HVAC Experts: Calculating Heat Pump Efficency

[LD1]

Just wondering if my system is operating at an acceptable level. It keeps my house warm just fine but I took some temp readings and couldn't find any of the formulas for calculating anything.

Ambient outsid temp this morning when I took the readings was 44*F

Air comming out of the registers is 96*F
Return Air at the vents is 72*F

Copper line coming into the heat exchanger is 166*F
Return line is 76*F

Blower is 1000CFM And all measurments were with the same instrument.

Just wondering if this sounds like an efficient system.
I like that the exchanger is dropping 90* between the inlet and return. Almost cooling the return down to the temp of the air blowing aross it.

What concerns me is that At the registers I am only gaining 52* over ambient outside air.

Thus unless I am not accounting for something, @ 20* outside, I would only be blowing 72* air inside.

With the given prices in propane I have had the unit set to switch to prop @ 17* and have not had a problem for the last 4 years. Yes My electric bills went up a bit, but not neatly as much as using 500 gal propane every year like the first two years I was here. I have went 4 winters on this current fill and it is still @ 30%

So just for My benefit, I was wondering how to do some of the calculations if anyone of you guys are HVAC experts.

Much Appreciated.

 

[arrow]

Heat pump efficiency is measured at the factory. Your heat pump efficiency may be plus or minus from the factory numbers. Here is a formula for measuring efficiency during the heating season which the general consensus agrees is more accurate than the COP (coefficient of performance) figures. Its like how much horsepower does your engine produce even though it is stated it has 245 hp from the factory. On a dynamometer is the only way to figure what YOUR engine produces.

The Heating Season Performance Factor - HSPF - is a measure of the overall heating efficiency of a heat pump.

HSPF = hs / Pws 3413
where

hs = heat produced during the season (Btu)

Pws = elctrical power consumed during the season (kWh)

A HSPF in the range of 5-7 is acceptable and around 2-4 for air heatpumps. Of course you do not have to go thru an entire season but during the heating season, compare your output to your kwh from one heating month to the next that have similar mean temps. You cannot figure your current heat output and expect to get that same differential when outside ambient temps get colder. Your heat pump is going to work harder as it gets colder. You will use more kwh's as a result and end up with a different number which could end up being less or more. It all depends on what YOUR heatpump characteristics are and is why efficiency is measured between heat (btu) produced vs. kwh needed to get to an efficiency point number.

 

[Sigarms]

I'm not expert, but here's my take on it...

I'm assuming you have a 2.5 ton (30,000 BTU) heat pump system?

Long story short, with any heat pump, in heating mode, the lower the outside temp, the system capacity for heating diminishes.

One thing you need to look at (which people rarely do) is look at the coefficient of performance (or COP) of the unit in question.

All manufacturers have this information available for each specific unit, just need to know where to look.

It keeps my house warm just fine

The first couple of questions I'd ask is how old is the unit, which make, model and serial of outdoor AND indoor blower (to verify the AHRI rating and ensure system performance) and have you seen any fluctuation in your utility/fuel bill?

Heat Pumps Performance and Efficiency Ratings

 

[LD1]

Heat pump efficiency is measured at the factory. Your heat pump efficiency may be plus or minus from the factory numbers. Here is a formula for measuring efficiency during the heating season which the general consensus agrees is more accurate than the COP (coefficient of performance) figures. Its like how much horsepower does your engine produce even though it is stated it has 245 hp from the factory. On a dynamometer is the only way to figure what YOUR engine produces.

The Heating Season Performance Factor - HSPF - is a measure of the overall heating efficiency of a heat pump.

HSPF = hs / Pws 3413
where

hs = heat produced during the season (Btu)

A HSPF in the range of 5-7 is acceptable and around 2-4 for air heatpumps. Of course you do not have to go thru an entire season but during the heating season, compare your output to your kwh from one heating month to the next that have similar mean temps. You cannot figure your current heat output and expect to get that same differential when outside ambient temps get colder. Your heat pump is going to work harder as it gets colder. You will use more kwh's as a result and end up with a different number which could end up being less or more. It all depends on what YOUR heatpump characteristics are and is why efficiency is measured between heat (btu) produced vs. kwh needed to get to an efficiency point number.

How would one go about figuring out how many BTU's are being produced without expensive equipment. I know I am flowing 1000CFM and I can measure all the temps. Is there a formula to figure BTU's based on how many degrees I am warming that 1000 cubic foot of air in that given minute??????
Pws = elctrical power consumed during the season (kWh)

 

[arrow]

btu output is measured by the hour. There is much math involved. The easiest way to do this is to see how many kwh's your hp is using to produce its rated output. Apply your kwh production when the heating system is not being used vs. when it is to get an idea of output of your unit. You are asking about efficiency correct?
You may also atempt to try a btu calculator you can find online. This will ask what temp you wish to have the inside. This will not necessarily tell you what your unit is producing but will tell you how many btu's are needed to heat the space.
BTU's can only be measured by instument in an enclosed space. For large house type spaces you need math. If it is of any help 3.4 btu is equivalent to 1 watt of electricity. Get the amount of btu's to heat your space to a certain temp using the calculator and compare how many kilowatts you actually use during heat pump season multiplied by 3.4. then compare the ratio of btu needed to produce the desired temp for the space vs. the needed amount of kwh converted to btu to produce that temp. 3-5 with water is good and 2-4 is good with an air hp.

 

[LD1]

Arrow, you have been uch help already so bear with me this will probabally be a longer post than I intend.

Basically I am trying to figure out at what point I should start using propane instead of the HP based on cost and BTU. We all know that the colder it gets, the less efficent a HP gets. And I want to moniter its output at different temps this winter to come to my conclusion.

Currently with a Propane backup @ 2.00 per gallon and 91,330 BTU in a gallon of prop, with a 90% furnace, I am getting ~ 82000 BTU for $2. Or I am paying $.024 er 1000BTU.

With 1 watt being = to 3.41 BTU, it takes 293 watts to make 1000BTU. With electric rates @ .10 that is .0293 per BTU straight elec. Which is only slightly better than PROP.

Now what I am wanting insight and conformation on is I found that 1BTU will heat 55 cu ft of air 1 degree per hour.

Using that, with the #'s I listed in the first post, I am getting a 24* temp increase @ my vents.With a 1000cfm blower, that is 60,000 cu ft per hour. 60,000 x 24 degrees divided by 55 I am comming up with 26,181 BTU for my heat pump given current conditions.

The heat pump uses 12A at 241V which is 2892W. Which is about 9 BTU per watt, or about 2.6 times more efficent than electric. Which measures out to $.011 per 1000BTU as compared to .024 with prop or .029 with elect.

This is what I want to continue calculating as the temp drops to see at which point I will be money ahead to switch to propane. Does this make sense or am I missing anything??

I am not factoring load loss, because regardless of what system I use, the hous is still the same and load loss will be the same. Last year, and in years past when my electric was .08 per KWH and propane was 2.50 and higher, it was a no brainer to use as little propane as possible. But propane has slowly came bach down and electric has went up since last winter so It got me thinking about this more and more.

Thanks forthe input.

 

[arrow]

The ratio of CA (cost avoidance) between your pump and propane is 2.6. Which means that you will be at a break even point when your propane use overcomes its ratio of electricity used to produce a given amount of btu to get to your desired temp. There is one basic tenant that filters into all of this. An hp does not produce heat, it moves it. Propane is a fuel to produce heat. You will only come to a break even point when you are asking the propane to produce the same heat (btu) as the hp is using electricity (3400 btu = 1 kwh) to produce the same inside temp from extraction. Right now your most critical profile will be of kwh used by your hp at different points of the year. You have all the rest of the figures to extrapolate efficiency ratings as they compare to propane use.
As this is part of my current job, I need to make presentations to various municipal CDP's and such about HVAC installations.. As I cannot get into too much math with groups in order to keep their attention, I need to simplify cost avoidance so everyone can understand. I hope you were able to glean something from my communication and apologize if I did not get to the meat of your question as a result of my own habits.

 

[LD1]

Not quite sure I understand what you were talking about in your last post.

The heat pump 2.6 times more eff. than electric heat and only ~2.1 times more cost effective than propane.

But are you basically agreeing with me to figure this the way I am based on a cost per BTU basis and go with the cheapest BTU's.

Basically I took the #'s and the math from my last post and put in a spread sheet that way I only have to change a few #'s and have the new results.

So assuming .1 per KWH and $2.00 for propane and 72 degrees maintained in the house, my break even point would be 83* heat out of my vents. This 11 degree spread based on above math gives me right @ 12000 BTU. With the given energy that the heat pump uses, that is the same cost per BTU as the propane would be @ that 11* swing.

So If I understand you correctly, You are agreeing that I need to use propane at this point and the heat pump above this point, provided that is is enough to heat the house. (It may be cheaper per BTU but just may not be enough BTU i dont know yet)??

Provided that the prices dont change, and if this is correct logic, I will me monitering the performance and temps as the outside air drops. Once I find the point that I am only increasing the inside air by 11 degrees, this is tho point that I will set the heat pump to. Which is currently @ 17*.

I forgot to mention also, that once Propane reaches 2.40 per gallon and if electric stays @ .1, it is a wash. Same BTU's per dollar. So if electric comes down any or propane shoots up a bunch, I may jsut run little electric heaters.

 

[Sigarms]

This is what I want to continue calculating as the temp drops to see at which point I will be money ahead to switch to propane. Does this make sense or am I missing anything??

Defrost cycle on the heat pump.

Once I find the point that I am only increasing the inside air by 11 degrees, this is tho point that I will set the heat pump to. Which is currently @ 17*.

On "average" a 2.5 ton 13 SEER heat pump at 17 degrees outside temp drops it's COP down to 2.3 and it's heating performance down to 16,000 BTU's.

I am not factoring load loss, because regardless of what system I use, the hous is still the same and load loss will be the same.

You haven't considered short cycling, and yes, depending on the load of the house vs the actual system capacity can have an effect on system performance.

 

[LD1]

Defrost cycle on the heat pump.

Good point. I forgot about that. This I guess when I get close to the switch over point, I will have to moniter how frequent and how long the defrost cycle lasts. But If memory serves me correctly, It is only about 60 seconds long, and it doesnt rnt the blower in the house or outside, and it only occures maybe 2 times per hour at max. So...If it did it twice per hour, all day, every day in the month, I think that is like $6.72 per month. But I'll moniter it closer this winter for sure.

 

[arrow]

Sigarms brings up a point that I am not sure you have considered or not. His assertion about short cycling could bring all this math being done to a moot point. If it is warm outside such as 60-65 and you want your house at 68, depending on your settings, it could cause an on-off situation with your pump. Ordinarily short cycling is not a normal occurence and they're are several causes but engaging the heat pump for only a couple -3 degrees of inside temp increase could cause this situation. If this be the case, then you need to run the propane.

 

[Chilly807]

Interesting discussion to say the least. We've never used our back-up system, since we know that the heat pump is more efficient, at least in our case.

Ours is geo-thermal, which I realize is a different set of variables altogether.

I hadn't considered that propane could be more cost-effective than a heat pump, but depending on the loss of efficiency in cold weather as well as the cost of propane it does make sense.

Having said that, there may be an easier way of figuring this. We know that our heat pump system, including the pump, circ pumps, etc, draws "x" number of amps when it's running. If you know the number of hours per day it runs, then you can easily calculate the cost of heat.

The propane is a bit harder. I'm thinking you don't have a flow meter for the propane backup, if such a thing even exists. If you do, you're golden, since it's easy enough to calculate the cost of using propane then.

Generally speaking, if our heat pump runs 24 hours a day, it costs us about $5 per day for heat. We augment that with burning wood, but I'm pretty sure the wood is more expensive if we had to buy it than running the heat pump. In other words, it actually costs us more to try to save money by burning wood. Unfortunately, we both dearly love wood heat, so it's a moot point.

To get back to the question, I'd try using the propane backup for a given period, and see what it costs you. Once you establish a cost/day for a given temperature, you can relate that to how many kw/h you use during a period of the same general temperature using only the heat pump.

Not an exact method by any standards, but roughly effective. We consider the heat pump/geothermal system to be cost-effective right up to the point where it can't keep up with the heat loss during extreme cold weather (relatively speaking). Anything below -20 to -25 C, we need to burn a bit of wood to keep warm. There is also a water heater backup which we could use, but never have (we have infloor radiant heat).

Sean

 

[LD1]

Interesting discussion to say the least. We've never used our back-up system, since we know that the heat pump is more efficient, at least in our case.

Ours is geo-thermal, which I realize is a different set of variables altogether.

I hadn't considered that propane could be more cost-effective than a heat pump, but depending on the loss of efficiency in cold weather as well as the cost of propane it does make sense.

Having said that, there may be an easier way of figuring this. We know that our heat pump system, including the pump, circ pumps, etc, draws "x" number of amps when it's running. If you know the number of hours per day it runs, then you can easily calculate the cost of heat.

The propane is a bit harder. I'm thinking you don't have a flow meter for the propane backup, if such a thing even exists. If you do, you're golden, since it's easy enough to calculate the cost of using propane then.

Generally speaking, if our heat pump runs 24 hours a day, it costs us about $5 per day for heat. We augment that with burning wood, but I'm pretty sure the wood is more expensive if we had to buy it than running the heat pump. In other words, it actually costs us more to try to save money by burning wood. Unfortunately, we both dearly love wood heat, so it's a moot point.

To get back to the question, I'd try using the propane backup for a given period, and see what it costs you. Once you establish a cost/day for a given temperature, you can relate that to how many kw/h you use during a period of the same general temperature using only the heat pump.

Not an exact method by any standards, but roughly effective. We consider the heat pump/geothermal system to be cost-effective right up to the point where it can't keep up with the heat loss during extreme cold weather (relatively speaking). Anything below -20 to -25 C, we need to burn a bit of wood to keep warm. There is also a water heater backup which we could use, but never have (we have infloor radiant heat).

Sean

I have no good way of metering the propane other than going off of the 91xxx BTU per gallon times my 90% eff furnace figures that I used earlier. In this is probabally a BEST case scenerio. It is based on getting a full 91000 BTU for each gallon and ASSUMING I am getting the optimal 90% eff out of my furnace.

But just based on past experience, The first year I lived at the house, that winter we used a whol 500 gal tank full of propane (400 actual gal @ 80%). Propane was a little cheaper than but at todays rate, that would be $800 for a roughly 6 month heating season here in central OH. Or $133 per month just for the propane. This was with the heat pump set @ 38 degrees F cutoff. My electric bills that winter over the 6 months averaged 120.

I lowered the setting to 17 degrees and the following two winters, my bills averaged 180. So it cost me $60 per month more in electric. At the end of those two winters my tank was @ 50%. Which means I only ised 30% (150gallons) for all of both winters. Which is $25 per month of propane. Add that to the $60 increase in electric bill and it is costing me ~ 85 per month to heat vs the 133 per month before. Almost a $50 savings.

Right now I am headding into my 5th winter on that same tankfull of propane and it is still @ 30%. My electric bills have been higher ($200-250/month), but that is due to things I have added. Such as a 30x50 garage that I am in all the time. Motion detector halogen lights that stay on more than I'd like, and a 3000 gal goldfish pond that has a 250w pump that runs all the time. Not to mention the elec rates have went up. So I cant really compair now to when I first moved in and the HP was set @ 38. But I can compare summertime bills and they have went up about 50-60 per month since then, so my winter time is probabally not far off.

 

[Chilly807]

Is there any information from the heat pump supplier as to the lowest ambient temperature it is designed to work at? They have made some inroads in the past five years or so, the new ones are rated to much colder ambient temps.

Our primary consideration is the rate of heat loss from the house. The heat source (ground temp) stays pretty consistent for the most part during the heating season, goes down to maybe 48-50 degrees over the winter.

During the design phase of our system, they built it around the premise that the heating system would provide 70% of the heat loss in BTU/hour from the house. In other words, if the heat loss was 10,000 BTUs, the system needed to be capable of 7000 BTUs. We have a 3 ton system, which equates to 36,000 BTU/hour.

Part of the challenge is the varying rates for fuels, be they electricity or fossil fuels.

Sean

 

[LD1]

All I know is that is is a rheem 2.5ton unit that was new when the house was new in 1996. So it is almost 15 years old. My house is only 1320 sq ft but also has a full basement that is not counted in the sq ft and is heated too.

It is also a 10 seer

 

[Richard]

Here's a question since you're talking about heat pumps....

Within a similar range, does a heat pump use as much electricity to heat in the winter as it does to cool in the summer?

(Presuming the heating element does not turn on)

I've always felt that they use more in winter but it dawned on me the other day, since they work in reverse, they should be 'similar' ??

 

[LD1]

I think they are the same. It is still running the same fans, and the same compressor. Compressing the refrigerant to the same pressure. It is just flowing in reverse.

Similar to a hydraulic motor. Same flow in both directions and same power.

I have also confirmed this with the meter. AC and heating mode are within a tenth of an amp. At least on my unit.

 

[LD1]

Just a little update

36 degrees out this morning.

72* air going in the return vent and 94* air comming out.

Using my above mentioned formulas (I made an excell spreadsheet sho it is simple as entering two numbers)

I came up with 24,000 btu's this morning @ the 36* outside.

Which is still 2.43 times more efficent than straight electric.

It is currently costing me .12 per 10,000BTU vs .24 with propane @ 2.00 per gal vs .29 with straight electric heat.

Si I am still looking good down to 36*

 

[Chilly807]

I'm going to wander way out on a limb here and say you're going to be "good" down to about -7 or -8 C. Beyond that and most air-source heat pumps begin to struggle to keep up.

At this stage, about all you can do is keep running the numbers as the temp drops.

Sean

 

[LD1]

I'm going to wander way out on a limb here and say you're going to be "good" down to about -7 or -8 C. Beyond that and most air-source heat pumps begin to struggle to keep up.

At this stage, about all you can do is keep running the numbers as the temp drops.

Sean

well that is about 17-19 degrees F which is about what it is set at now