eg 2% vs 20,000 steps. And this requires nitrogen, and it takes a 
long time...

Nick