AC Load w/heat strips

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billsims

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Just received book (Commercial Calculations)but am uncertain about requirements.

If I am calculating service load on a church assembly hall and it has(2) 5 ton heat pumps and (2) 3 ton heat pumps, name plate for 5 ton unit calls for 60A for compressor and 90 amp for air handler. Name plate for 3 ton calls for 60A and 30 Amp.
If I am correct then the demand on service drop would be 480Amps, therefore a 400 amp service would not be acceptable. Forgot to say that this service is Single Phase.
Thanks

[ January 10, 2006, 03:10 PM: Message edited by: billsims ]
 
Re: AC Load w/heat strips

You need the FLA of the units and the amperage of the strip heat. You do not use the breaker size of the units to arrive at the service size.
Your post says A/C load and electric heat. These are not heat pumps?

[ January 04, 2006, 04:58 PM: Message edited by: bob ]
 
Re: AC Load w/heat strips

Are you sure you don't have the amperage backwards for inside and outside?
 
Re: AC Load w/heat strips

This is heat vs ac, non-coincidental loads, use the one that is the higher load in your calculations. What is the KW on heat and the amps on ac?

[ January 05, 2006, 04:44 PM: Message edited by: Jhr ]
 
Re: AC Load w/heat strips

Originally posted by Jhr:
This is heat vs ac, non-coincidental loads, use the one that is the higher load in your calculations. What is the KW on heat and the amps on ac?
Click to expand...
But if it's a heat pump, they are not non-coincidental loads. The heat strips and the compressor can run at the same time.
 
Re: AC Load w/heat strips

But if it's a heat pump, they are not non-coincidental loads. The heat strips and the compressor can run at the same time.
Click to expand...
What we need if its a heat pump is the heat pump kva and the supplementary heat kva. Now if the supplementary heat is not on at the same time as the heat pump, then the heat pump kva need not be added, this he would have to ask the AC man. Not been an AC man I got this info. off the net

This is how a heat pump works according to " how stuff works.com": Imagine that you took an air conditioner and flipped it around so that the hot coils were on the inside and the cold coils were on the outside. Then you would have a heater. It turns out that this heater works extremely well. Rather than burning a fuel, what it is doing is "moving heat."

A heat pump is an air conditioner that contains a valve that lets it switch between "air conditioner" and "heater." When the valve is switched one way, the heat pump acts like an air conditioner, and when it is switched the other way it reverses the flow of Freon and acts like a heater.

Heat pumps can be extremely efficient in their use of energy. But one problem with most heat pumps is that the coils in the outside air collect ice. The heat pump has to melt this ice periodically, so it switches itself back to air conditioner mode to heat up the coils. To avoid pumping cold air into the house in air conditioner mode, the heat pump also lights up burners or electric strip heaters to heat the cold air that the air conditioner is pumping out. Once the ice is melted, the heat pump switches back to heating mode and turns off the burners. :cool: :cool:
 
Re: AC Load w/heat strips

That's one use of the heat strips. Another time they are used is when it gets really cold. The amount of heat that a heat pump can produce decreases as the outside temperatures get lower. Unfortunately, your heating BTU needs go up as the outside temperatures get lower. So you quickly reach a point where the heat pump can't keep up. The auxillary heat will be switched on at this point to help out the heat pump, so both are running at the same time.

Some HVAC guys make the heat pump turn off when the aux heat is turned on. This is dumb, as you're turning off your most efficient heat source even though its output is diminishing. They can also wire them to not use the aux heat during defrost, but I really wouldn't like 10 minutes of COLD air coming out of the ducts periodically in winter.
 
Re: AC Load w/heat strips

Then in this instance the HeatPump load for calculating service intrance would be a total of all the units combined?? I'm being told by an inspector that I can derate, but I don't see how since this is a commercial building.
 
Re: AC Load w/heat strips

Bill
220.30C2 states that you use 100% of the compressor and electric heaters unless the controller prevents all from operating at the same time.
 
Re: AC Load w/heat strips

Most of the responses you have received in the Forum have some truth in them. There are several things we need to know for a heat pump problem.


First of all, are there supplemental electric heaters integral with the unit and on the same circuit as the indoor unit,

or are the supplemental heaters separate from the heat pump but installed in the duct work and on a different circuit?


Also, as some of the posts in the forum indicate, the efficiency of heat pumps decreases somewhere below 40 degrees F and the supplemental heat may be switched on ? often this happens in increments so all of the electric heat is not employed simultaneously. The heat bank may also be turned on during ?de-icing? to keep the outdoor unit from freezing over in the winter. There are different designs in regard to the way the heat bank functions, so we need to know if the design will allow the heat bank to run simultaneously with the heat pump compressor.


If the electric heat banks are contained within the heat pump unit, with no external heat banks, the manufacturer?s information should help you determine what the maximum heating load is and what the maximum cooling load is, then use the larger of the two.


If the heat banks are separately installed and on separate circuits, you will need to determine if the control circuit allows the heat pump to run simultaneously with the electric heat, and if so you should add the heat pump load to the supplemental heat loads when calculating the heating load.


In colder parts of the country, the additional electric heat banks are essential. In some milder climates, that may not be the case. Also, some of the more efficient systems that use ground water return loops or other geothermal options may not need the supplemental electrical heat in some climates.


When you look for the load on the unit, be sure you use the nameplate load information, not the suggested circuit breaker size, for sizing the electrical service. The manufacturer should have information in the installation manual and possibly on their website to help you determine the specific load requirements for each model.


You are correct in your statement that since all units can be turned on simultaneously, they will all need to be added into the service size. I am not sure what is included in the amperages you have listed, so cannot tell you if that is the correct calculation or not. Make sure you know what the actual load is on each unit, and if electric heat is included. If the electric heat is not included in the heat pump cabinet, are there other circuits supplying separate electric heaters in the ductwork? If so, include them in the heat side of your calculation.


Any feedback on this is welcome, I hope it will be of some help.
 
Re: AC Load w/heat strips

[QUOTE The heat bank may also be turned on during “de-icing” to keep the outdoor unit from freezing over in the winter. ] [/QUOTE]

It doesn't keep the outdoor unit from freezing it tempers the cold(absence of heat)air that is blown into the home during the reversal of refrigerant flow. Correct?
 
Re: AC Load w/heat strips

Most of the responses you have received in the Forum have some truth in them. There are several things we need to know for a heat pump problem.
Click to expand...
Exscuse me :cool: :cool: .
 
Re: AC Load w/heat strips

Bill, I just finished working on a "Florida Heat Pump" on a geothermal system here in Kansas that had specs similar to those for your 5 ton unit. The heating units were in two banks, one of which would come on with the heat pump, the other came on only the emergency heat came on.

The heat pump was non coincident with emergency heat. From all testing we could do, it appeared that the max. heat pump CB rating of 60A included the first bank of heaters. The 90A CB for the heaters included both banks of heat strips.

If yours is similar the load of the heat would be non coincident with the heat pump and would therefor be what you would need for your calculation. (For your comparison, the heat pump was pulling around 20A by itself under test conditions and the emergency heat pulled 85A.) ;)
 
Re: AC Load w/heat strips

Originally posted by suemarkp:
Some HVAC guys make the heat pump turn off when the aux heat is turned on. This is dumb, as you're turning off your most efficient heat source even though its output is diminishing. They can also wire them to not use the aux heat during defrost, but I really wouldn't like 10 minutes of COLD air coming out of the ducts periodically in winter.
Click to expand...
It should be kept in mind that there comes a point where the heat produced by the heat pump costs more to collect than what it would cost to get a similar amount of heat from your alternative source.

Although the point will vary depending on the cost of the alternative heat source it seems it would be prudent to turn off the pump at that point. :)
 
Re: AC Load w/heat strips

That point would occur when the Coefficient of Performance (COP) is near or below 1. MOst heat pump charts I've seen of COP -vs- temp has a COP above 2.0 at 0 degrees F. I think you're going to hit the point where they don't recommend running the heat pump because the cold temperatures can damage the system before you'll hit the COP = 1 point.

See page 22 of this document for an example of the COP's at various temperatures for a York heat pump: http://www.yorkupg.com/PDFFiles/036-21570-002-D-0605.pdf
 
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