Heating the house

I want to talk a bit about home heating.  I'm not an expert, but I've done a little research into it, and thought it would be good to share what I've learned.  I have a similar post about home cooling here.

When we bought this house, it came equipped with a 4 ton, 10 Seer heat pump using R-22 Freon refrigerant, and with an electric furnace back-up. 

You may not be from an area where heat pumps are commonly found, so I'll share a quick explanation.  A heat pump is basically a reversible air-conditioner.  Have you ever walked past a window-mounted air conditioning unit on the outside?  It will blow hot air at you that has been removed from inside the house.  If you turn it around in the winter, you can remove heat from the air outside and warm the inside of your house!  That's what a heat pump does, using a liquid with a very low boiling point - Freon.

Take a look at the image below.  A heat pump is being used as an air-conditioner in the summer.
Step 1- the compressor pumps gaseous refrigerant from indoors up in pressure.  The compression adds quite a bit of heat to the refrigerant, which makes its way to the condenser.
Step 2 - A fan blows ambient outdoor air over the condenser, cooling the refrigerant, and condensing it to liquid.  This process removes the latent heat of vaporization from the liquid, releasing an enormous amount of heat from the refrigerant and sending it outdoors.
Step 3 - the warm liquid refrigerant from the condenser passes through an orifice called an expansion valve.  This reduces the pressure and causes the liquid to begin expanding, and boiling into a low-pressure gas, and like a discharging CO2 fire extinguisher, it gets cold.
Step 4 - warm air from indoors is blown across an evaporator, which boils all the refrigerant into a gas.  The heat of vaporization is transferred from the refrigerant to the indoor air as the refrigerant boils off and absorbs the heat of vaporization from warm air blown over the coils by a fan.  The cool air returns to the indoors, and the refrigerant returns to the compressor inlet.  Pretty simple.

In the winter months this process is reversed. The evaporator becomes the condenser, and the Condenser becomes the evaporator.  In this manner, heat is transferred from outdoors to indoors.  With the addition of a reversing valve, you can see how a refrigeration unit is able to remove heat from the cold outdoors, and transfer that heat to your warmer indoors. The left cycle is cooling, and the right cycle is heating.

Edit:  I just located a really good video about heat pumps.

There comes a point, however, where the outside ambient temperature is so low that it can no longer vaporize the refrigerant.  Heat transfer from outside to inside comes to a halt.  Well before this temperature is reached, the heat pump loses efficiency, and is no longer able to keep up with heat losses from your home.

This outside temperature where the heat pump begins having difficulty using refrigeration to heat the house is where backup heating comes into play.  At an outdoor temperature of 25-35 degrees F, the thermostat should switch over to a backup furnace, and lock out the heat pump.  The temperature at which this transition happens is adjustable in the thermostat programming.

From what I understand, heat pumps can sometimes be stand-alone units in milder climates, such as coastal regions.

Our previous lower-efficiency heat pump had also suffered serious damage.  A small copper capillary tube in the condenser vibrated over the years and developed a fatigue crack.  The crack occurred while we were on vacation, and the heat pump ran for perhaps a week without proper refrigerant or lubrication.  It got pretty hot, and baked the paint off the compressor housing.  I'm fairly certain that this caused at least a minor drop in the unit's efficiency.

The previous backup furnace was an electric furnace.  It had two 240 Volt heaters, each pulling 41 Amps.  Ohm's Law states that Volts x Amps = Power.  240 Volts x 82 Amps = 19680 Watts, whenever the temperature was below 40 degrees.  Freaking 20 Kilowatts of power usage December through March.  No wonder the winter electric bill was $450 per month! 

If we weren't living in a hydropower region, it would be 2 or 3 times that cost.  Of course nobody in their right mind would use an electric furnace where electricity was so expensive, so there's that...

This summer, we decided that it was time to replace the burned up heat pump with a new high-efficiency unit rated at 16 SEER.  We also decided to replace the electric furnace with a propane-fired furnace.  I would have preferred natural gas, but the utility has decided not to expand to our street.  The nearest residence with gas service is only half a mile away... so close!

The heating and air conditioning guys did a marvelous job installing the heat pump and furnace.  The real chore was routing propane lines to the furnace and fabricating ducting for the inlet air and exhaust gas where there had been none before.

The heater is so efficient that the exhaust gas temperature is low enough that PVC pipe can be used for the exhaust duct!  I was a bit surprised by that!

I also had to place on order a larger (500 gallon) propane tank.  We decided to purchase it rather than rent.  The advantage of owning the tank is that we can shop propane vendors for the lowest price, and of course, no rental fees.  The old 100 gallon tank that previously fed our cook-top and gas fireplace was relocated to the shop, so now I have 200 gallons storage at the shop.

My portion of the job was to dig a trench 18" deep from the propane tank location to the connection outside the house.  The larger propane tank requires a minimum of 15 ft clearance to a structure or property line.  We tried to get the tank further away from the house, and into the trees just a little.  This required a 70 ft run of trench, which was not an easy trench to dig.

I rented this thing here:  A Toro TRX-16.

This did most of the work from the grass to the location of the tank.  Our soil is quite rocky, and as it dragged the dirt up, it tended to pile rocks in between the tread and the rolling elements of the tracks.  At that point, one tread would bind up and the thing would twist until I got the rock removed.  You operate this by moving it backwards.

Not a bad trench for an amateur.

Unfortunately, I couldn't get the trencher close to the house.  There were sprinkler lines to worry about, and the old propane tank was right in the way.  It was just too risky, so I had to hand-dig the last 10 feet or so.  The wife suggested the tarp to keep the dirt and rocks out of the grass. This part was a legit pain in the butt.

I'll probably transplant a few saplings in front of it. 

I'm looking forward to seeing how this new heat pump and propane furnace combination impacts our heating expenses.  Hopefully we haven't switched from writing a big check to the electric company to writing a big check to the propane company!