Heat Pump vs. Natural gas heater Cost Efficiency

[dcsmith]

I have a Pentair Ultratemp 140 Heat pump (140,000 BTU, 230v @ 50amps) and a Pentair 400,000 BTU natural gas hearer (4 Therms per hour). My current cost for electricity (just electricity-no other fees) is $ 0.145640 per kWh. My cost for natural gas is $1.296980 (just gas - no other fees) per Therm.
My calculations for the equivalent amount of heat output is $4.77 per hour for electricity and $5.19 per hour for natural gas - making the heat pump more efficient.

Am I off base here?

 

[PoolStored]

Am I off base here?

This can help you validate your numbers. Allows you to enter your gas and electrical costs and compare.

Heat Pump Calculator

See how much you could save by switching to one of our pool heat pumps in just three easy steps. Use our Pool Heat Pump Calculator to see your savings today!

www.pentair.com

 

[mas985]

First, the two units heat at different rates so comparing a cost per hour is not really fair. Second, if you are using 230v x 50a for the energy use of the HP, that will be over estimating the cost of the HP. That is a rating, not usage. Energy usage is dependent on COP which in turn is dependent on environmental conditions. Also, you need to take into account the heater BTU efficiency because NG heaters are rated by Input BTU while HP are rated at nominal output BTU.

I have a spreadsheet in my signature that can help with the calculations:

 

[dcsmith]

This can help you validate your numbers. Allows you to enter your gas and electrical costs and compare.

Heat Pump Calculator

See how much you could save by switching to one of our pool heat pumps in just three easy steps. Use our Pool Heat Pump Calculator to see your savings today!

www.pentair.com

It didn't work. It got hung up a message "loading". It only asked for electric information. It was more like sizing a heat pump.

 

[PoolStored]

Interesting. You are second member to report an issue today! Works for me, last step allows I out of costs. Maybe clear your browser cache…

 

[dcsmith]

First, the two units heat at different rates so comparing a cost per hour is not really fair. Second, if you are using 230v x 50a for the energy use of the HP, that will be over estimating the cost of the HP. That is a rating, not usage. Energy usage is dependent on COP which in turn is dependent on environmental conditions. Also, you need to take into account the heater BTU efficiency because NG heaters are rated by Input BTU while HP are rated at nominal output BTU.

I have a spreadsheet in my signature that can help with the calculations:

View attachment 479780

First, the two units heat at different rates so comparing a cost per hour is not really fair. Second, if you are using 230v x 50a for the energy use of the HP, that will be over estimating the cost of the HP. That is a rating, not usage. Energy usage is dependent on COP which in turn is dependent on environmental conditions. Also, you need to take into account the heater BTU efficiency because NG heaters are rated by Input BTU while HP are rated at nominal output BTU.

I have a spreadsheet in my signature that can help with the calculations:

View attachment 479780

This is exactly what I'm looking for. How do I access the spreadsheet in your signature?

 

[dcsmith]

First, the two units heat at different rates so comparing a cost per hour is not really fair. Second, if you are using 230v x 50a for the energy use of the HP, that will be over estimating the cost of the HP. That is a rating, not usage. Energy usage is dependent on COP which in turn is dependent on environmental conditions. Also, you need to take into account the heater BTU efficiency because NG heaters are rated by Input BTU while HP are rated at nominal output BTU.

I have a spreadsheet in my signature that can help with the calculations:

View attachment 479780

First, the two units heat at different rates so comparing a cost per hour is not really fair. Second, if you are using 230v x 50a for the energy use of the HP, that will be over estimating the cost of the HP. That is a rating, not usage. Energy usage is dependent on COP which in turn is dependent on environmental conditions. Also, you need to take into account the heater BTU efficiency because NG heaters are rated by Input BTU while HP are rated at nominal output BTU.

I have a spreadsheet in my signature that can help with the calculations:

View attachment 479780

Found it.

I plugged in the Pentair numbers for both heaters and came out with a major win for the "Cost Winner" being the heat pump. $7.54 per day vs. $16.29 per day for gas. For some reason, that doesn't sound right.

 

[mas985]

With the pump/pool spreadsheets link here:

Hydraulics 101 - Have you lost your head?

Edit - Before reading this article, you may want to first familiarize yourself with this Pool School article on Pump Basics Hydraulics 101 - Pump Head Curves, Plumbing Head Curves and Operating Points Hydraulic Head In fluid dynamics, head is a concept that relates the energy in an...

www.troublefreepool.com

Keep in mind that COP drops with temperature. I think Pentair has a table on their website.

 

[JoyfulNoise]

Doesn’t the COP vary with ambient air temperature? You may need to factor that in to your calculations if you haven’t already.

Heat pumps are great for maintaining a pool’s water temp especially when you use a solar cover. The only downside is you need to run your pool pump and HP all the time (or at least most of a 24 hour period) if you want the water temp to be very constant. With a gas heater, you get instant heat (relatively speaking) when you want it. Mark’s spreadsheet does a nice job of pointing that out.

Since you have a pool and spa, I’m guess you predominantly use the gas heater for the spa and the heat pump for the pool. If so, that would likely be the most optimal setup for heating.

 

[dcsmith]

With the pump/pool spreadsheets link here:

Hydraulics 101 - Have you lost your head?

Edit - Before reading this article, you may want to first familiarize yourself with this Pool School article on Pump Basics Hydraulics 101 - Pump Head Curves, Plumbing Head Curves and Operating Points Hydraulic Head In fluid dynamics, head is a concept that relates the energy in an...

www.troublefreepool.com

Keep in mind that COP drops with temperature. I think Pentair has a table on their website.

That's what I used. I did make some minor changes using 80 deg. (air temp), 63% RH, 80 deg. water temp. This yields a reduced COP of 5.6 and 134,000 BTU. The numbers still come out to more that 2:1 winner for Heat Pump. $8.04 vs. $16.29. Is this accurate? BTW: All I'm looking at is the Pool w/o cover.

 

[dcsmith]

Doesn’t the COP vary with ambient air temperature? You may need to factor that in to your calculations if you haven’t already.

Heat pumps are great for maintaining a pool’s water temp especially when you use a solar cover. The only downside is you need to run your pool pump and HP all the time (or at least most of a 24 hour period) if you want the water temp to be very constant. With a gas heater, you get instant heat (relatively speaking) when you want it. Mark’s spreadsheet does a nice job of pointing that out.

Since you have a pool and spa, I’m guess you predominantly use the gas heater for the spa and the heat pump for the pool. If so, that would likely be the most optimal setup for heating.

That was the original intent. However, if the heat pump is TWICE as cost efficient as the gas heater, I'm considering using it for the pool. I'm thinking I have some numbers wrong in his spreadsheet. I have revised the COP and temp/humidity parameters a bit. Didn't make much of a cost difference.

 

[mas985]

Is the NG heater you are looking at only 84% efficient? Newer ones seem to be higher than that.

Also, if you are planning to use the heater in winter, then you should look at much cooler temperatures where COP is lower.

 

[dcsmith]

Is the NG heater you are looking at only 84% efficient? Newer ones seem to be higher than that.

Also, if you are planning to use the heater in winter, then you should look at much cooler temperatures where COP is lower.

That's what the Pentair MasterTemp documentation seems to indicate.

 

[mas985]

I was thinking of the high efficiency version which is 96% but probably cost a lot more too.

One thing to consider is the heat up time with your spa. It is going to take a lot longer (3x) with the HP than the NG heater.

 

[dcsmith]

I was thinking of the high efficiency version which is 96% but probably cost a lot more too.

One thing to consider is the heat up time with your spa. It is going to take a lot longer (3x) with the HP than the NG heater.

Yes, I know. However, this is what Pinch-a-Penny could get me two years ago. For this discussion, I am interested in heating the pool only. I was just surprised to see the heat pump twice as cost efficient as the gas heater.

 

[mas985]

That isn't always the case, the break even is somewhere are around 5:1 $/therm:$/kwh. For me, NG is cheaper.

 

[JoyfulNoise]

For me, NG would be cheaper too. Our electric rates aren’t terrible but they’re not super low either. Gas is plentiful in our state and we have a pretty robust gas infrastructure so the $’s per therm are pretty low. Our climate is also not well suited for heat pumps as the air is extremely dry all the time. June is our driest month and the RH can get down into the low single digits. Sure the air is “hot” but it doesn’t hold much heat energy so I doubt we’d every get the best performance out of a heat pump.

Then again, heat pumps are getting better every year as they introduce some of the more advanced technology like variable speed compressors. Noise is another factor too (I already have two old and noisy AC compressors so adding another is not optimal) but even in that area heat pumps are a lot quieter than they used to be.

 

[dcsmith]

That isn't always the case, the break even is somewhere are around 5:1 $/therm:$/kwh. For me, NG is cheaper.

Now you’ve got me really confused. Your spreadsheet indicates for the same amount of temperature rise, the heat pump is twice as efficient.

What am I missing?

 

[wgillette]

However, if the heat pump is TWICE as cost efficient as the gas heater, I'm considering using it for the pool.

How were you using your heat pump before?

I've been working this worksheet today as well. I have a cover and a Pentair 140H/C. I ran the numbers with a low COP and higher COP to compare.

 

[PoolStored]

I've been working this worksheet today as well. I have a cover and a Pentair 140H/C. I ran the numbers with a low COP and higher COP to compare.

Why is your BTU loss per day different between the two columns if you are just comparing COP differences?