Expected temperature rise

[alben]

I will reiterate what is in my signature.

Minnesota climate.
17K gallon 18X36 inground pool.
75K BTU natural gas heater.
1 HP pump.

My question is what can be expected for a temperature rise of my pool? It seems like it takes forever to heat my pool. I do not know what is normal. I did an experiment where I measured the pool temperature. Then I measured the temperature of the water from the return jets. There is a 2 degree rise.

What concerns me is that my heater is 30 years old (yes, 30 years old!) and perhaps the exchanger is plugged or not working properly. I replaced the burner tray 5 years ago. The flames from the burners are nice and blue. The heater when on consumes 55 cubic feet of gas per hour.

Perhaps the real problem is that the heater is undersized?

 

[chem geek]

Ignoring heat losses (which you can figure by seeing how quickly the temperature of the pool drops when the heater is off), a 75K BTU output heater would raise 17,000 gallons of water by 0.53ºF per hour. In practice, your heater is probably 80% efficient or less so probably 60K BTU/hr or less for output heating). A 2ºF increase you are measuring implies a roughly 4 hour turnover or around 70 GPM flow rate. 55 cubic feet of gas per hour is roughly 55,000 BTU/hr so that sounds too low since your 75K BTU heater rating is higher than that.

Depending on how quickly you are expecting your pool to heat up, I'd say your heater is undersized for your pool. For my 16,000 gallon pool, the heater is 250K BTU/hr input and 200K BTU/hr output (80% efficient) and increases the pool temp at around 1.5ºF per hour at the most and that's reasonable for us.

Richard

 

[alben]

Thanks for the quick reply. The 55 cubic feet per hour was based on reading the gas meter at two points in time (over several hours) while the heater was on continuously. Then calculating.

Doing a bit of research apparently 1 cubic foot gas is approx 1020 BTUs. So really my 75K BTU heater should be consuming about 73 cubic feet of gas per hour, right? Maybe there is an adjustment on the gas valve somewhere?

You confirmed my suspicions that the heater is under sized. On cooler days it seems that the heater is "just keeping up" and maintaining the overall pool temperature through a 24 hour period. On warmer days, I am getting maybe a 1 degree per six hour rise (4 degrees in 24 hours) in overall pool temp.

I think your answered my question, my heat exchanger is probably working fine (not plugged/scaled up) in that it is doing what it is supposed to do. It is just way undersized. Right?

 

[luckyman_apd]

The gas heater that was here when we bought the house was rusted out and it leaked, so we never used it and just got rid of it. I installed a solar heater instead, but I still was losing heat every night. Long story short, look into a solar blanket. It will help keep the heat in at night. I got an 18 x 36 at Lowes, yes Lowes, for $112. Well worth it and I never have more than a 1 degree loss overnight. I haven't lost anything overnight in the last week. If your heater is undersized, a blanket may help it be less undersized, if that makes any sense

 

[alben]

I am using a super tight mesh safety cover for safety reasons (kids/pets/wild animals). So a solar blanket is not an option for me. No room for solar panels either.

Anyway I just took a new gas meter reading today. The reading that I gave in my earlier post was from 2 years ago. To my surprise I am consuming a lot less gas now. I timed 3 min 36 sec for 2 cu ft of gas. This works out to 33.3 cu ft per hour. Our gas rates are $0.00908 per cu ft (in case you are curious).

I am consuming less than half the gas that the heater should be. So I am thinking that my gas valve on my heater needs to be adjusted. It also explains why I don't hear the loud "whoosh" I used to hear when the heater turned on. Now it is just a faint "whoosh".