2.5 specific Dx gshp heat exchanger questions

first, i've read all the gshp and geo thermal threads on this forum, and still have a few questions.
i've just moved a house on whidbey island, WA. i have to trench about 400' for power, and another 400' for water. as long as i'm trenching, i'd like to go deeper to set horizontal loops for a 2.5 ton gshp. i've already done the manual j calculations for the finished (remodeled and reinsulated) house, this seems appropriate. the trench lengths are about right for that sized system and my soil type. now for the tricky part. pulling this project together financialy has been a stretch. i've emptied my retirement acounts, run up the heloc, and will be maxing out my credit cards, to get this house certified for occupancy. at that point, i'll be able to get a standard mortgage, and borrow enough to pay all that off and free up the funds to remodel.
what that means, is i need to do this in 2 stages. the loop, which i'll do myself, and the gshp install, and startup, which i'll contract out.
ok, enough back story, now the questions:

if i setup a Dx loop, what size/rating of copper pipe would i use?

and

would i need a specific Dx gshp, i.e. viking, or could i use a standard air to air hp, and hook it up to my loop instead of an outdoor heat exchanger?

everywhere i've looked, i find the "gshp's are NOT a DIY project" answer. this seems silly. i'm a fairly handy guy who's not intimidated by a little math. i can sweat a pipe at least as well as the next guy. i'm just as capable of pressure testing. so the 3rd, bonus question is: is there something special that needs to happen when setting up the loop beyound the capabitilties of your average amateur?

I haven't done one but the wizards over in Oaklahoma do it all in polyethylene tube.

The GSHP is an air to water/ glycol device. Other choices are always possible but generally not economical. If you really want some good oil on it, track down and join the Ground Source Heat Pumps Association in Oaklahoma City. They can send you a stack of literature on how to go about it.

right, that's the way i was originally planing to go, water based with hdpe pipe. but the Dx is short for direct exchange, wherein you run refridgerant through copper tubes underground. theoreticaly, this makes for a simpler system. by eliminating the ground water/refidgerant heat exchanger, and the water pump. it seems like it should be a no brainer improvement. yes the copper pipe will be spendy, but i should be able to more than cover the cost difference by doing it myself. when looking into the costs of a professionally designed and installed system, vs doing my own math, sourcing my own hp, and installing my own loop, i could use silver pipe and come out even.
my understanding is that direct exchange is new, and relatively untested. i'm ok with that. if i weren't fairly comfortable with risk, i wouldn't have moved the house, i'd just buy one in place, like most folks do. in this case, my risk is that the system will not prform optimally. or that i get a fiasco that breaks down daily. i'm ok taking that risk. i'll control as many factors as i can, then roll the dice. even though this is new and untested, it's a principle that's been around forever, just a new twist.

you will be far far far better off water source if you're doing it your self as over time it is much more diy friendly as compared to Dx, and should there be a leak or damage tot he Dx loop you are totaly FUBARed.

ok. i'll bite. how would i not be fubared with a leak in a hdpe loop? it's not like i can just pour in some stop leak. i'll grant you that just getting a 2000' reel of hdpe should be easier to place, slightly. it's january, and hdpe has a memory in the cold...
i thought i'd found the place that could help me spec the copper tube/pipe necessary to pull this off. the geothermal boards i've found are interested in selling me a system for 20% of the value of my home, the diy boards are full of the blind leading the blind, and folks not interested in doing basic online research. i find that i need something else. if i don't get answers soon, i'll be forced to go hdpe, as my trenches will be closed in about 2 weeks. if anyone out there has any input as to the pipe specs for a dx system, now would be a great time.
thanks,
riggerjack

If you go with DX exchange , you facing one big problem, and that is, huge surface and amount of piping needed to achieve proper heat transfer and on the other side relatively small amount of refrigerant from compressor,
That mean low velocities in heat exchanger and big problem with oil return, and second, huge difference in heat exchangers volume (between ground loop and internal). That also mean huge amount of refrigerant in circuit. If you planing to use it also for cooling, that mean huge difference in required refrigerant in both operating modes. That is pretty challenging to solve and, in same time, ensure problem free operation.

It is much easier to use solar photovoltaic collectors and accumulators for water ground loop pump power assistance than make such system reliable.

If you still wish to do DX heat pump, than you should first select HP manufacturer and model and then consult their instalation documentation.

This is one instalation manual. Here you could find some more information's, but be aware that this is manufacturer of equipment point of view.

Quote:

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Originally Posted by Riggerjack

if i don't get answers soon, i'll be forced to go hdpe, as my trenches will be closed in about 2 weeks. if anyone out there has any input as to the pipe specs for a dx system, now would be a great time.
thanks,
riggerjack

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You should think about that before starting digging a trenches. You Americans are known as good planers, but it appears that you started from wrong end of story.

According to my conversion table 2.5 ton equals 8.8kW.

Is this evaporator capasity or condenser capasity? In either case this is a farly large heat pump.

I'm assuming 8.8kW condenser capasity. Assuming cop at 3.5 this gives an evaporator capasity of 6.3kW.

Now, to determine a suitable ground loop dimension we ned to know the refrigerant, heat transfer coefficient, evaporating temperature and liquid temperature as a bare minimum.

I'm assuming 30W/m, -8C evaporating, 35C liquid temp and R507 refrigerant.

This set of assumptions requires slightly above 200 m, 600' of pipe.

A 7/8" pipe, 200m long gives a pressure drop of a totally unaccetable 1.2 bar when there is only gas flowing in the pipe. Gas speed is 8.2m/s. However the speed in the pipe will be lower in the section of pipe where there is a mix of gas and liqiud. On the other hand will a mix of gas and liquid have a higher pressure loss at same speed compared to only gas. I have no possibility to calculate the true pressure loss in this application. My best guess would be the true pressure loss to be lower than the corresponding pressure loss when there is only gas flowing.

Alternatively 1-1/8" pipe. Gas speed 4.6m/s and pressure loss of 0.3 bar. However 200m of 1-1/8" copper pipe isn't cheap.

One other thing to be VERY aware of is to NOT lay down this loop in the same trench as the water supply. I know one DIY guy that dit that and the first winter the water supply froze.

30W/m pipe gives 60w/m trench. This is a farly high value that easily is able to freeze the trench completely if this is a place where it is some sort of normal winter temps with snow and sub zero temps.

Edit:
One alternative is to dig another trench and have two paralell loops. Then you could maybe get by with two 5/8" or two 3/4" loops, say 120m pipe each for the same capasity. This will give lower pressure loss and maybe cheaper pipes.

well how easy is it to you to add water/propolyne glycol as compared to ***** ;) for starters, you can all ways bypass the ground loop to a new one on the fly with water source, so many more options my friend.

I'm no expert on this subject but my thoughts against using an underground DX system are based on experience of air/air heat pumps.

If the underground DX pipework runs at anywhere near freezing conditions then it is to be assumed that the surrounding ground will be brought down to freezing as well.

As the pipework is in contact with the ground materials, whether trench filled sand or similar then there is no room for expansion of the frozen ground other than into the structure of the pipework.

Any defrosting will need to be for extended periods to ensure that the whole area around the pipework is completely free of ice. If not then the melt water will refreeze and increase the pressure on the pipe.

So there is a potential for the underground pipework to be crushed, increasing resistance, until it is completely blocked. Or, stones etc having worked their way into the pipe trench puncture the pipe due to the ice pressure.

I may be wrong but these possible scenarios would make me head for a water based system which runs at above freezing point.

thank you. this was what i was looking for. substansive reasons NOT to go dx. my original plan was water glycol, and i'd never heard of dx until a few weeks ago. it seemed like a simpler way to go. eliminating water pump and heat exchanger seemed like a great way to go. your reasons are persuasive, and i can now go back to my original plan, unhaunted by what-if's. i'll let you know how vthis turns out in the end.
again, thank you for your input.
riggerjack

Another factor too is ground chemistry, copper corrods easier then most think where as plastic in a protected ground environment lastes good as forever!

Quote:

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Originally Posted by Brian_UK

I'm no expert on this subject but my thoughts against using an underground DX system are based on experience of air/air heat pumps.

If the underground DX pipework runs at anywhere near freezing conditions then it is to be assumed that the surrounding ground will be brought down to freezing as well.

As the pipework is in contact with the ground materials, whether trench filled sand or similar then there is no room for expansion of the frozen ground other than into the structure of the pipework.

Any defrosting will need to be for extended periods to ensure that the whole area around the pipework is completely free of ice. If not then the melt water will refreeze and increase the pressure on the pipe.

So there is a potential for the underground pipework to be crushed, increasing resistance, until it is completely blocked. Or, stones etc having worked their way into the pipe trench puncture the pipe due to the ice pressure.

I may be wrong but these possible scenarios would make me head for a water based system which runs at above freezing point.

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Pretty right but in fact there is room for expansion. What can happen is the ice pushes the earth away during buildup so when a defrost occurs a gap opens up and contact between pipe and earth is lost. Once this happens there is a massive drop off in performance and the customer is left wondering why it used to work but doesn't now..

Thanks for that Multi, I wasn't quite sure what would happen underground.

I know that I have have had heat pump evaporator coils crushed by ice formation and could see a similar event happening.

Plumbers sometimes lay copper in the ground for potable water transport (the ground never freezes here...thank God otherwise I'd move somewhere where it doesn't) but it is usually sheathed in a plastic sleeve which protects it from direct contact with the ground and reduces corrosion issues, but the sleeve effectively cancels out any thermal benefit in using the copper.