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Old 03-05-2010, 10:52 PM   #1
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Comparison and Heat Pump Selection help...


I thought I had this figured out but know I don't know. I was given some information but I only have the results but not the full calculations.



What I have: Cost for 1 million BTU's of heat
  1. Electric ($.0965/kwh) = $30.75
  2. LP ($2.35/gallon) = $25.65
However, I was just given propane at $3.59 a gallon! What are the full formulas and or calculations that were used to arrive at these numbers? I was given these by my local power company. Just using division I got $39.18 per 1 million BTU now for propane. Without the calculations I can clearly see electric heat is cheaper than propane per BTU.

How much additional savings would I have going with a two-stage heat pump vs. a single stage? Where I start having questions is related to the delta T. I plan on having the low balance point somewhere around 35 degrees and bound to go lower with the price of gas. At 35 degrees the Delta T rise is about 22.1 but is only 21.8 in high stage why? Is it lower or is that additional? This is for a Goodman 18 SEER. Is this two-stage concept more for cooling than heating?

At some unknown point, the "warm comfort" feel of the cool air blowing out with the low delta T will feel too cool for us, although we still have "heat." At that point we will switch to hydronic heat.

One advantage of my thermostat is I have control & flexibility over the 1st and 2nd stage set points and what is the second stage. I can either have 1st and 2nd stage heat pump OR single stage heat pump and 2nd stage hydronic heat. This will clearly give a "warm comfort" but higher cost. Two-stage heat pump can always be programmed manually or using automation to switch to "emergency heat" which will give us our "comfort" back.

Which configuration in the long run is best? Single or Dual Stage Heat Pump?

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Old 03-05-2010, 11:32 PM   #2
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Comparison and Heat Pump Selection help...


I use two setbacks per day one starts 15 minutes before I leave for work which lowers temperature 15 degrees, essentially turning it off. The other is when we go to bed, the same 15 degrees. We currently have hydronic heat only (need to buy heat pump, purpose of this thread) on one half and a single stage heat pump (potentially needing replacement) on the second half of house. When the systems recover my automation system turns them on at the same time the hydronic has reached desired temp in half the time.

So where are the savings? Are mathematically the same amount of BTU's used just spread over a longer time? Are the only potential savings during periods when temp is being maintained, so is a two-stage heat pump only useful if you don't do setbacks?



From Spec sheets:
  • 16 SEER at 35 degrees single stage delta T 21.1 at 2.37Kw
  • 16 SEER at 35 degrees 1st stage delta T 23.2 at 1.78Kw and 2nd stage delta T 23.0 at 2.46Kw
  • 18 SEER at 35 degrees 1st stage delta T 22.1 at 1.73Kw and 2nd stage Delta T 21.8 at 2.46Kw
Am I reading this right

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Old 03-06-2010, 06:00 AM   #3
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Comparison and Heat Pump Selection help...


Well. First. 1,000,000 BTUs of heat delivered to the house by electric resistance heat at .0965 KWH is a cost of $28.27(293KWHs equals 1,000,000 BTUs).
1,000,000 BTUs of heat delivered to the house by LP at 95% efficiency and a per gallon rate of $2.35 is $26.89, at 80% efficiency its $31.93

At $3.59 a gallon it comes out to $41.08 at 95% eff, and $48.78 for 80% efficiency.

Use an electric resistance heat efficiency of 100%. And use your boiler or furnaces rated efficiency.
With a boiler or furnace. Use 92,000 BTUs to the gallon for LP. Divide 1,000,000 by 92,000. Then multiply that by(100 divided by burner efficiency) to get the gallons you will use to put 1,000,000 BTUs into your home. And then multiply by your LP rate.
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Old 03-06-2010, 06:06 AM   #4
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Comparison and Heat Pump Selection help...


Second.

2 stage equipment is for comfort. Not for savings on heating or cooling cost.
Any real savings you get from 2 stage. is from being able to raise the stat in summer, or lower it in winter. And still be as comfortable as you were with thew lower setting in summer, and higher setting in winter.

When you say hydronic back up. Will that be baseboard, cast iron rads, or a hydronic coil in the air handler. it makes a big difference on how you should set up the system to operate.
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Old 03-06-2010, 06:22 AM   #5
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Comparison and Heat Pump Selection help...


If you use a hydronic coil. Then you can have a much high efficiency operating system.
Since you can bring on the hydro heat with the heat pump, and have a warmer discharge air temp at lower outdoor temps. And still get the savings from the heat pump. Over using just LP.
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Old 03-06-2010, 12:00 PM   #6
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Comparison and Heat Pump Selection help...


Quote:
Originally Posted by beenthere View Post
If you use a hydronic coil. Then you can have a much high efficiency operating system.
Since you can bring on the hydro heat with the heat pump, and have a warmer discharge air temp at lower outdoor temps. And still get the savings from the heat pump. Over using just LP.
This is great! The Rinnai hydronic air-handler works with either a single stage or two-stage heat pump. My thermostat can turn off heat pump at a preset low temp.

So comfort is air quality and mostly humidity control?

We are cold natured so heat is set at 72 evenings and 75 in morning, cooling maybe 76 so the fringe benefit isn't for us. Set back away is 55 degrees in winter, summer set back hasn't been determined since we moved in August of last year. Probably 85

Will add steam humidifier
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Old 03-06-2010, 12:17 PM   #7
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Comparison and Heat Pump Selection help...


Humidifier requirement is determined your homes construction. Not what type, or how many stages of heat you have. So if it needs a humidifier now, it will need one when you put in your new system. Unless you tighten up your home.

Weather or not you need a steamer, depends on how much humidity your home needs. Which is determined by how leaky your house is.

Only time to shut off the heat pump is when its COP is too low. And with a $3.59 per gallon LP cost, that would be COP around 1.2, which is probably somewhere around 0 to -10F.

By using the LP as aux heat, while the heat pump is running. You can utilize the higher efficiency of the heat pump under your areas ambient conditions. And control water temp to the hydro coil. To get a nice warm air at lower temps. But still have a long run time to provide both even temp through out the house,and also long run times help to prevent that cool draft feeling as soon as the heat is off.

With your automation system. Can you use a temp sensor to bring on your tankless, And circulate just enough water to give it an extra 10 or 20 of temp rise when the outdoor temp drops below X degrees?

if so, you can set it up to be a very comfortable, and efficiency system..
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Old 03-06-2010, 09:34 PM   #8
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Comparison and Heat Pump Selection help...


Quote:
Originally Posted by beenthere View Post
Humidifier requirement is determined your homes construction. Not what type, or how many stages of heat you have. So if it needs a humidifier now, it will need one when you put in your new system. Unless you tighten up your home.

Weather or not you need a steamer, depends on how much humidity your home needs. Which is determined by how leaky your house is.

Only time to shut off the heat pump is when its COP is too low. And with a $3.59 per gallon LP cost, that would be COP around 1.2, which is probably somewhere around 0 to -10F.

By using the LP as aux heat, while the heat pump is running. You can utilize the higher efficiency of the heat pump under your areas ambient conditions. And control water temp to the hydro coil. To get a nice warm air at lower temps. But still have a long run time to provide both even temp through out the house,and also long run times help to prevent that cool draft feeling as soon as the heat is off.

With your automation system. Can you use a temp sensor to bring on your tankless, And circulate just enough water to give it an extra 10 or 20 of temp rise when the outdoor temp drops below X degrees?

if so, you can set it up to be a very comfortable, and efficiency system..
Yes I can. To do this I would definitely have to go single stage. The thermostat will treat hydronic coil as second stage. I would then just have to set the second stage differential at a point where it will kick in an add the additional heat.

Thanks so much this board is GREAT!

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