Please review my system sketch!

[varadhammo]

Hi all,

I've put together a conceptual sketch of what I'm planning on doing for my GARN system. I have two buildings which have radiant floor zones and DHW. I'm trying to minimize my heat loss to the ground (one building is 100' each way to the GARN, questionable insulation quality...) by only sending full tank temperature through the trench when DHW calls, otherwise mix to low temp at the GARN. This is just a rough sketch, I haven't filled in the details (valves, unions, etc) or the electrical side of things (except the basic operating modes). Am I on the right track? It's a pretty simple system, but I'm new at this, so I want to make sure I'm not missing anything basic! Any input/suggestions/criticism is most welcome.

Thanks very much,
Jake

 

[stee6043]

My first thought is that you're adding too much complexity to the system by trying to phase the supply from the shed in two "modes". I suspect your pex heat loss is going to be very close to the same regardless of whether you're pushing the DHW supply through or the in-floor supply. If you can estimate ground temperature in the winter I bet you can find a quick online estimator to illustrate your "savings". btu's/hr saved will be low (my bet only) and not worth the effort.

If you're that worried about the pex I'd suggest you tear it up and replace it for a long term proper solution.

 

[arbutus]

I have an open boiler in a shed with a system that needs low temp water. I pump the boiler water to the A side of a hx in the house. The B side of the hx is part of the primary loop. Three secondary loops: backup lp boiler, panel radiators with TRVs, and radiant floor. The radiant floor is mixed to proper temp with a Taco I series 3 way valve with outdoor reset.

Having walked on a nice green grass path in the middle of February here in the UP, I would highly recommend installing quality insulated underground lines. Yes, your delta T underground will be lower when running radiant only, but the ground is at best 50F, always. Lots of lost heat, for the life of the Garn, if you don't fix it.

 

[ewdudley]

If you can estimate ground temperature in the winter I bet you can find a quick online estimator to illustrate your "savings". btu's/hr saved will be low (my bet only) and not worth the effort.
If you're that worried about the pex I'd suggest you tear it up and replace it for a long term proper solution.

Having walked on a nice green grass path in the middle of February here in the UP, I would highly recommend installing quality insulated underground lines. Yes, your delta T underg

Heat loss would be proportional to deltaT, so if (for example) mixed supply temperature is 120 degF, unmixed supply temperature is 180 degF, and ground temperature is 50 degF, then if you're running mixed supply most all of the time then heat loss would be cut almost in half.

But half of a lot is a lot. Like stee an arb are saying, you should seriously consider approaching if from the other direction by improving the quality of your underground lines.

If you sharpen your pencil and do your homework, you may find that with radiant floor loads you will be able to take full advantage of high system deltaT and therefore run much smaller underground lines. A 70 degF deltaT or more should be easily achievable, which would mean you can deliver 100,000 btu per hour with less than 3 gpm, which is doable with a round-trip of 200 ft of 3/4 inch PEX.

 

[varadhammo]

The underground PEX we have is the eze flex 5-layer stuff from Z-supply in Michigan. I don't know yet how it will perform, but I do have my doubts. About 2/3 of the trench still isn't backfilled (the other third was installed before the foundation was backfilled so it's probably 5-6 feet underground). I'm planning on having that 2/3 of the trench spray-foamed (on the outside of the 6" white corrugated stuff) before filling it in.

My first thought is that you're adding too much complexity to the system by trying to phase the supply from the shed in two "modes".

As for complexity, it doesn't really seem like much cost wise, maybe labor, but it's really just adding two zone valves and a bit of relay logic... seems worth doing to minimize loss, since the system is really only running in DHW mode a small percentage of the time. Or am I missing something?

 

[stee6043]

Heat loss would be proportional to deltaT, so if (for example) mixed supply temperature is 120 degF, unmixed supply temperature is 180 degF, and ground temperature is 50 degF, then if you're running mixed supply most all of the time then heat loss would be cut almost in half.

But half of a lot is a lot. Like stee an arb are saying, you should seriously consider approaching if from the other direction by improving the quality of your underground lines.

If you sharpen your pencil and do your homework, you may find that with radiant floor loads you will be able to take full advantage of high system deltaT and therefore run much smaller underground lines. A 70 degF deltaT or more should be easily achievable, which would mean you can deliver 100,000 btu per hour with less than 3 gpm, which is doable with a round-trip of 200 ft of 3/4 inch PEX.

While the loss is directly related to the delta between the two it won't be anywhere near half.

I'm simply suggesting that these controls aren't going to be worth it to save a few hundred or even thousand btu/hr. Again, my opinion only. I'm from the KISS camp usually.

 

[BoilerMan]

I like your idea of spray foaming as much of the underground as practicable.

I'd consider moving the circulators into the boiler shed. Unpressurized systems need the circulator close to the boiler pushing water toward the restriction (zones). I like the idea of the dual temp to minimize the heat-loss, for the extra pipe and two ZV's it should pay for itself in wood/labor savings quickly.

You could also consider using a FPHX and making the system side pressurized. It would add another circulator, but save on chemicals for treating open systems like a GARN.

I think your on the right track, the use of delta-T circ on the radiant is a good choice and should minimise flow, this comes at a cost of having the water in the underground longer, possibly giving up more heat on the return.

TS

 

[ewdudley]

While the loss is directly related to the delta between the two it won't be anywhere near half.

A deltaT of 70 degF versus a deltaT of 130 works out to 54%. So I guess it comes down to quotidian semantics: if 54% is not "anywhere near half" then what would you accept as "almost half"?

 

[stee6043]

A deltaT of 70 degF versus a deltaT of 130 works out to 54%. So I guess it comes down to quotidian semantics: if 54% is not "anywhere near half" then what would you accept as "almost half"?

You used what for R value of the pex package? And you accounted for what kind of heat transfer (water, clay, loose sand) around the insulated pipe? Is there also a layer of air in some of the pex pipe rigs you can buy?

I wasn't trying to start a debate. But the pipe has some insulating properties and we know nothing about his soil. But I will concede. You can have the 54%. I'd love to see the OP actually measure it someday and report back.

 

[varadhammo]

I'd consider moving the circulators into the boiler shed. Unpressurized systems need the circulator close to the boiler pushing water toward the restriction (zones).

Hmm, I've thought about this. In my setup, the GARN is the highest point in the system. For one of the buildings to be fed, the highest floor is about -16 ft. from the GARN storage tank water level, the other building, about -6 ft. My understanding was that I'd achieve the greatest net positive suction head by putting the circulators down in the buildings. Is there some other factor you're referring to besides NPSH when you say that they'd work best close to the boiler?

stee, I hope I get to the point where I can measure my temp drop for you someday soon For the record, the PEX rig does have airspace and a foil radiant barrier on the wrap insulation (5 layers). The corrugated is thick, higher quality stuff than your run of the mill drain pipe. So at least water infiltration shouldn't be an issue.