Interesting applicatin of Boiler to Geo system.

SpeyFitter

So the company I work for does a fair number of geothermal water to water installations every year. In just about every installation, we typically use an IBC condensing boiler for back up & supplemental heat for the radiant, and to heat the indirect hot water tank since geo systems don't push the water temps neccessary to do that (other then using the desuperheater system to garner some added heat from the geo system for the domestic hot water).

But we have a few commercial projects we've done, and one of them I went to yesterday to commission an IBC, was done by an Engineer. So typical as in all engineered systems there are all kinds of extra safety switches, zone valves, components, and the like. Many things that add cost and complexity to the systems which in some cases aren't neccessary. But right away when I started figuring out the system since I had never been to the site, there was something I haven't seen before.

The engineer had the boiler supplementing the ground loop of the geo pumps, and in a "parallel" configuratin i.e. no primary secondary, no hydraulic seperatin, just one big pump injecting off of the boiler into the geo loop.

Now every suite (it's a condo buildilng with some commercial suites below the residential suites) has it's own ground to air source heat pump of which it pulls it's own geo water that is circulating in the building (for heating and cooling). And the Geo pumps in the Mechanical room pull geo water to preheat 2 large domestic hot water tanks. The Domestic hot water tank is then moved through an A.O. Smith Cyclone hot water tank to bump it up to 140 as "insurance" that there will be enough hot water if you get the lingo.

The thing I can't figure out is what application this boiler has. It injects heat (pulls a little water out, puts a little warmer water back in) into the geo source main just upstream of where the heat pumps oull their water out, obviously so it gets this "warmth" to incresae it's efficiency and make the compressors life that much easier.

But there are so many variables that I wonder how this system will work. For example, you can get a rough result based on conductivity tests of the soil, as well as previous expierience and careful detailed engineering, as to what the temperature profile of the soil might put out as the year progresses from fall to spring, but I think you actually have to regularly monitor it the first year to see this without boiler interaction. And then you have to apply the boiler in such a way to ensure that it is just lightly bumping up the temperature and not heating the earth!

Anyways, I also had to phone the Boiler manufacturer and inform them of this application and the potential for it running at subfreezing temps at some point (it's got glycol in it). IBC said it would be fine, and it would obviously condense like mad. When I fired it up yesterday it WAS condensing like mad - the condensate trap was giving a nice pee stream AND the exhaust was virtually dry (no steam!)! It had about 50 degree return temps, I had it putting out about 65 just to get the boiler up and running.

I am going to have to make contact with the engineer to see what he has in mind.

shoudabeenaplumba

I saw something like this

On a new building for NH Art Institute in Manchester NH.  Every room/zone had a heat pump, and the building water was constanly circulating at 70 degrees.  Three boilers for hot water and to bump the ground temp when needed.  

I think it pulled from the ground loop to the boiler, then dropped back into the loop at the same point, just as you were describing but there was a large hydroseperator near the wall after the ground loop but before the boiler. 

Mark Eatherton

I too saw this just this week...

Doing a consult for a friend who is looking at doing the mechanicals on a 25 K sq ft home with GSHP. I questioned the piping methodology, and there was no control logic to determine when to shut down the loop field, consequently, it WOULD be entirely possible to pump natural gas BTU's down hole. Out of site, out of mind???



I have no issues with drawing a mod con that low in operating temperature, but I do question the hydraulics and logic to make it work...



This reminds me of the "good ol' solar days" when every former vacuum cleaner sales man turned into a solar expert over night by reading a couple of magazine articles and books. This industry (gshp) is headed for the same disasters that came out of the solar industry due to a lack of standards. The building inspectors haven't the faintest idea what they are looking at, and the consumer just assumes the contractor knows what they are doing,



The engineer who had drawn this system up for pricing didn't even understand the concept of P/S piping. He had the loop pump piped in series to the parallel pumps serving the 8 remote GSHP's throughout the house.



Future train wreck with a 30% tax credit on it....



ME

shoudabeenaplumba

Not an HVAC guy, so please don't get offended if/when I ask stupid questions.

Was just an extension of the primary loop with no heat exchanger? I don't understand how a big system like that is supposed to work. Does the loop go out through the building and come back? How does the 1st heat pump fare compared to the 8th on the loop?



Maybe the trick is the boiler on setback, so when the primary loop needs to be 90 instead of 70 the burner tunes up, but with no monitoring of the return loop temp I can't see how that could work. Or maybe the heat pumps pull extra out to insure return temp is always going to be the same, therefore, no extra heat going out. Are they multi stage HP? I'm sure there's a curve figured for when the boiler should fire to use more gas than electric, maybe even an algorithm to account for the differing price of the two fuels.

Rich_L

Wasting BTU's

I don't see any way you can help but waste energy and therefore energy dollars heating the ground doing an application like this.

The nature of the GSHP beast pulls the natural ground temp down throughout the heating season. It's customary in my area (Iowa) to see ground loops go sub freezing. In my opinion that's part of the proof these systems are doing what they are designed to do, move heat from the ground outside to the inside of the structure. Ground source heat pumps, not to be confused with heat pumps designed for service water loops, are designed to operate at these temps. Efficiencies unquestionably will drop off as the ground loop temps drop too. By adding heat to the ground loop I don't see anyway you can help but put at least a little of that added heat back into the ground. Best case scenario you may be able to slow the grounds heat loss. How desirable is that?

One method I've seen is to add a mod-con boiler to boost the radiant loop on a water to water GSHP during extremely cold temps. This also takes some careful planning to avoid taking over the load that you want the heat pump to primarily cover.



Regardless I still like those GSHP's!



Rich L

Henry_6

Geothermal Supplemental Heat

I agree with Rich. The heating supplement should only be added to the distribution side of a W to W geothermal system for proper economic benefit. Have we forgotten that heat travels to cold always? The Europeans design systems that routinely operate in a latent heat evaporation state (< 32f for transfer fluid temperature) which then yields more than one BTU for each pound of fluid. This makes the radiant floor application very efficient because the evaporating pressure is reduced, but amplified because it is below freezing. The compressor is then operating at a lower condensing pressure, because the radiant floor never should exceed 85f. If you control the geothermal system with an outdoor reset control, it is possible to have Coefficient of Performance (COP) which is in the 4.0-5.0 range. If you raise the temperature of the evaporator by heating the input heat, this goes away. In Scandinavia, many applications are designed to maintain permafrost so that buildings will not sink out of sight in the summer. After all, this is the land of the Midnight Sun. An easy way to relate to COP is that when you feed the W to W geo pump a Dollar, a COP of 4 equals $4.00 of thermal energy delivered, and 5 equals $5.00. The design of systems in Germany and Scandinavia is primarily for heating and DHW, but cooling is possible and needed in commercial operations. I just wish that we Americans would not engineer everything for low bid!

Henry_6

Geothermal Supplemental Heat

I agree with Rich. The heating supplement should only be added to the distribution side of a W to W geothermal system for proper economic benefit. Have we forgotten that heat travels to cold always? The Europeans design systems that routinely operate in a latent heat evaporation state (

AHerra

Engineers Need to Stay Away

 We have been doing Geothermal for 4 yrs now and have taken our lumps by referring to manufacturer's recommendations on installs. I am a firm believer the GEO WORLD (not including installers) has no idea of Water to Water installs. Radiant BTU sizing, water flows.  If the initial loop is designed well and is large enough there is absolutely no reason for boiler back up heat on storage tanks unless you are using hot water coils on air handlers. I am in the Chicago area and have earth loops just checked last week still in the 40's.  The sole purpose of a GSHP is to save money. So lets not let some engineer sitting in an office that has absolutely no install experience tell us to add heat to a loop. Lets take it upon ourselves to size the loop correctly and give the job needed not the job based on what the customer wants to spend.

AL

eluv8

another idea

This topic brings me back to a thought I had of using the loopfield as a thermal dump for a solar dhw system. Winter only of course, so Summer when you really need it you cant use it. If only we could use the energy in the summer as well.

Gordan

Latent heat evaporation state

Henry,



Could you please explain what you mean by this latent heat evaporation state? Is the latent heat available without actually freezing the transfer fluid? Where does it come from?



The typical mindset seems to be that low loop temperatures are acceptable rather than actually desirable, and yield lower CoP, not higher. Is that all wrong?