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Old 09-09-17, 09:24 AM   #1
jeff5may
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Default Repurpose an old central 2 ton

I recently purchased a 2 ton outdoor unit for 50 bucks. I'm going to be doing some experimenting with it, to figure out what kind of gains can be had with the stock setup, before doing anything strange to the thing. Looking for ideas from you all.

The original owner had it running in his home and decided to clean the indoor A-coil, which was filthy dirty and full of years worth of lime deposits. Doing so, he etched a pinhole and let out the magic gas. He called the local HVAC firm and was sold into a modern replacement. He scrapped the indoor coil and sold me the outdoor unit.

The unit is a 10 seer Goodman air source heat pump. Cap tube with distributor, Bristol compressor. I believe the heat pump is rated around 7 or 8 hspf.


Last edited by jeff5may; 09-10-17 at 08:09 PM..
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Old 09-09-17, 11:17 AM   #2
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Since it's a heat pump, just add a TXV and coax coil to get a self contained geothermal unit.
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Old 09-09-17, 05:21 PM   #3
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Quote:
Originally Posted by jeff5may View Post
I recently purchased a 2 ton outdoor unit for 50 bucks.
2-Tons is a pretty good size unit.

The really big pay-off comes from putting the coils in the ground or in water. Otherwise, over-sizing the HXs and using R290 would give you marginal improvement in efficiency.

With regards to your house, do you have any idea what your heat loss/gain is in max heating or max cooling?

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Old 09-09-17, 11:32 PM   #4
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Ok, what I am going to do first is run the outdoor unit like a big heat pump water heater to measure its baseline performance. I have a water heater gleaned from the rental trailer that is still water tight. It was getting clogged with lime, and tenant tried to flush it. Doing so, the drain spout got gunked up and clogged. My dad figured it was old enough to change, so we stuck a new one in the trailer.


30 gallon

I blew some compressed air into the drain spout, then muriatic acid, to get the gunk out. I haven't looked inside the tank yet, but both heating elements still work. Usually when the tank gets thick deposits inside, the calcium builds up around the bottom element and it burns out.

I have a question about this plan: what size and length of copper tubing is enough to make the thing operate at high efficiency? I am planning on taking the bottom element out and running a pipe and Union into the heating element bung like Memphis did with his. That way, I can log temperature and flow of water and refrigerant to tell how much heat is being produced in various configurations, as well as overall cycle times and such.

I already have a couple of 2 ton TXV's I can use with this rig, as well as an A-coil from a 2.5 ton unit, among other components, laying around. After a decent operating profile is charted from the stock outdoor unit, I will change the cap tube metering device with a TXV and chart the new operation profile.

For practical purposes, let's say I can heat the greenhouse or barn with this unit. I won't be installing the rig where I live, because I rent. The town house I'm renting has its own (undersized) central heat pump, and I have a mini window shaker heat pump (perpetually set at 70 degF) hung in an upstairs bedroom already. All of the stuff at home works well down to around -10 degF, which never happens here for long, if ever. If I were to venture a guess, I could probably count the number of days that bottomed out that low on one hand in the last 2 decades.

Another idea: my parents have a pond about 15 yards away from their house. The same pond is about 20 yards from the barn. I could possibly rig something to use the pond as a heat dump or source in place of the stock outdoor HX to do some testing with. The only problem with that idea: the pond is part of the front yard, surrounded with lawn. Digging up the front lawn would definitely be frowned upon.

I'll be going over there soon, and will edit some pics and commentary into this post.
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Old 09-10-17, 09:06 AM   #5
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Jeff5may

I'm suprised you have asked the question; "I have a question about this plan: what size and length of copper tubing is enough to make the thing operate at high efficiency?"

You had posted Oct 10 2013

Acquarioe,

"I believe 10 meters is a magic number. The airtap hpwh unit uses this length of tubing for its hwt snake condenser. Many, many hobbyists have used this length for their water side exchangers. As long as the tubing is large enough for the refrigerant to flow, I believe one could heat or cool a skyscraper with a 10 meter exchanger!"

I'm for certain you're looking for some corroboration.

It's true for a small pump the 30' of copper 5/16 dia.works well (5000 BTU) as this is what I had found. However for 24,000 BTU this would be different.

Case in Point:

In my solar hot water tank I have 50' of 7/8 dia. copper tubing. It works but not the best.

If you consider the heated copper tube immersed in water there would be a film of heated water surrounding the tube, slowly rising via convection presenting cooler water to the surface of the heated tube. I you have an abundance of heat inside the tube the convection action may not be enough. This is what I have in my case.

It would be best for this amount of heat be mechanically moved. I would humbly suggest for a system capable of a 2 ton output be part of a heat-exchanger with a circulation pump. The movement of water over the heated surfaces via a pump would greatly enhance the efficiency instead of relying on convection.

This is what I had found working with the HP hot water tank and solar hot water tank as well as the 5T GeoThermal heat pump.

Randen

Last edited by randen; 09-10-17 at 09:12 AM.. Reason: corrected date & punctuation
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Old 09-10-17, 10:08 AM   #6
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That's exactly why I asked the question. For a 5000 BTU batch tank, the heat transfer and mass flow rates are small enough that a single "Slinky" tube condenser doesn't have to be huge to be effective. Below about a ton of refrigeration, the "mini" systems hit the range where the system is more effective than it costs pretty quickly. Adding more efficiency past a certain point becomes exponentially more expensive. Not so true with the 2 tons of capacity. I'm not trying to cram 50 meters of copper tubing through a little bung and spiral it all inside a residential water heater tank.

The 2 ton system I will be tinkering with falls above the "mini" size class. It's a super common capacity for apartment complex and condo communities as well as smaller single family homes. In the commercial sector, this size condensing unit powers beer caves and glass door beverage coolers around the world. To most small business owner and upper middle class individuals, as well as most home owners, the outdoor units are priced low enough to justify replacement without a lot of consideration. As such, working units can be found second hand very easily and are highly affordable.

Call me a scavenger, tell me about how you HAVE TO replace your primary vehicle every 7 years and your HVAC system every 10. Tell me about the risk of having to do maintenance or being stuck on the side of the road, or burning up in the summer, or freezing in winter... I'll just nod my head and let you justify your monthly payments for all that stuff. Me, I pay rent and live my life like a nomad. Rather than go on vacation for a minute somewhere, I pack up and go: living there is so much more satisfying. This is America: even when there are no jobs, there are ways to earn money and live well. Even when the housing market is heavenly and tied up, there are still affordable places to live. Believe it or not, I have been there and done that. My retirement will be spent mentoring and consulting, paying it forward to the kids, watching them walk that beam way up in the air where I used to. Watching TV and playing bingo just doesn't do it for me. But I digress...

I am not dead set against using a BPHE to feed the water heater. For my purposes, it would for sure be more directly measurable, compact, and replaceable than a tube slinky. But for a DIY project, it would need a lot more brazing and pipe fitting than the slinky rig. A continuous run of copper tubing with a single u-bend at the end has only one point of failure and a super low chance of ever fouling, whereas the BPHE has all kinds of fittings that can leak, plus the relative certainty of needing cleaning in the future.

To refine my question, can someone relate to me the equivalent relative effectiveness of the two types of exchange methods? For an entirely wrong example, an answer would be something like "2 runs of 1/4 inch tubing 10 meters long would have about the same exchange rate as a 10 plate bphe 5x12 inch flowing 5 gallons per minute at blah blah dT". I don't need a dissertation, just some ballpark figures to relate the cost vs. effectiveness factors involved. Needless to say, an immersion element would rely completely on convection and direct radiation. A bphe would rely on a small circulation pump, which for this discussion is another element of complexity (and point of possible failure/maintenance/expense/control) to consider.

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Old 09-11-17, 07:15 AM   #7
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I will be watching this one..
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Old 09-14-17, 12:05 PM   #8
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OK, so I've been doing some homework on brazed plate heat exchangers. I've found the entire sizing process is dominated by two types:

A: Gurus who will give you 3 answers to the same question
B: Spec sheet readers who say only one specific model in the universe will work

Being somewhere in between these two types, I have come to these conclusions:

-Smaller surface area per plate units cost a lot less than larger ones
-Extra plates don't cost add much cost if you want them
-Similar units are not rated the same between manufacturers or market sectors (on the surface) AT ALL
-Cross-referencing and comparing similar units must be done using engineering data, charts, graphs, math, and combinations of these elements

Considering these factors and parsing through way too many articles and sheets, I've found that for my situation, there is a fairly large range of products that will fit my 2 ton unit. Total indoor-side efficiency and destination temperature are the two main factors that play against each other. At higher destination temperature, a larger heat exchanger is needed.

On the small and somewhat less efficient side, for condensing duty only, a 3x12x24 plate unit will do the job. This is SWEP B8 size. In this width, running the BPHE in evaporator or condenser duty, a longer profile is needed for good heat transfer. From the guru angle, less longer plates are better than stacking more shorter plates. So in the 3 inch width, an 18 inch long 20 plate unit (SWEP B15 profile) is needed.

On the larger and more efficient side, a 5x12x16 plate unit will do the job as a condenser. For better heat transfer at not much expense, a 20 plate is better. For dual duty, a 5x12x30 plate unit (GEA FP5x12-30) is good for lower temp water, while a 40 plate unit is better for higher temp water. The 30 plate GEA model crosses to a SWEP B10-50 unit or an Alfa Laval CB25-34 or CB26-34 unit. The 40 plate GEA model crosses to a SWEP b10-50 unit or an Alfa Laval CB25/26-44 model. From these few models, pretty much all of the knock-off units can be compared.

The price range of most of these BPHE units starts out around $150 shipped to my door. Some of the larger units are right below $200. Prices go up from there if you are looking for name brand stuff. Taking a chance on used or old stock units is closer to $120 and up.

Does this sound right? I figured these things would be more expensive...
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Old 09-14-17, 06:53 PM   #9
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Another thing that helps reduce water heater build up is a magnesium sacrificial anode.
Just on the off you don't enjoy messing with the plumbing at the rental.
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Old 09-15-17, 03:40 PM   #10
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Oil Pan,

The rental trailer is full of gray plastic pipe. It was a nightmare at first. The stuff that has failed has been replaced with pex. Crossing my fingers this year...

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