Inverter v. Condenser

Dave Yates (PAH)

Were they studying true variable speed systems or compressors with unloaders?

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Frank Dobbs

Brave new world: Inverter v. condenser

So many technologies, so little time...

Granted that a circulating hydronic system provides much better quality heat than forced hot air. So that even with central air you would want hydronic heat, even tho that means installing two systems (if budget permits).

However, does the introduction to the US of inverter heat pumps, such as the Mitsubishi Y series, change the equation a little?

--Comfort

Is the quality of the force air heat much more pleasant because of constant, continuously modulated air flow and other design improvements? Or, conversely, is hydronic radiant heat still markedly more plesant?

--Efficiency

Would a condensing gas boiler save potfulls of money compared to such an excellent heat pump?

I do know that in the 80's I made condos out of two NYC rowhouses that a contractor had installed GE heatpumps in. The engineer who calculated the annual operating cost found them no more expensive than ordinary boilers at the time. Actual bills backed up his calculations.

My architect is installing the Mitsubishi in a commercial building and thinks it might work in my NYC rowhouse. However, he would be the first to admit that he is not an HVAC expert.

Feel free to stomp, if you think the idea is lame-brained.

Dave Yates (PAH)

changing the rules

Frank,

Inverter systems have been in use in European countries for more than a decade. As a result, the technology (while more complicated than plane-Jane refrigerant-based units) is time-tested and reliable. The on-board self-diagnostic logic pretty much diagnoses the problem if it's a component failure. We now have three voltages to manage internally, so troubleshooting is a bit more demanding. 220v single phase to 300v DC, which is inverted to 200v three phase.

Yes to both of your comfort questions.

Cost savings will be dependent upon fuel cost and their comparison. We're being told that the SEER ratings for Inverter systems rivals those of ground source heat pumps. However, the test proceedures for A/C and heat pumps does not yet allow for modulating and will only recognize flat-out Btu production. As a result, we must rely upon the manufacturers to spoon-feed us the numbers.

Given a choice, I'd go for hydronics every time.

Have your contractor take a peek at the Mitsubishi MXZ30 Inverter heat pump. A single outdoor whisper-quiet condenser with a single compressor that feeds three separate refrigerant circuits. Multiple choices for the three indoor units - up to 30K Btu's combined. You can connect three individual indoor (typically wall-hung) units and the control wiring consists of but two wires - digital DC communication that can be daisy-chained from the condenser to unit to unit to unit. Outdoor unit will ramp up or down to meet connected load. No electric resistance heat needed for cold weather operation because the compressor will ramp up its speed to meet that demand too.

During operation, the condenser checks outdoor air temp while the indoor units each monitor air temps and humidity (each unit can be set to maintain set humidity levels too) in their zone. Upon a call for cooling or heating, the equipment monitors how quickly those temps are changing and that drives the speed of the system. The refrigerant flow is modulated via a linear-slot expansion valve and that's done at the condenser, so both lines must be insulated.

As I understand it, US manufacturers are moving into the Inverter market. Ducted systems pose balancing problems once air-flow changes too much, so they'll have that problem to deal with, but there's no reason why the indoor blower can't be left to run at higher speed while the refrigerant and outdoor motors are modulated. At some point in time - no doubt when fuel costs rise enough - we'll stop heating and cooling on a whole-house approach and adopt more sensible methods - like wall-hung mini-splits.

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jerry scharf_3

Any newer research on variable capacity condensers

Dave,

The only research I know of that compares overall efficiency for variable vs fixed capacity compressers was done over 5 years ago at Lawrence Berkeley Labs. The study concluded that on an efficeincy basis, there was little advantage of variable run continuously over fixed run intermittantly. I used that to decide on the Freus compressor rather than looking for something like these units. (An EER of 18 and a estimated SEER in the low 20s helped.) Do you or Mitsubishi know of any other comparable independent studies of this?

Of course, this does not address the comfort improvements, multiple expansion coils, etc.

I whole heartedly agree that SEER is only effective when comparing similar systems, so it's hard to compare any nontraditional system. (sounds a bit like AFUE)

Some of the things that the Mitsu units do I have built through proportional dampers and controls. The house required serious HEPA filtration runing 7/24 for other reasons, so that drops out the cost of moving the air from the A/C side in my special case.

thanks,
jerry

Frank Dobbs

Ed,
Thanks for your thoughtful response. It was very helpful (almost a FAQ, actually.)


> Yes to both of your comfort

> questions.


> Cost savings will be dependent

> upon fuel cost and their comparison. We're being

> told that the SEER ratings for Inverter systems

> rivals those of ground source heat pumps.

> However, the test proceedures for A/C and heat

> pumps does not yet allow for modulating and will

> only recognize flat-out Btu production. As a

> result, we must rely upon the manufacturers to

> spoon-feed us the numbers.


If it's true that inverters are as efficient as ground source heat pumps, I found this report on the DOE website:

http://geoheat.oit.edu/pdf/tp80.pdf

It says that ASHRAE prefers a weather-driven method, like the bin method, to SEER, etc. The study uses the bin method to compare the gs heat pump to 3 other equipment typse using weather data for 2 different cities. The GSHPs were a lot more efficient than air source heat pumps, especially on the heating side. They also, on average, saved 38% on cooling and 22% on heating from a forced air system. Given my load factor and energy costs, I would multiply their savings by five, but of course we really don't know for sure that inverter technology really is as good as GSHP's.

However, if inverters could save 22% on heating from a conventional boiler, that would be one half or more the saving one could expect from a condensing boiler.


> Given a choice,

> I'd go for hydronics every time.


If I understand it, the inverter reduces the two advantages of condensing hydronics -- comfort and efficiency, however not completely.

Hydronics has the disadvantage of additional cost, the inconvenience of retrofitting two different systems, and the challenge to modernist, minimalist decor that radiant panels or baseboard present. But you feel so strongly about hydronic that you recommend it no matter what?

> Have your

> contractor take a peek at the Mitsubishi MXZ30

> Inverter heat pump. A single outdoor

> whisper-quiet condenser with a single compressor

> that feeds three separate refrigerant circuits.

> Multiple choices for the three indoor units - up

> to 30K Btu's combined. You can connect three

> individual indoor (typically wall-hung) units and

> the control wiring consists of but two wires -

> digital DC communication that can be

> daisy-chained from the condenser to unit to unit

> to unit. Outdoor unit will ramp up or down to

> meet connected load. No electric resistance heat

> needed for cold weather operation because the

> compressor will ramp up its speed to meet that

> demand too.

>

> During operation, the condenser

> checks outdoor air temp while the indoor units

> each monitor air temps and humidity (each unit

> can be set to maintain set humidity levels too)

> in their zone. Upon a call for cooling or

> heating, the equipment monitors how quickly those

> temps are changing and that drives the speed of

> the system. The refrigerant flow is modulated via

> a linear-slot expansion valve and that's done at

> the condenser, so both lines must be insulated.

> As I understand it, US manufacturers are moving

> into the Inverter market. Ducted systems pose

> balancing problems once air-flow changes too

> much, so they'll have that problem to deal with,

> but there's no reason why the indoor blower can't

> be left to run at higher speed while the

> refrigerant and outdoor motors are modulated. At

> some point in time - no doubt when fuel costs

> rise enough - we'll stop heating and cooling on a

> whole-house approach and adopt more sensible

> methods - like wall-hung mini-splits.

>

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> 98&Step=30"_To Learn More About This

> Professional, Click Here to Visit Their Ad in

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