Calculating per-hour cost of natural gas heater.

[PrivatePilot]

Ok, I'm well versed with calculating electricity usage - I can calculate (down to the penny) how much anything electrical costs to run, but I'm not finding the math as easy when it comes to our pool heater. I'm looking to do some comparisons this year on the costs associated with running our natural gas heater at night versus running our solar setup but paying peak electrical rates for the pump during peak solar hours.

In order to figure out what's the best method to go with I need to know the per-hour cost of the gas heater.

So, I have a RayPak 130,000BTU heater.

Our natural gas rates are a jumble of different fees and such (delivery, cap and trade fees, transportation/delivery fees, etc) but I looked at a bill from last summer and we used 140M3 and the actual gas cost $47, so that would come out to about $0.33/M3.

Unfortunately in the jumble between therms, cubic meters, and BTU's my head starts to spin.

Thanks in advance for the help.

 

[needsajet]

First up, make sure the 130,000 Btu per hour is the input rating. Gas appliances will have input and output figures, and it's the input figure you need, difference mostly being heat lost through the stack. Your 130K is probably the input rating, reflective of gas consumption rather than heat energy put into the water. If so, output rating might be around 105,000 to 110,000 Btu/h

Use only the marginal rate from your gas bill. For example, you pay your customer charge for having the gas service. This doesn't go up if you use more. So consider only the variable costs, probably volume of gas times price per volume and price for storage and delivery. Add any discount or tax % to the number.

So now you have an incremental usage rate per cubic meter, aka marginal rate.

Gas varies in useable heat energy content, but for round numbers, one cubic metre of natural gas contains around 35000 to 36000 Btu, so for a 130,000 Btu/h heater, you might pick 3.7 cubic metres per hour which would get you very close, or even 4.0 is reasonable and conservative. Your utility is probably compelled by law to tell you the heat content of the gas they sell, if you ask, or maybe it's shown on the bill in gigajoules. One gigajoule = 1.055 million Btu.

If your gas meter reads in cubic metres, it might be helpful to turn all other gas consumption off, start the pool heater, then record time and number of units. The utility will use a correction factor against the meter reading, so this is also approximate, but might give you a cross-check estimate.

If heating requires you to pump in excess of normal filtration needs, then you'd need to add the electricity for the pump. If by any chance you have a power vent, you'd need to add the electricity for that, but it won't be much.

 

[Esloser]

I’ll see if I can hunt down the math for you. I’ve seen it in a thread on here before and used it in my own calculations.

Here it is:

The formula is pretty simple.

Take the gallons of the pool and multiply by 8.3. That gives you the weight of the water. Take 20% off of the BTU rating of the heater. (example, 400,000 BTU heater - 20% = 320,000 BTU's)

Divide the BTU number by the weight number and that will give you the number of degrees of temp rise per hour under ideal conditions.


Example: 5000 gallon pool with a 200,000 BTU heater

5000*8.3 = 41,500 lbs of water

200,000-20% = 160,000 BTU's available to the water/hour.

160,000\41,500 = 3.8 deg/hour of temp rise.

Keep in mind this does not account for any heat loss due to air temps being colder than the water or heat gain from the air being warmer than the water.

If you are heating with Nat gas, your billing will give you a cost/therm for the fuel. To make the math easy, Lets assume it is $1.00/therm. (a therm, sometimes referred to as CCF (100 cubic feet) is approximately 100,000 BTU's)

Using the above heater at 200,000 BTU's of input, it will consume about 2 therms/hour for a hourly run cost of $2.00.

Dan

 

[PrivatePilot]

If you are heating with Nat gas, your billing will give you a cost/therm for the fuel.

That's the problem - it's not measured in therms here, it's measured in cubic metres.

The thing is, I'm not sure how to calculate how many M3 of gas the heater uses every hour. If I did, yes, the math would indeed be simple.

 

[needsajet]

Approximately 3.7 cubic metres per hour (provided 130,000 btu/h is the input rating).

 

[PrivatePilot]

Hmm, OK, thanks - so $1.22/hour then by my calculations. That's more than I expected, especially after I round in the cost of running the pump as well, although off peak is only ~$0.06/kWh, but it's a cost nonetheless.

Well, that narrows it down then - running my pump for my solar panels during the day (at even the peak rates of $0.15/kwh, with my VS pump using about 1kw/h when in solar mode) is still WAY cheaper.

 

[PoolGate]

Hmm, OK, thanks - so $1.22/hour then by my calculations. That's more than I expected, especially after I round in the cost of running the pump as well, although off peak is only ~$0.06/kWh, but it's a cost nonetheless.

Well, that narrows it down then - running my pump for my solar panels during the day (at even the peak rates of $0.15/kwh, with my VS pump using about 1kw/h when in solar mode) is still WAY cheaper.

Does $1.22 include all gas costs? My gas actual costs are twice my per therm charge (gas) for distribution and every other extra thing they add into the bill.

 

[PrivatePilot]

Does $1.22 include all gas costs? My gas actual costs are twice my per therm charge (gas) for distribution and every other extra thing they add into the bill.

I took the full dollar value of one bill and divided it my the cubic meters used, so yes, although probably not 100% accurate (as some of the delivery fees and such are sliding depending on consumption) it at least gives me a ballpark figure.

 

[needsajet]

You shouldn't include your daily/monthly service cost. You have to pay that whether you heat the pool or not. (unless the pool heater is your only gas appliance)

 

[skimmerswimmer]

For reference, I have a 335k btu Raypak natural gas heater on it's own meter and it costs about $3 per hour to run.