First, I really appreciate all the wisdom here.  I've learned a lot reading through all the old threads.

I'm currently having a geo system installed (3 ClimateMaster Tranquility 27 systems: 2 ton packaged, 3 ton packaged, and 4 ton split, standing column open loop).  Because of the way the house is designed, it works out best to have three zones, each with its own system.

Anyway, my current questions have to do with the desuperheater(s).  The 2 ton, the 4 ton, the buffer tank (80) and the water heater (80) will all be at one end of the basement, while the 3 ton will be about 70 feet away at the other end.  The installer says we should hook up the third DSH, even though it's so far away.  The manuals say "DO NOT" go over 50 ft.  But if there's heat available, I'd like to use it.

Questions:
1. Why the 50 foot limit?  Efficiency?  Limit of the circulator pump?
2. If PEX is used, does that affect how hard the pump has to work?
3. Is it reasonable to add a small extra pump on the line to assist with circulation?  Would that kill any cost benefit of DSH?
4. Is there any downside to trying to use it even at 70 ft?  Warranty implications?
5. Is the DSH stealing heat from the rest of the house, or is it capturing heat that would otherwise be lost?

Thanks for any advice.

MC in MA

1. Efficiency and limitations of pump 2. Not if it is sized right 3. Back to efficiency 4. If there were DSH problem they might question the install. 5. In winter it is borrowing in summer it's free.... how much you get in summer depends on EWT of well. That said the 70ft run looks less attractive. EWT below say 65F in summer nets little DSH.

5:

In cooling mode, it is my understanding that a DSH can capture some of the "free" heat removed from the house that would otherwise go into the loop.

In heating mode, the DSH uses about 10% (the figure commonly used here) of the heat that could be otherwise be used to heat the house. The benefit is that the heat is produced at the efficiency of the geothermal unit. Depending on the system, the DSH may automatically turn off when higher stages engage so that all of the available heat can be used to heat the house when conditions demand this.

Other issues aside for the moment, I wonder if any additional heat can be gained from a DSH so far away from the buffer tank. There will be some heat loss from the buffer tank to the DSH, and then back again from the DSH to the buffer tank. I suppose if the total loss is less than the DSH contribution it may be ok (need to factor in the cost to run the DSH pump as well.) Anyone have heat loss data over long runs?

You can size the pipe and insulate it to compensate. But it may not be worth it. Or maybe a small storage tank under a sink at that end of the house would be useful anyway - quick hot water is nice to have.

Our climate is heating-dominated. I think the cooling load is about 2/3 the heating load.  With the open loop and the extra capacity, I don't expect the EWT to go up too much in the summer.  I expect we'll be on stage 1 dehumidification mode most of the time, when we need the A/C at all.  So, I don't expect that much hot water help in the summer or swing seasons, but it would be nice to get some.  The specs say I should get 1-2 thousand btuh at 50-60 EWT in cooling mode, even on stage 1 from that system.  But I suppose the spec is for running the whole time.

What's the math here?  How many BTUs does it take to heat 80 gallons of water from 50->125.  Or maybe I should say how many BTUs does it take to increase the temperature of 80 gal of water 1 degree.  How much will I lose over 70 insulated feet?

Obviously, I can monitor the temperatures and see if we're getting any advantage in practice, and shut it off if necessary.  Maybe it will only work well enough to make a difference in heating mode.

MC in MA

Posted By jonr on 16 Sep 2010 05:35 PM
You can size the pipe and insulate it to compensate. But it may not be worth it. Or maybe a small storage tank under a sink at that end of the house would be useful anyway - quick hot water is nice to have.

Mild months may present a problem.  Is it too late to order that particular unit without a DSH and save the money?

I think I'd skip the DSH on the unit 70 feet away.

Heat transfer math is relatively simple: 1 gallon of water is 8.3 pounds; heating a pound of water 1 deg F requires 1 btu. 80 gallons heated from 50 to 125 requires 50,000 btus.

Figure 5-10% of unit capacity as available for DSH. In a northern climate owing to lower EWTs those figures might fall to 4-8% (4 in summer, 8 in winter), but that is pure conjecture.

perhaps you might find it useful to know that heating op cost goes up slightly where DSH is employed. might be better to skip DSH where benefit is marginal.
Joe

Brainstorming here - if you have the DSH anyway, and only if you get approval from the manufacturer (I doubt they would give it), I wonder if you could somehow use the DSH as a hot water circulation pump to keep hot water at the far end of the house when the DSH is running.

Hi MC, I'm also located in MA and have a CM tranquility 27 6 ton unit with SCW design. My DSH is also ~70 feet from my 80G electric water tank. So far it has worked well in the winter months. As mentioned earlier with the low ETW temp and shorter cycle times the DSH is useless in the summer.
The installer used PEX for the DSH. Although I haven’t measured it. Looks like ½”. I’ve been thinking of re-running the PEX line to reduce the length by 10’-15’. I think the installer wanted to create a neat appearance of the DSH water lines but this added to the length significantly.
I think it’s worth giving the DSH a try.
John

http://welserver.com/WEL0167/

A couple of questions. How much hot water do you use? If not much then can you justify the expense? How much will that 70' unit be running to even give you heated water? Are the tanks positioned to limit run lengths? I'd like to move mine but their location means short runs = higher efficiencies.

I say run the pipes now. Just include enough shutoffs so you can disable it.