Heat Pumps
Air to Air
What is a Heat Pump ?
Since the 60's this question has been asked a lot so you are certainly not by yourself.
Heat pumps are a very good source for heating and very economical as well. The biggest draw backs are they may not be properly installed and the most common complaint is the air coming out your registers is not as hot as your furnace, thus you very well may complain of being too cold. Technically the heat pumps air may be around 95 degrees but the furnace air is around 125 degrees. Which air is hotter?
If your accustomed to the "Hot Air" of the furnace then you may not be satisfied with a heat pump.
The below should help in you decision of a heat pump. If you have a question please send me an email via this site and I will get back. Email me at click on this to email me
Some terminology you might run into, "Air to Air".
Air to air simply means air goes across the outside condenser coil to absorb heat or get rid of heat depending on the mode it is in and the air across the evaporator coil, the indoor unit, picks up heat or cooling, (removing heat from the air or adding heat ), to distribute to your conditioned rooms so you may be comfortable. A water to air means the condenser coil uses water to do the same thing, (that may be a water cooled unit or Geothermal unit), & the indoor evaporator uses air to distribute heat or cooling as mentioned before.
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The Heat Pump provides heating & cooling and heating is provided at a very economical rate. Heat Pumps are good & economical even in cold climates.
To see where & how long the Heat Pump does it's job one needs to look at the hours per year at temperatures for heating (see example on Questions-Answers-Tips at the MENU SELECTION page) & the Heat Pump balance chart (below) that tells you the Heat Pump will perform down to a given temperature. Temperatures below the balance point one needs make up heat be it in the form of gas, electric, oil & so on (according to your location).
What is the cost of the fuel proposed? You need to look at the projected annual cost of fuel to see what would be the most (better choice) with a good payback.
In air conditioning you will notice that the OUTSIDE unit gets hot while the indoor coil gets cold. The "Heat Pump" reverses this process.The OUTSIDE unit now blows cold air during it's winter season and the inside coil gets hot.
Through it's own process it takes HEAT from the outside air and blows it into your home via the gas or refrigerant cycle. There is plenty of heat in the cold air outside air even though you think it's cold. That is what a heat pump does is take outside air heat & puts it into your home.
However the colder it gets outside the less "Heat" the heat pump puts out yet your home requires more heat to keep you warm, (from Manual "J" calculation Heat Loss). The Heat Pump Balance chart will show the heat pump will heat your home until it reaches a certain temperature as shown on the balance chart. Usually the Heat Pump will do it's job down to somewhere around 40 degrees then other types of heating are needed to make up the increase of heating BTU's as it gets colder. Below where the two lines cross on the balance chart means the heat pump will not due the heating and the colder it gets the more the heat pump will not keep up with cold temperatures. You may need an an example of 70,000 BTU's of heat and the heat pump only delivers 36,000 BTU's at 38 degrees. Not enough heat unless you have planned & installed some other type.
Some use electric heat for supplementary, (also known as auxiliary), heat others use gas, oil, and other types of heat at those colder temperatures. That will depend on your area and it's cost for fuel. The following sample balance chart will help explain further.
Auxiliary heat in some areas is mainly for some heating if the heat pumps compressor goes out or there is a mal-function with the heat pump unit.
The heating difference must come from somewhere, (gas, electric, oil, etc), if your comfort is to be realized. That's one of the reasons that a "Load sizing calculation" and a "Heat Pump Balance Chart" should be done.
The "Heat Pump" heating efficiency is rated as C.O.P., (Coefficient of performance). That simply means wattage converted to Btu's. The amount of unit Btu's divided by (wattage x 3.41btu's).
Another heat rating is called the "HSPF", the Annual Heating Season Performance Factor. A unit's heating HSPF is listed in the "ARI" Directory. The HSPF number can be used to estimate the annual heating energy consumed by the heat pump. Including the indoor & outdoor fan motors, the crank case heater, & the controls.
The air conditioner part of the system is rated as SEER. The larger the seer the more btu's per electrical comsumption. (Ex: 12 SEER is btu's per watt of electricity that runs the machine.)
One needs to be aware of the rise in air temperature in a furnace and a heat pump.
Especially when replacing a furnace with the heat pump. This should be explained to the home owner before hand.
A furnace may be around 125 degrees in it's air temperature to a room while the heat pump may be 95 degrees. Both are warm or hot and will keep your comfort but the heat pumps air seems to be colder to the hand when a customer checks the feel of the air blowing out of the grille. Comfort will still be there but customers need to be aware of the difference. Either one needs to be properly installed and especially the heat pump with the balance chart
The heating temperature of the heat pumps air is not as hot as the gas furnace therefore some people may complain at first. The heat pumps air is in the 90°'s but for very cold climates the MOST efficient form of heating is only good to the application rating of approximately 38° to 40°, after that another more economical fuel, (usually a gas furnace), kicks in & the heat pump stops. The colder it gets outside the less heat the heat pump puts out. Sometimes that "Extra" heat is in the form of resistance or electrical heat which is the most costly fuel you can find.
Spec data sheets on the heat pump!
Specification data is available from the dealer and shows good information for the proposed unit.
The below specs are examples of a 5 ton Heat Pump showing the rated cooling btu's & heating btu's at various outside temperatures.
Don't confuse this as only heat pumps have this type of info. No matter what the equipment there is a specification sheet on it.
Cooling:
Ex: Outdoor temperature of 95° and 67° entering wet bulb & at 80° dry bulb at 2000 cfm the Btu capacity would be 59,700 Btu's.
That same unit at 115° outside would be 53,200 Btu's
Air conditioning specs on a 5 ton heat pump, single or 3 phase unit
Heating data is as follows:
Notice the colder it gets outside the unit capacity is less. That's why a heat pump balance chart is done so you can see where the heat pump no longer does the job & other types of heating "KICK in" at colder temperatures
Heating specs on a 5 ton heat pump, single or 3 phase unit
Heat specs on a 5 ton heat pump
The Heat Pump balance chart is also done with these facts. Below is an example of a Heat Pump balance chart. (For AC people it can be found in the Manual J book).
One takes the Heating load calculation & makes the 1st line by drawing from the residence Heat Load to a 65 degree temperature, ( 65 degrees is always used), from the spec sheet take the Heat Pump data & draw the 2nd line. You can then see where the Heat Pump does it's job to where the two lines cross. Below that line you need other heat to keep up with the colder temperatures and may very well not use the heat pump at these colder temperatures.
Usually other more economical fuels take over then, ( depending on your area ).
The most economical fuels for heating are: (1). Heat pump, (2). Natural Gas, (3). Propane Gas, (4). Oil, & in last place is (5). Electrical heat, (aka wire resistance heat). I am not familiar with boilers or radiators.
The Heat Pump only satisfies your comfort to the "Balance" point ( where the two lines cross then straight down is the balance temperature - meaning the heat pump will do the comfort job down to that temperature ). Temperatures lower than the balance point require additional fuel for heating and is needed since the heat pump will not produce enough heat for your comfort and probably cut off if designed properly and let a more economical heat and fuel take over.
Every installation of a heat pump should have a balance chart for the customer & the installing contractor. Heat Pumps at and above the "Balance point are probably the most economical form of fuel you can get and that leads to the "Bin Method" for hours a year at given temperatures. Annual operation cost of the higher efficiency units be cooling or heating & the cost of their annual fuel usage or requirement will determine for you which is the best purchase. The AC & Heat Dealer should be able to provide this information.
Heat Pump balance example |
as the outdoor temperature starts to drop so does the capacity of the Heat Pump system. On those colder days where the capacity drops below the required heat loss, (balance point), additional heat must be supplied. That heat can be electric, gas, or what you and the dealer may agree to.
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The building heat loss is in red. Unit capacity in black. The point at which the unit capacity line & the calculated heat loss intersects is the balance point. This example has heat loss at 34,500 BTU/hr & unit capacity at 34,500 BTU/hr at 32.5 degrees
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at 10 degrees the heat pump will require an additional 36,500 BTU/hr to maintain comfort. The heater selected must deliver at least 36,500 BTU/hr at the designed point and thats with the output of the "Heat Pump".
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Emergency or Auxiliary Heat
In this example:
10 degrees is the full capacity required of 57,500 BTU/hr. and at 80% this would require an input of 46,000 BTU/hr. Usually this is electric heat and is to maintain some comfort until the compressor or unit is repaired.
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In colder climate areas electric heat may make up the difference of heat needed
Emergency or Auxiliary heat requirement may vary from 60% to 100% of the heat required at design temperature conditions.
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If you have any question please contact me.
Download or print a heatpump balance chart form (PDF) click >
HeatPumpBalChart.pdf
Heat Pumps should be sized according to the "Cooling" load calculation & in accordance with Manual "J". That is primarily due to maintaining latent or moisture control in the air conditioning season.
Air conditioning for humid areas would be in trouble if a heat pump was sized according to the heating.
It has been brought to my attention that areas that have very little air conditioning may be sized according to a heating calculation but I still wonder for air conditioning do those areas have any humidity to deal with during the very little cooling season.
I don't know of any manufacturer that say's to size according to the "Heat Load" regardless of your temperature and if there is please send an E-mail who that manufacturer may be.
