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Minimum outside temp for heat pump

604 views 4 replies 3 participants last post by  Steven123cool  
#1 ·
Hi. I have a heat pump with electric aux heat. After years of bring hit with high electric bills during the winter, last month was just under $1000, I decided to swap out my old Honeywell thermostat with an Ecobee which gives me better control over aux heat. The Ecobee gives me the option to set the minimum temp that the heat pump will operate. Any recommendations around how low I can set the temp? The default is 35°. I was thinking 25°.
 
#2 ·
OK - lets explore this a bit...

Let's say a 3 ton heat pump - what this means is the unit is able to cool a home down using 36,000 BTUs on a normal day. As it gets much hotter outside, the actual BTUs start to drop a bit. It is just a fact of refrigeration. Now what happens when we go into heating mode. Start at 60°F - this is about when you need to heat the home. The unit is pretty close to matched the opposite of the cooling spectrum. So we would expect 36,000 BTUs of heat. We also need to understand that a heat pump uses refrigeration or compression to our advantage. At that 60° we get a 3 to 1 ratio, meaning for every kilo-watt of electricity we use, we get 3 kilo-watts of heat. This is what makes a heat pump so great. But now what happens when we hit 50°F outside? Well the compression ratios change and that 3 to 1 is no longer 3 to 1, as there is less heat to gather from the outside as there was at 60°F. So now we are in the area of 2.5 to 1. Then when we hot 40°F outside we are closer to 2 to 1, once we hit in the area of 30° that ratio is around 1.5 to 1. Now how are the BTUs in this same exchange? That 36,000 is no longer even close, more like 30,000 at 50° and around 25,000 BTUs in then 40° area. The colder it gets, the less heat it can grab from outside. You will see ads stating the heat pump can run all the way down to 0°F, sure it can, but that ratio is .5 to 1 and the 36,000 BTUs is now 10,000 BTUs. Again, just an estimate or idea what is happening using these numbers. Exact numbers have many variables attached. So on a heat pump the cost of operating at 30° is rather expensive, nearly as much as electric heating element. This is why a heat pump in Philly or anyplace where the temperatures get below 40° for a length of time is a dumb idea. Contractors are selling this and making good money doing so, and how they sleep at night is beyond me. I would not set it to 25° as it is more wear and tear on the heat pump and the cost of operation is about the same if not more than running the electric heat strips. 40° to 45° is best. Even 35° is on the low side.

There are other facts that need to be known also. On average, a home with a 3 ton AC would work perfect when we have 90° outside temps and want 70° inside temps, or 30° difference. The problem is, in the heating mode, we have more than a 30° difference in temperature, so it requires more than the 36,000 BTUs to heat when we get below 40° outside. Another reason a heat pump is a poor option for your area. Now if you were in Texas or Southern California, we do not get that cold day, so heat pumps are awesome when natural gas is not available.

Then the million dollar question is, how can you reduce that heating bill? Change the AHU (Air Handling Unit) to a LP furnace, get a LP tank or if Natural gas is available, go that route. Natural gas is 1/3rd the cost of LP or heat pumps, and simpler to run. As for the Ecobee or Nest, not a fan - seen too many issues with those two thermostats, The Honeywell T9 or T10 is much better and cost is about the same, it also has better features, is made by a company that has manufactured thermostats for over 100 years vs 10 - 20 years.

Look into converting to LP or Natural gas, for you, it would be a huge payback. On the average 1800 square foot home, a normal Natural gas bill to heat in the coldest temperatures we would see is around $150 a month. My 1200 square foot home costs around $100 to heat and it is 68 year old home, average insulation - fair to good windows. The same home with 100% electric, which is what would be running right now would cost me around $850 for the same heating Therms. Would not take long to get a payback and upgrade, no mater how old the home is. Do not let these salesman or Want-A-Bee techs tell you otherwise. It is hard to fight the facts, which is all I or others provide on this site. We have nothing to gain, so no reason to lead you in a bad direction on purpose.

Good Luck

Eggs
 
#3 ·
Thank you Eggs for such a detailed response. The house was built as all electric as the development was not piped with natural gas. Other than our 100 gal LP tank for the fireplace, there is not really the room for a large LP tank. One of our neighbors opted for an oil furnace for back up heat, but oil costs are high as well. We are likely stuck with what we have. Looks like were going to get hit with more single digit temps next week :eek: There goes Feb's electric bill!
 
#4 ·
Just so you know - oil is less than 1/2 the cost of electric heat.
The 100 gallon LP tank would deliver only (91,500 BTUs/gal) 9,150,000 BTUs -
I use just under 4 therms per day or 400,000 BTUs
IF I went to Dual Fuel - LP / heat pump (LP for the coldest days) 9.1M BTUs would give me 23 days of run time.
So a 350 gallon tank wound be around 3 months, that 350 gallon tank is not all that much larger than what you have now. It would get you through the coldest days and save a ton of money. Just a mater of finding a location for the tank. You do not need a 800 gallon or 1,000 gallon tank as it is only for maybe 3 months.

Oil on the other hand is 140k BTUs per gallon, so a 100 gallon tank would take my house 35 days, assume your neighbor has a 300 gallon oil tank, it is about the same size as the LP tank.

Funny, just had class two nights ago, going over this same info to my students. People just do not realize this stuff until they do the math, but it help knowing how to do the math.

For the record - ( South East Wisconsin )
1 KW/h = 3412 BTUs or 100,000 BTUs = 29.3 KW/h or in my area (0.22 per KW/h) $6.36
1 gallon of fuel oil - 140,000 BTUs (3.91/gal) so 100,000 BTUs = $2.79
1 gallon of LP = 91,500 BTUs ($2.10/gal) so 100,000 BTUs = $2.30
1 them of Natural gas. = 100,000 BTUs so 100,000 BTUs = $0.93

This means - in my home - Electric cost of heating per month is $769 - LP per month is $278 - Oil per month is $337 - Natural gas is $113
In your home - break this down - $1,000 electric (subtract normal power usage of $150) $850 = 0.1757/KW or 4,838 KW/h for heating or 16,506,545 BTUs
Electric = $850 - LP = $379 - - Oil = $460 - - Natural gas = $153
Would not take long to save enough money to pay for a new furnace at almost $500 a month.
These are real actual numbers - BTUs used, not taking into consideration of efficiency losses which would be in the area of 5-7%.

interesting?

Just food for thought - I can see the brain working from here.

Good Luck

Eggs