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Old 08-15-14, 10:28 PM   #1
NiHaoMike
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Default "Downsizing" an oversized A/C

Let's suppose you have a building with a way oversized (central or large package) A/C (or heat pump), either due to recent energy efficiency renovations or as a result of a bad HVAC engineer. The A/C is otherwise in good condition and has many more years of service left.

Solution 1 - Replace the compressor and TXV with smaller ones. Expensive in parts even if you buy surplus, also very labor intensive.

Solution 2 - Replace the whole unit. Even more expensive and selling the old unit is unlikely to offset much cost.

Solution 3 - Change the refrigerant to a lower pressure refrigerant (e.g R22 to R134a) to lower the capacity. Labor intensive, especially if the oil has to be changed to POE.

Solution 4 - Add a VFD. Not cheap if you have to buy the VFD new, but very economical if you can get a surplus VFD. Relatively low labor cost and doesn't even need an EPA certification. Also get the benefits of a variable speed system. The high tech naysayers don't like this solution, but why listen to them?

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Old 08-16-14, 01:31 AM   #2
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How recent is recent? Is the contractor still in business?

Solution four is extremely labor intensive in configuring the control programming. The labor involved in controls adjustment can far exceed a simple compressor swap-out. A working compressor swap-out is nowhere near as labor intensive as a burn-out replacement.

You may not need an EPA certification, but if semantics are part of the consideration, you may need to be a licensed electrician depending on your jurisdiction.

The very specific example in solution three does not matter if you have an EPA certification or not. The use of non SNAP approved alternate refrigerant in an HCFC or CFC equipment is illegal even if you have a license.

Also, the fairly new system's warranty will likely be voided as turning a system down excessively lowers gas velocity and may lead to oil return issues.

I may have solution 5 to propose depending on your answer to my question which may let you have solution 2 completed quite economically.

Last edited by ICanHas; 08-16-14 at 01:45 AM..
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Old 08-16-14, 07:48 AM   #3
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It's a hypothetical situation that I don't have, but is all too common. Let's say the unit is maybe 4 years old and the company who installed it is still in business just like many "hack" HVAC companies.

This is a DIY site and electrical stuff is generally more within reach of the average DIYer than refrigerant plumbing. In most areas (at least in the US), it is not necessary to have an electrician license to work on the wiring in your own house - how does Home Depot sell so many electrical parts to the general public?

There is a heat pump out there that runs on a VFD just fine. It was a R22 unit converted to run on R134a, so I figure it must have been really oversized to begin with.



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Old 08-16-14, 10:16 PM   #4
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Quote:
It's a hypothetical situation that I don't have, but is all too common.
1.)Let's say the unit is maybe 4 years old and the company who installed it is still in business just like many "hack" HVAC companies.

2.)This is a DIY site and electrical stuff is generally more within reach of the average DIYer than refrigerant plumbing.

3.) In most areas (at least in the US), it is not necessary to have an electrician license to work on the wiring in your own house - how does Home Depot sell so many electrical parts to the general public?

4.) There is a heat pump out there that runs on a VFD just fine. It was a R22 unit converted to run on R134a, so I figure it must have been really oversized to begin with.

1.) Option 5 here. It's not terribly old. Perhaps one of the salesman made less than wise decision. If you're a great negotiator and well versed in self representation, you maybe able to compel them to correct the sizing by establishing that sizing was incorrect at the time of install and that it was merely a delayed discovery of defect in workmanship of selection. You just hire someone to do proper sizing, then have a legal expert write them a letter requesting resolution and accomplish a resolution for lesser monetary $$ than VFD + permit cost.

2.) That's a presumption based on assuming what's easy to you is easy to everyone. The breadth of reach is contingent upon the skills, experience, comfort level and tools available for use to the individual(s). Your dPIC, DSP program codes posts are beyond comprehension to many DIYers. For some DIYers, all this digital bullcrap is extremely labor and resource intensive (computer stuff, wires, interfaces, soldering, software)

3.) I said that in SOME. Which jurisdictions have you verified to come to your conclusion it is allowed in most places in the US? At least around here, homeowners may do some work, but still require permit and inspections and the AHJ may or may not pass inspection on jury rigged modification to permanently installed equipment. Availability of materials at Home Depot/Lowe's is not an assumption of legality and "buy it and do whatever you want". You can buy bath tubs, sinks and such that requires plumbing permit to install as well as black iron pipes mainly used for gas piping, which may or may not be legal to DIY depending on your locale. Furthermore, your AHJ may not pass improper or butchered install.

4.). Ok great. An existence of a working unit doesn't mean absence of problem in every install. Look up return gas velocity. How far you can push the envelope before problem develops depends on the system.

Reducing compressor reduces return gas velocity and on systems with not much room for loss in return gas velocity before oil return issues. In this case compress damage wouldn't be covered by warranty.

How far, and how long you can operate under reduced speed is system dependent. So, if your over sized unit is used like a mini-split and it runs basically around the clock between 10-20% capacity, you might actually be losing oil from compressor.

From Copeland Emerson Climate AE-1495
"
Piping
Unlike a variable speed compressor whose mass flow
and gas velocity changes with its speed
, the digital
scroll’s pumping capacity is equal to its 100% capacity
while it is pumping. For this reason the gas velocity
remains high even during periods when the capacity
demand is low. Because the mass flow and gas velocity
remain high, piping may be designed as if it were
designed for a non capacity controlled compressor. For
vertical piping a trap every 20 feet should be sufficient
to ensure proper oil return. This recommendation is
based upon a minimum 1500 fpm velocity or higher.
When the digital scroll compressor is part of a tandem,
a double riser should be considered to assure that the
velocity remains above 1500 fpm when only the digital
scroll is running.

Start Up and Shut Down
To improve the starting characteristics of the digital
scroll compressor, the the Emerson controllers delay
loading the compressor for 0.1 seconds. Likewise, to
eliminate the reverse rotation sound at shut down the
compressor is unloaded 0.5 seconds before shut down.
Compressor Cycling

Because of the digital scroll's seamless capacity
modulation from 10% to 100%, capacity short cycling
should not be a problem for single compressors.

However, if the digital compressor is in tandem with a
non modulated scroll, short cycling of the non modulated
compressor may be a problem if the system control is
not designed and set correctly. The Emerson digital
controllers have a built in two minute anti-short cycle
timer to prevent short cycling."

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Old 08-16-14, 11:42 PM   #5
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I'm thinking of an off the shelf surplus VFD, though I suppose an open source VFD wouldn't be quite out of the question. Some EV builders have done just that.

A simple 555 or timer module can be used to force full speed every few minutes for oil return.
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Old 08-16-14, 11:50 PM   #6
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Cool

For the average DIY homeowners, I believe component level electronic circuitry assembly and programming crap is EXTREMELY labor intensive and tedious. I know you're simply looking for the argument to present VFD as the most feasible option. I disagree to agree with your position.

Where do you get the idea that compressor change out is very labor intensive while the above that even many controls techs would not be able to do gets tossed around so lightly?

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Old 08-17-14, 12:06 AM   #7
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It's just wiring together a few modules - VFD, timer, and thermal controller. It's like building a DIY EV, but much simpler. An open source VFD could be even simpler (one module does it all) if assembled units are made available.

I wonder if you try to oppose every new technology that comes out...
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Old 08-17-14, 10:30 AM   #8
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Quote:
Originally Posted by NiHaoMike View Post
...I wonder if you try to oppose every new technology that comes out...
I think you are getting close to the truth of the matter.
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Old 08-20-14, 04:01 AM   #9
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Quote:
I wonder if you try to oppose every new technology that comes out...
I only oppose "savings" products with economic, safety and/or climatic disadvantages overwhelms advantages.

They are basically:

- Flammable gas filled air conditioning and refrigeration product(negligible, if any performance advantage but extreme hazards to people)

- "green devices" that inject harmonics into power line and contribute to increasing total demand distortion for power systems (kWh wasted per kWh generated increases, thus doing more harm than good)

- Appliances that are uneconomical to repair due to embedded firmware controlled power electronics (many China made stuff. Proprietary software dictated hardware that will be scrapped due to software related issues rendering it irreparable)

- Foreign made "green" products made using toxic chemicals that simply outsource pollution(you're just polluting elsewhere rather than at home)

- The environmentally harmful "green" products (it's all about the green look of the money. manufactured with harmful chemicals or contains harmful substances)

Pollution, green washing, environmental marketing, embedded systems, digitally controlled DSP photovoltaic grid-tie inverter, MPPT, toxic chemicals, RoHS, microelectronics, trichlorosilane, silicon tetrachloride, cadmium, CdTe, itai-itai disease, Reduction of Harmful Substances directive

Last edited by ICanHas; 08-20-14 at 07:31 AM..
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Old 08-20-14, 10:20 AM   #10
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Quote:
Originally Posted by ICanHas View Post
I only oppose "savings" products with.......
ICanHas,

Since this is a "Project Oriented" site, I'm really curious what projects you are actually working on, and do you have any photos of what you are doing??

-AC

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