dslreports logo
site
 
    All Forums Hot Topics Gallery
spc

spacer




how-to block ads


Search Topic:
uniqs
10
share rss forum feed


ITICharlie1
Premium
join:2003-01-22
St. Louis MO
reply to pandora

Re: Decent 20-30 KW standby generator

I just have to ask, why do you need one so large? Is it for a business? I can run everything except the central AC on my 7000 watts gas genny.
--
Don't let my reality hinder your imagination!


fifty nine

join:2002-09-25
Sussex, NJ
kudos:2
said by ITICharlie1:

I just have to ask, why do you need one so large? Is it for a business? I can run everything except the central AC on my 7000 watts gas genny.

Maybe he wants to run his AC.

pandora
Premium
join:2001-06-01
Outland
kudos:2
Reviews:
·ooma
·Google Voice
·Comcast
·Future Nine Corp..
reply to ITICharlie1
said by ITICharlie1:

I just have to ask, why do you need one so large? Is it for a business? I can run everything except the central AC on my 7000 watts gas genny.

Two 4 ton heat pumps are my ONLY heat source. Both must be able to operate. Two electric heaters can use up to 4.3 KW a piece. 20KW is the floor for my home to remain comfortable. 20 KW is the highest wattage an air cooled generac will provide, after that it's liquid cooled.
--
"If you put the federal government in charge of the Sahara Desert, in 5 years there'd be a shortage of sand." - Milton Friedman"

Mr Matt

join:2008-01-29
Eustis, FL
kudos:1
Reviews:
·Millenicom
·Embarq Now Centu..
·Comcast
·CenturyLink
I would expect that the power required by both of your heat pumps running in the defrost mode simultaneously would be 24 KW or approximately 100 Amps or 50 Amps each. The maximum continuous power from a 20 KW generator is 83 Amps. If you want to be able to use most of your appliances at the same time, you should consider at least a 36 KW Generator, which will provide 150 Amps @ 240 Volts. Since you are having a 400 Amp service installed, for the difference in price I would consider a 48 KW generator.

pandora
Premium
join:2001-06-01
Outland
kudos:2
Reviews:
·ooma
·Google Voice
·Comcast
·Future Nine Corp..
said by Mr Matt:

I would expect that the power required by both of your heat pumps running in the defrost mode simultaneously would be 24 KW or approximately 100 Amps or 50 Amps each. The maximum continuous power from a 20 KW generator is 83 Amps. If you want to be able to use most of your appliances at the same time, you should consider at least a 36 KW Generator, which will provide 150 Amps @ 240 Volts. Since you are having a 400 Amp service installed, for the difference in price I would consider a 48 KW generator.

I think 30 KW is about it for an above ground 1,000 gallon propane tank in terms of gas flow to drive the generator.

As to defrost, I tend to doubt that much power is required.
--
"If you put the federal government in charge of the Sahara Desert, in 5 years there'd be a shortage of sand." - Milton Friedman"

Mr Matt

join:2008-01-29
Eustis, FL
kudos:1
Reviews:
·Millenicom
·Embarq Now Centu..
·Comcast
·CenturyLink
What is the name plate amps on the condenser and air handler of your heat pumps. Your heat strips already draw about 18 Amps at 240 Volts. Each condenser will probably draw about 25 Amps and each air handler exclusive of your heat strips 5 Amps. When in the defrost mode your current requirement should be around 48 Amps per heat pump. Contact your HVAC contractor and confirm.

The last thing you will want to do is purchase a generator that cannot carry the offered load. I would not recommend guessing because the information is already on the specification plates on your air handler and condenser. My 3.5 ton condenser requires 20 Amps my air handler requires 5 amps and my 4.8 KW heat strip requires 20 Amps for a total of 45 Amps while in the defrost mode. My 2.5 ton condenser requires 40 Amps while in the defrost mode.


cowboyro
Premium
join:2000-10-11
Shelton, CT
reply to pandora
said by pandora:

As to defrost, I tend to doubt that much power is required.

It's whatever heat pumps+strips+air handler require.
From the specs you gave it will be around 10kW *each*.
You may be able to block the strips if you are willing to cope with ice-cold air being blown at full blast for few minutes (trust me I have tested with aux heat off), but it is very unpleasant. For all practical purposes it's like having the A/C on.

pandora
Premium
join:2001-06-01
Outland
kudos:2
Reviews:
·ooma
·Google Voice
·Comcast
·Future Nine Corp..
reply to Mr Matt
said by Mr Matt:

What is the name plate amps on the condenser and air handler of your heat pumps. Your heat strips already draw about 18 Amps at 240 Volts. Each condenser will probably draw about 25 Amps and each air handler exclusive of your heat strips 5 Amps. When in the defrost mode your current requirement should be around 48 Amps per heat pump. Contact your HVAC contractor and confirm.

The last thing you will want to do is purchase a generator that cannot carry the offered load. I would not recommend guessing because the information is already on the specification plates on your air handler and condenser. My 3.5 ton condenser requires 20 Amps my air handler requires 5 amps and my 4.8 KW heat strip requires 20 Amps for a total of 45 Amps while in the defrost mode. My 2.5 ton condenser requires 40 Amps while in the defrost mode.

Heat strips can't be used on any generator solution I can afford. They are 20KW a piece, and I'd need a 55-60 KW unit. Not affordable. Heat strips are for auxiliary heat only.

The heat pump is a Goodman DSZC18048 condenser, with a CHPF4860D6 Horizton Cased A Coil 4-5 Ton, a MBVC2000AA Modular Blower, a TX5N4A 3-5 ton TXV, R410A, a F200E1037 HoneyWell Media Air Cleaner. F200 Series. 20'' x 25'' and a HKR-20C Slide in Electric Heat Strip 20KW with Breaker.

When the generator is on, emergency heat will be turned off as it is 20 KW (the size of the generator).

The heat pumps are rated to provide power down to -10F.
--
"If you put the federal government in charge of the Sahara Desert, in 5 years there'd be a shortage of sand." - Milton Friedman"

pandora
Premium
join:2001-06-01
Outland
kudos:2
Reviews:
·ooma
·Google Voice
·Comcast
·Future Nine Corp..
reply to cowboyro
said by cowboyro:

said by pandora:

As to defrost, I tend to doubt that much power is required.

It's whatever heat pumps+strips+air handler require.
From the specs you gave it will be around 10kW *each*.
You may be able to block the strips if you are willing to cope with ice-cold air being blown at full blast for few minutes (trust me I have tested with aux heat off), but it is very unpleasant. For all practical purposes it's like having the A/C on.

The Generac DLM permits me to turn off the auxiliary heat (and even the heat pumps) as needed based on load. Heat pumps are set to priority 1, strips to 2. The auxiliary heat will not be available during any defrost cycle while using the generator.

However, the air handlers will not blow cold air. There is a plugboard where a minimum temperature from the heat pump can be set when in heat pump mode, if the heat pump doesn't provide sufficient heat, the air handler will turn off until it does.
--
"If you put the federal government in charge of the Sahara Desert, in 5 years there'd be a shortage of sand." - Milton Friedman"


cowboyro
Premium
join:2000-10-11
Shelton, CT

1 edit
said by pandora:

However, the air handlers will not blow cold air. There is a plugboard where a minimum temperature from the heat pump can be set when in heat pump mode, if the heat pump doesn't provide sufficient heat, the air handler will turn off until it does.

You really need to understand how it works. If the coils in the air handler can't suck heat from the house the outdoor coils will never defrost. You are defrosting the outdoor coils using heat from the house.
Refrigerant enters the evaporator, it super-cools and absorbs heat from the air. It is then going in the compressor at indoor air temperature and super-heated through compression, releasing the heat. If it cannot absorb heat it cannot release heat. Without air flow the evaporator *WILL* freeze. Frozen coils can lead to compressor damage.

Mr Matt

join:2008-01-29
Eustis, FL
kudos:1
Reviews:
·Millenicom
·Embarq Now Centu..
·Comcast
·CenturyLink
reply to pandora
You might want to consider doing what my brother in law did for emergency heat. He has a cottage where the power occasionally fails for several days. The cottage was originally heated by electric baseboard heaters. That was not satisfactory during a power failure so he installed a direct vent propane heater which uses a millivolt thermostat that does not require AC power. When he is out of the cottage he uses the propane heater. The heaters were not expensive and can provide a lot of heat rather than using your heat pumps during a power failure. Consider one or more units like this one:

»www.amazon.com/Home-Comfort-DV21···01AH8K68

pandora
Premium
join:2001-06-01
Outland
kudos:2
Reviews:
·ooma
·Google Voice
·Comcast
·Future Nine Corp..
said by Mr Matt:

You might want to consider doing what my brother in law did for emergency heat. He has a cottage where the power occasionally fails for several days. The cottage was originally heated by electric baseboard heaters. That was not satisfactory during a power failure so he installed a direct vent propane heater which uses a millivolt thermostat that does not require AC power. When he is out of the cottage he uses the propane heater. The heaters were not expensive and can provide a lot of heat rather than using your heat pumps during a power failure. Consider one or more units like this one:

»www.amazon.com/Home-Comfort-DV21···01AH8K68

That could work. I'll have to see what happens over time.
--
"If you put the federal government in charge of the Sahara Desert, in 5 years there'd be a shortage of sand." - Milton Friedman"


tmh

@verizon.net
reply to pandora
said by pandora:

Two 4 ton heat pumps are my ONLY heat source. Both must be able to operate.

Since you already have propane, would it not be simpler (and cost less) to put in a 48k BTU heating stove (or two)? These will work both with and without power.

After Sandy, a 40k BTU stove in my basement kept temperatures in the mid 60's on the main level. That's with the valve midway (I estimate about 30k BTU). The power was off overnight. It was in the 30s outside.

ncbill
Premium
join:2007-01-23
Winston Salem, NC
Reviews:
·AT&T Southeast
reply to Mr Matt
The OP has just essentially doubled his house size IIRC.

Even multiple through-the-wall heaters likely won't be enough to keep his large home comfortable.

I would still urge the OP to swap his current air handler for one with a propane furnace - maybe just the downstairs (heat rises).

I'm sure his local HVAC would have no problem pulling a gas line to the handler and putting in the vent pipe (PVC, as noted) through the nearest outside wall.

said by Mr Matt:

You might want to consider doing what my brother in law did for emergency heat. He has a cottage where the power occasionally fails for several days. The cottage was originally heated by electric baseboard heaters. That was not satisfactory during a power failure so he installed a direct vent propane heater which uses a millivolt thermostat that does not require AC power. When he is out of the cottage he uses the propane heater. The heaters were not expensive and can provide a lot of heat rather than using your heat pumps during a power failure. Consider one or more units like this one:

»www.amazon.com/Home-Comfort-DV21···01AH8K68


TheMG
Premium
join:2007-09-04
Canada
kudos:3
Reviews:
·NorthWest Tel

1 recommendation

said by ncbill:

I would still urge the OP to swap his current air handler for one with a propane furnace

I don't know why the OP is so against that. It is perhaps the easiest and most effective solution.

As I said, he's going to be burning through propane very quickly with that large generator running the heatpumps.

I did some quick math, and if it is correct, the OP's heatpumps at optimum advertised efficiency would consume 4kW to produce 72,000BTU.

Generator he purchased running at 1/2 load will be consuming 1.86 gallon of LPG per hour (manufacturer does not give specs for lesser loads, but presumably the OP will have some other electrical loads beyond the heat pumps operating in the house).

A 90% efficiency propane furnace will consume about 0.84 gallon per hour to produce those same BTUs of heat output. That's less than half!!!!

And that's the BEST CASE SCENARIO. Still need to take into account that the large generator still consumes a lot of fuel even under light loads when the heatpumps aren't running, also that the heatpump is less efficient as outside temperature decreases, the heat strips for defrosting, etc, etc.