Geothermal...

Status
Not open for further replies.
1793

1793

Senior Member
Location
Louisville, Kentucky
Occupation
Inspector
I have had a customer approach me about running the lines for a Geothermal unit.

The Geo folks tell me that they need 2-60amp DP for heat strips and 1-40amp dp for compressor.

Does anyone have any experience with this type of heating? I would like to get some help on the load calculations.

Thanks in advance,

Norb

[ January 04, 2006, 05:29 PM: Message edited by: 1793 ]
 
Re: Geothermal...

I'm not familiar with geo-stuff really but I think I could figure what the load would be if I had enough info.

What's a DP?

Isn't it (2 x 60) + 40 = 160 amps if it were 1 leg single phase.
 
Re: Geothermal...

Dang that is a lot of juice. I thought they were suppose to be the primo thing now days. What size is the unit?

Maybe I have this confussed. Is this the thing that requires a lake or pond?

Paul
 
Re: Geothermal...

Geothermal uses a pipe trenched into the ground for cooling and heating. A typical system may have a 500' loop of pipe. (A guy here at work installed his own with a backhoe. Another guy had his professionally installed.)

Since the compressor is a motor, a 40A breaker probably means the motor only needs 20A while running. (The breaker is sized for starting current, like any motor).

The 2 60 amp breakers are for supplemental heat. On the coldest days, the geothermal won't be sufficent to heat the whole house. So you need a supplemental source.

Should the heating load be sized for 1.25 X the load for continuous use? If so, I guess the 60A breaker already has the 1.25 factored in.

And the heating is often submetered. Many utilities charge a cheaper rate for electric heat.

Steve

[ January 05, 2006, 09:19 AM: Message edited by: steve66 ]
 
Re: Geothermal...

Norb -

What part of the country are you in? I understood that one of the big advantages of Geoexchange heating was that the ground temperature was high enough year around that you didn't need supplemental heat they way you do with a heat pump. Is this a fairly shallow depth horizontal loop system?

Mike
 
Re: Geothermal...

I am in Kentucky along the Ohio River. I have a call in to the Heat & Air to get some more information about the unit to be installed. When I get more of the details I will post.

Norb
 
Re: Geothermal...

Is this a self contained unit? In Illinois and Wisconsin we typically would have an outdoor heat pump feed a blower coil on an indoor electric furnace. The furnace heat would only run when the geo couldn't produce enough. But that would mean compessor power outside and furnace power inside.

For the other guys Geothermal can be Closed Loop (an array of buried pipes) or Open Loop using water from a well, lake or river.

It's still cheaper to use gas here and since POCO uses gas to generate elctricity it will probably stay that way.

I suggest use the heat pump for cooling, install a generator to heat the house, use the electricity free, sell the excess to the POCO, and take the tax write-off for co-generation.
 
Re: Geothermal...

I just happen to be wiring a high end custom home with geo-thermal heating. The backup strips for my job require a 240v, 30a circuit and a 240v, 60a circuit with the compressor requiring a 240v, 40a circuit. The circulation pumps for the underground geo-thermal lines are feed off of the compressor unit which, of course, is provided by the heating contractor; at least in my case anyway :)
 
Re: Geothermal...

You guys are going to be installing more and more of these as natural gas prices rise.
 
Re: Geothermal...

I live in Ohio and had a GEO system installed, but with gas as aux heat. The power seems right for the backup electric heat, the backup heat will only come on briefly during periods of extended cold weather (last year aux heat came on twice). The system that you posted has a two stage aux backup heat. Probably will never or rarely get to the second stage. All electric Geo systems have a very high SER rating (15 -17), which with gas prices is very affordable. Also, the GEO system will heat your hot water. There is some good info on the internet concerning GEO systems.
 
Re: Geothermal...

5Spot:
The backup strips for my job require a 240v, 30a circuit and a 240v, 60a circuit
Click to expand...
Do you recall the Kw's that the 60amp supplied? I have a call into the?Heating Company but I have not heard back yet. I would like to start getting supplies and I don't want to make a second or third trip to the supply house.

Thanks

Norb
 
Re: Geothermal...

1793, maybe I can help get you in the right direction. I have a closed loop GS. Like some of the previous posts, I agree that it should consist of two standard sized strips of heat either 10 or 12.5 or 15KW. Since 15KW is too large for it to be on a 60 amp branch circuit and 12.5KW is not that common, I would aim for the 10KW. So you have 41.67 amps on each strip. 424.3(B) requires the branch circuit at 125% of nameplate. Minimum #6 cu for each. Now the compressor at 125% and the fan motor at 100% unless the nameplate says different. Have fun!
 
Re: Geothermal...

Do you recall the Kw's that the 60amp supplied? I have a call into the?Heating Company but I have not heard back yet. I would like to start getting supplies and I don't want to make a second or third trip to the supply house.
Click to expand...
I tried for the same information when I had my heat pump installed. I wanted to do the electrical to save money and do it better. However, getting correct answers from the company was difficult. For my 20 KW aux heat, they said run two 60A circuits. But because of derating issues (4 conductors in a raceway), I really wanted to know exactly what the nameplate amps really were. I can do the math for 10 KW, but the nameplate amps for the heat strips didn't really come out to 10 KW, it was a little less, I think 38.5 amps each??

I ended up really needing a 50A circuit and a 60A circuit. One was bigger because the controls and fan are also on the same internal circuit as one of the heat strip feeds.

The other thing to watch in the instruction is the size of the equipment ground. It called for a #6, and I only ran a #8 along with the two branch circuit. Code says a #10 would have been sufficient. So I spliced a #6 in my junction box to the #8 EGC in the circuit. Kind of silly, but I believe technically compliant. There were two grounding lugs in the air handler, so perhaps two #10's would have also been allowed, but the install instructions were not at all clear.
 
Re: Geothermal...

Geothermal heat extraction is not new, I remember teaching a course on it to HVAC mechanics back in the mid 70's when energy was a big isssue. The method used was to drill a deep well with sufficient water supply. The water temerature at the deep depth was fairly constant. The water was brought to the surface, passed through a heat exchanger where the heat was extracted and used for heating. The system was similar to a heat pump, but instead of extracting heat from the ambient air, heat was extracted from the underground water.
 
Re: Geothermal...

1793, As John C says, the Geo thermal is basically driven from an energy exchanger system such as a heat pump split system. The supplemental heat elements are interum heating until the refrigerant exchange gets up to speed.
Your concern as i understand is the elements needed and the feeder to match. The heating call is the HVAC contractor's sizing for compliance to the local AHJ climate zoning specifications and house conditioned volume.

There is a possible Murphy 'gotcha' if the specs show the unit as a 2 or 3 stage and the HVAC only installs minimum code. The feeders should be sized for the capability to meet the unit overall specs.
Compressor tonnage (1 ton= 12 kBtu or 3.5 kW) is the key to matching what is needed for the living area volume in the Climate Zone requirement per the local AHJ permit. (i.e. For a 2000 sf house in North West sea level locations a 700 sf per ton or 3 ton heat pump is generally adequate.) Size the start up stage elements on the high side for higher elevation Zone 2 areas for quick heat response using something like a 15 kW (5kw + 10kw) combination. For considering lesser climate zone requirements and higher compressor exchange rate efficiency an 10 seer is generally energy code minimum compared to a higher 16-18 seer rating. A scrolled compressor only uses one motor for both fan and refrigerant exchange functions.) The interior air handler is separate and has an additional feed. A 60a, 240v 1 ph for 10kw, 30a for 5kw, 40a disconnect will handle 3 ton compressor loads easily. Again, this is based on a smaller house as compared to other size standards and in different areas.
rbj ,Seattle

[ January 17, 2006, 03:06 AM: Message edited by: gndrod ]
 
Re: Geothermal...

I think the KW of the supplemental heat will vary from installation to installation. It is going to depend on the size of the house, the climate or region of the installation, and the geothermal installation. A lot of other factors could also come into play.

Steve
 
Re: Geothermal...

Norb,
Steve's right about needing supplementary heat before the system gets going. You mentioned heatstrips, so I think a good rule of thumb is that a 5kw heatstrip per 1000 sf of house may be needed for the climate zone and house volume size compensation. 15 kw worth of strips in a 2500 sf home may work nicely for a warm climate zone area until the geo thermal comes up to speed. So essentially the exchange blower, heatpump compressor, and heatstrips will be a coincident load regardless of the compensating energy savings source being tapped.
I take it that there is not an HVAC sub on this project. There should be energy specs required by the AHJ and plans to determine the HVAC equipment tonnage. If not then the equipment being supplied for NPR and house heating volume requirement and heat loss calculation is what you need for the proper answer.
rbj, Seattle

[ January 09, 2006, 10:30 PM: Message edited by: gndrod ]
 
Status
Not open for further replies.
Top