Standby generator sizing with geothermal

F

fireryan

Senior Member
Location
Minnesota
Im working on sizing a standby generator for a home with geothermal. The compressor unit or heat pump says is draws 44.8 amps at 240v. So that is 10,752 watts. The auxiliary heat in the unit is 9.6kw so total with them two alone is 20,352 watts. My question is when auxiliary heat kicks on in these geothermal units does the heat pump still run while the auxiliary heat is on or are they separate from one another? This will obviously affect my generator sizing significantly, never really dealt with geo thermal units so not sure how to size this
 
fireryan said:
Im working on sizing a standby generator for a home with geothermal. The compressor unit or heat pump says is draws 44.8 amps at 240v. So that is 10,752 watts. The auxiliary heat in the unit is 9.6kw so total with them two alone is 20,352 watts. My question is when auxiliary heat kicks on in these geothermal units does the heat pump still run while the auxiliary heat is on or are they separate from one another? This will obviously affect my generator sizing significantly, never really dealt with geo thermal units so not sure how to size this
Click to expand...
That comes down to the specific unit but in general, no, and you can certainly lock that down on most models. I will say on geothermal, it ain't designed right if tapping into aux heat, but I do understand if say a compressor fails, etc. This is why I would work the settings to just lock one or the other out.

But no, you are very wrong if you think 44.8A*240V is 10,752W on a compressor. Maybe dust off your education books.
 
fastline said:
it ain't designed right if tapping into aux heat
Click to expand...
100%

Heat pumps have really improved in the last decade or so, and no matter how cold it gets, if you are down deep enough there is plenty of heat to tap into. Up here in the anthracite region all the old underground miners talked about how steady the temp was. There was a tunnel across the street from me with a short shaft leading down into it. They could walk all the way to Stockton to go to work in the tunnel. That's like 3 or 4 miles. It went under Broad St and some of the store basements had a door or a gate to access the tunnel, or for miners to buy whatever. They told me it was comfortable all winter
 
fastline said:
That comes down to the specific unit but in general, no, and you can certainly lock that down on most models. I will say on geothermal, it ain't designed right if tapping into aux heat, but I do understand if say a compressor fails, etc. This is why I would work the settings to just lock one or the other out.

But no, you are very wrong if you think 44.8A*240V is 10,752W on a compressor. Maybe dust off your education books.
Click to expand...
How is the wattage wrong? P=IxV
 
fireryan said:
How is the wattage wrong? P=IxV
Click to expand...
VA= IxV
Watts=IxVxPf

So your watts will be a bit lower than your VA. But I would use Watts for any calculations for a generator. Really though, that's all kind of a minor point. You want to make sure you have some extra capacity to start the heat pump. This is kind of tricky and there is really no way to know for sure. I suggest seeing if the manufacturer States anything about what size motor it can start, or offers any guidance.
 
@electrofelon is pointing out the key issue with using IxV for a compressor. That I number is a nominal rating that ignores that fact that motors have power factor, a motor's current draw changes with load, and motors often have a much higher starting current draw.

If you put a motor rated to draw 44.8A on a generator rated to supply 45A, it is very likely that the generator would be unable to start the motor, but that the generator would be oversized for keeping the motor running.


-Jonathan
 
I swear I will resist the urge to go down that generator rabbit hole.......but in brief, a compressor has an LRA spec, friggin use it! If you have a 'real' generator, it will have specs for starting loads, or even starting plus base loads.

You might want to share more about where you are getting your specs for the geo unit, because something does not smell right to me.

I found a small rabbit hole.....A generator, or any IC engine's efficiency is a function of load. Higher load = better efficiency. I'm super proud of these people that brag of having a 100KW CAT generator for their home. They don't tell you it costs $50/hr at idle! In short, you really want the generator to live in the sweet spot, and accept some load shedding if you want good eff.

The reason I mention is if that geo compressor is not VFD controlled, I would either do it, or install a real, digital soft start on it. Otherwise you get to oversize a generator just for 100ms worth of inrush, which is dumb.
 
I don't think there is always a cut and dried answer for a heat pump system. At least the geothermal ones don't need to defrost as in a typical air source system you'll have both the aux heat and the compressor running during defrost cycles. But heat pumps are only so big. If a 5 ton system is large enough to heat the house on a cold day, then aux heat should not be needed. But if you need 6 tons of heat at the cold design temp and the system only makes 5, then the aux heat will be running some on the colder days. The only way to know the answer to that is to ask the heat pump installer what the design heat load is (or look for a Manual J calculation which determines this). Emergency heat is usually by definition only used when the compressor has died, so it is the Aux heat case that needs to be determined.

If the user can manually load shed (manual ATS versus auto), then they could turn off the strip heat to ensure it doesn't run.

If HVAC and Electrical installers would post their calculations, it would make life easier for future contractors when changes are made.
 
suemarkp said:
I don't think there is always a cut and dried answer for a heat pump system. At least the geothermal ones don't need to defrost as in a typical air source system you'll have both the aux heat and the compressor running during defrost cycles. But heat pumps are only so big. If a 5 ton system is large enough to heat the house on a cold day, then aux heat should not be needed. But if you need 6 tons of heat at the cold design temp and the system only makes 5, then the aux heat will be running some on the colder days. The only way to know the answer to that is to ask the heat pump installer what the design heat load is (or look for a Manual J calculation which determines this). Emergency heat is usually by definition only used when the compressor has died, so it is the Aux heat case that needs to be determined.

If the user can manually load shed (manual ATS versus auto), then they could turn off the strip heat to ensure it doesn't run.

If HVAC and Electrical installers would post their calculations, it would make life easier for future contractors when changes are made.
Click to expand...
IDK how far you have dipped into the geo calcs but it can get pretty intense due to thermal conductivity in the soil. There are ways to optimize that, even improve it. IE, a 5T design may very well lay down in the peak of Winter, in a cold snap. An improvement is simply to increase the loop sizing or otherwise heat capacity. I've even been around hybrid systems that use both vertical and horizontal loops.

IMO, I rarely find much competence with installers beyond simple generic calcs, and rarely are actual site conditions factored.

I do realize this shoots way beyond the EE/EC envelope typically, in which it 'should' land on the HVAC designer, but it seems rare that they get the math right. Again, aux/resistive heat should not be needed unless there is a system failure, IF designed right.
 
fastline said:
Again, aux/resistive heat should not be needed unless there is a system failure, IF designed right.
Click to expand...
Maybe with geothermal (or a new house where things can be designed and not retrofitted), but my last house had a 5 ton heat pump and a Manual J load of about 60K BTU (after I upgraded some insulation and all the windows). Problem is that an air source heat pump (at least back then) did not make 5 tons of heat except at outside temps of 47F or higher. The heat pump was actually fine until around 35 degrees in which case the aux heat started kicking in. Even worse was how often the thing needed to defrost in those 35 to 40F temperatures which is most of the winter here in Seattle. Newer "cold climate" heat pumps have greatly increased their output at lower temperatures, but if you have a need for more than 60K BTU in winter and have ducts that limit you to a 5 ton's air flow, you don't have a lot of choices.

My current house is worse. It has the same or slightly higher heating requirement, but the ducts can only support 4 tons of airflow. Fortunately, this house has natural gas so it could switch to gas when it got below 40F. Now I'm 100% gas because the 5 ton heat pump that was installed blew up and now I have a 4 ton AC since this installer pointed out the ducts were small.
 
suemarkp said:
Maybe with geothermal (or a new house where things can be designed and not retrofitted), but my last house had a 5 ton heat pump and a Manual J load of about 60K BTU (after I upgraded some insulation and all the windows). Problem is that an air source heat pump (at least back then) did not make 5 tons of heat except at outside temps of 47F or higher. The heat pump was actually fine until around 35 degrees in which case the aux heat started kicking in. Even worse was how often the thing needed to defrost in those 35 to 40F temperatures which is most of the winter here in Seattle. Newer "cold climate" heat pumps have greatly increased their output at lower temperatures, but if you have a need for more than 60K BTU in winter and have ducts that limit you to a 5 ton's air flow, you don't have a lot of choices.

My current house is worse. It has the same or slightly higher heating requirement, but the ducts can only support 4 tons of airflow. Fortunately, this house has natural gas so it could switch to gas when it got below 40F. Now I'm 100% gas because the 5 ton heat pump that was installed blew up and now I have a 4 ton AC since this installer pointed out the ducts were small.
Click to expand...
Yeah, air source heat pumping is very different than geo in that the ambient air temps are not tied to the system performance at all. Rather, it comes down to system duty cycle relative to the variable involving thermal conductivity of the soil.

But you did point out a very vital vulnerability I try to point out to people with heat pumps in extreme cold. If that compressor, or really any part of the system fails, you are either stuck with no heat, or aux heat only. In my personal opinion, and the way I roll my own, I have a plan B with fossil fuels. You simply will not get a tech to touch your issues when it's -10F in a blizzard! Even having a wood burner spliced into the central system can save your butt. Always remember aux heat is one relay or open coil away from no heat.

One of the SUPER nice things with geo units is the compressor and evap are indoors so you may actually find a tech willing to mess with it in the cold due to the conditions.

The newer air source units with VFD and overspeed are able to do really well down into the teens, but I would not be betting on them in extremes. Those condenser boards are outdoors! I've been called plenty to save someone's azz in the extremes.
 
You must log in or register to reply here.
Top