A heat pump system is similar to a conventional home A/C system.† But it also has the ability to heat your home in addition to cooling it.† In the summer it cools the home, and in the winter it reverses the same process and heats it.† These are very popular systems in areas where the winters are very mild, unlike the greater Kansas City area.† But they are rapidly gaining ground in this area due to intense marketing.
Heat always moves from hot to cold.† The greater the temperature difference between the two areas is, the faster the heat moves from hot to cold.
In order to understand how this applies to the heat pump, we must first explore how an A/C system works at a very basic level.† There is an indoor coil and an outdoor coil.† The indoor coil temperature is about 40 degrees.† So when you blow the house temperature air through it, it absorbs heat from the house temperature air, because heat moves from hot to cold.† The absorbed heat is then sent to the outside unit to be disposed of.
The heat pump works identically to an A/C unit in the cooling mode.† But in the heating mode it simply reverses the process.† The outside unit absorbs the heat from outdoors and sends it to the inside coils to be disposed of.† So we utilize that heat to heat our home.† (When it isnít real cold outside).
Obviously, the colder it gets outside, the less heat there is out there to be sent inside.
Heat pumps are touted to be more efficient than natural gas furnaces until the outside temperature gets down to about 30 degrees.† But that doesnít mean that the heat pump will be able to heat your house to the desired temperature when the outside temperature is 30 degrees.† Hereís why.† In our climate, the furnaces are typically sized to deliver 2 Ĺ to 3 times as many Btu/h of heat as the A/C systems are rated to remove.† For example, a home here with a 36,000 Btu/h A/C system will often have a 100,000 Btu/h furnace.† This is because of the difference in temperature between the outside air and the house air in the winter compared to the difference between the outside temperature and the house air in the summer.†
In this area, we typically size the furnace to be able to put out enough Btu/h of heat to maintain an indoor temperature of 70 degrees when the outside temperature is 6 degrees.† (Remember that heat moves from hot to cold.† And the greater the difference in temperature between them, the faster the heat moves from hot to cold.)† The difference between 70 degrees and 6 degrees is 64 degrees.† Thatís a lot of temperature difference.
And in this area, we typically size the A/C to be able to remove enough Btu/h of heat to maintain an indoor temperature of† 75 degrees when the outside temperature is 96 degrees.† Thatís a temperature difference of only 21 degrees, compared to the 64 degree difference we see here in the winter.
Hereís the problem with sizing a heat pump in our area.† The heat pump puts out the same amount heat in the winter (on a day when the outside temperature isnít too cold) as the amount of heat it removes in the summer.† BUT, the heat pump system MUST be sized for the required Btu/h output of the A/C, because itís a very bad thing to oversize an A/C system.† So if our house needs 36,000 Btu/h of A/C and 100,000 Btu/h of heat, a 36,000 Btu/h heat pump must be installed.† And that is far less Btu/h than we need to heat our house with the heat pump alone, because we need 100,000 Btu/h to heat our house when the outside temperature is at 6 degrees.
Due to the heating disparity between a furnace and a heat pump in our area, the heat pump alone usually becomes inadequate to heat our house at an average outdoor temperature of 47 degrees.† At that point a backup heat source usually has to kick in to make up for the shortfall of required heat from the heat pump.
So we must have a backup source of heat.† And that source will typically be either a gas furnace or electric heat strips installed in an ďair handler.Ē† An air handler is a big box like a furnace with no gas burners in it.† Instead of the burners, it has electric heaters called ďstrip heatersĒ.
Of the two choices of backup heat, the electric heat strip choice is by far the most expensive route to go from an energy consumption standpoint.† The cost in dollars per Btu/h of heat output is much greater with the heat strips than it is with natural gas.
But the electric utility companies donít want to miss out on the revenue generated by heat pump applications.† So they offer special rates per kilowatt hour to people with heat pump systems in some areas.† This makes the heat pump a much more economically viable choice than it would have been without the special utility rate pricing.† Iíve heard tales of some people saving a lot of money with heat pump systems, especially when we have a very mild winter.† But Iíve also seen the flip side of the coin, where the winter electric bills were horrendous.† These were typically people with electric heat strip backup systems though.
Us city dwellers are lucky in that we have natural gas piped to our homes, because itís a lot cheaper than liquid propane (LP gas) that the people in the outlying areas are forced to use.† So heat pumps are nothing new to people in those areas.† Itís been the best alternative to LP gas for those people for a very long time.†
From a reliability standpoint, there are a lot more things to go wrong with a heat pump system.† The outside unit is running all year, so it takes a lot more of a beating than a dedicated A/C outside unit.† And if the system starts to develop Freon leaks, we have issues in both the summer and the winter.
As for the annual savings in energy bills possible with the heat pump, that can go either way depending on what the electric and gas rates do.† And the rates can change at the drop of a hat.† There are no guarantees that the electric company will continue their special pricing for heat pump owners, especially when some of the parent corporations own both electric and gas utilities.† The cost of heating your home with a conventional gas furnace vs. a heat pump system can also vary depending on how cold a winter we get.
UPDATE 6/23/11† The electric co. here was reportedly forced to take part of the heat pump discount off the table by the State regulators a few months ago, because the gas co. had been complaining that it gave the electric co. an unfair advantage in winter energy sales.
There are a couple of additional disadvantages to having a heat pump.† The heated air produced by the heat pump isnít nearly as hot as the air produced by a natural gas furnace.† Itís actually below our body temperature, even though itís above room temperature.† Any air being moved across our skin has a cooling effect on our body if the temperature of that air is below our body temperature.† Many people just canít take that.
The other disadvantage is that itís extremely difficult for conventional humidifiers to create enough humidity to add the desired amount of moisture to the house when the heating system is operating in the heat pump mode.† This is because the air temperature produced by the heat pump simply isnít hot enough to create enough humidity from the water.
One other drawback is that you canít use a setback thermostat to drop the temperature in the house when youíre asleep or at work in order to save money on your heating bills, because the heat pump just canít put out enough heat to raise the temperature in the house by several degrees in a reasonable amount of time without calling on the backup heat for help.
Overall, Iím not a big fan of heat pumps for many of the reasons listed here.† But, would I buy one for my own home?† I might consider it, because I wouldnít have the repair costs to worry about, and I know it would be properly maintained and tweaked for the best possible performance year round.† And I would monitor it to nip any problems in the bud as soon as they showed up.† But I would NEVER have a heat pump system with back up electric heat.† I would wisely choose natural gas for the backup heat source.† At least that way, if the electric rates spiked, I could just flip a switch and use the natural gas heat source.†
Up to this point, the heat pump systems weíve looked at were the conventional ďair sourceĒ models because these make up the vast majority of systems installed in our area.† But there are a couple of other types being installed, although they are much more expensive to buy.† They are however much more efficient to operate.† One of those is the ďground sourceĒ heat pump, which removes the heat from the ground around the house in the winter and sends it inside to heat us.† The other is the ďwater sourceĒ heat pump which removes heat from a body of water and sends it inside to heat us.† In both cases, (the ground or the water), there is much more heat available in those sources than there is outside during most of the winter.† The ground temperature here in the winter will remain at 55 degrees past a depth of 13í or so all year round.† So this is a great source of heat in the winter.† And itís a good place to get rid of the heat from the house in the summer as well.
Greater Kansas City including:
Johnson County, Kansas Kansas City, Kansas Kansas City, Missouri
Lake Quivira, KS
Mission Hills, KS
Mission Woods, KS
Overland Park, KS
Prairie Village, KS
Roeland Park, KS
Spring Hill, KS
Westwood Hills, KS
Copyright 2012 Leonard Arenson Heating & A/C