Heater run time

[valdisr]

I’ve been wondering if the way my pool heater is running is going to cause it to fail early.

I have a 400k BTU heater with a 3k gallon pool. This is open year round and set to 80. I have an automatic cover which helps retain heat. The heater comes on for about 5 minutes at a time each hour or so to keep it at temperature.

I wonder if it turning on/off multiple times like this a day is bad for the heater? Would I be better off to schedule it to heat to say 85 in the morning, then back down to 80, then 5 hours later back to 85 to make the heater run less often but for longer?

 

[ajw22]

I’ve been wondering if the way my pool heater is running is going to cause it to fail early.

I have a 400k BTU heater with a 3k gallon pool. This is open year round and set to 80. I have an automatic cover which helps retain heat. The heater comes on for about 5 minutes at a time each hour or so to keep it at temperature.

That should not wear out the heater.

I wonder if it turning on/off multiple times like this a day is bad for the heater? Would I be better off to schedule it to heat to say 85 in the morning, then back down to 80, then 5 hours later back to 85 to make the heater run less often but for longer?

With a small pool and a large heater why not heat on demand when you anticipate using the pool?

Running the heater on demand or every few hours will save you energy cost that you are just releasing into the air.

Maintaining 85F 24/7 means you let heat BTUs be lost to the air that you constantly replace by running the heater. If you heat on demand then you only spend for the BTUs you want when using the pool.

 

[valdisr]

We pretty much heat it when we want to go in. Pentair doesn’t want it below 68 (Heater Condensation - Further Reading) so I’ve kept it at 75 before but now 80 as i can get it up to temperature in less than an hour during the cold months. I haven’t noticed a large difference in propane usage with 75 versus 80.

 

[DrewLG]

The heater comes on for about 5 minutes at a time each hour or so to keep it at temperature. .... We pretty much heat it when we want to go in. Pentair doesn’t want it below 68 (Heater Condensation - Further Reading) so I’ve kept it at 75

Hmm, Pentair's instruction in that area isn't very clear, since I understand it differently.

From the wiki link you referenced, their first bullet point says you MUST NOT operate the heater continuously while the water temperature is below 68 degrees, but their second bullet point says you MUST operate the heater continuously while the water temperature is below 68 degrees:

• Operating this heater continuously at water temperatures below 68° F. (20° C) will cause harmful condensation and will damage the heater and void the warranty.
• When starting the heater for the swimming season with a water temperature below 50° F (10° C), the heater may be used to heat the water; however, make sure that the heater operates continuously until the water temperature reaches the heater’s minimum setting of 68° F (20° C).

To make sense of that apparent contradiction, I assume that "operate continuously" means something different in the first bullet point than in the second. So I don't read the above as "your heater will be damaged every time you start it with water temperature below 68 degrees". To me, it seems that they're simply saying this:

• Don't use the heater to maintain the water at a temperature below 68 degrees (e.g., by keeping it just above 32 degrees to prevent freezing).
• In other words, whenever water is heated from a temperature below 68 degrees, make sure that the heater operates continuously until the water temperature reaches at least 68 degrees.

Maybe I'm wrong. I should probably contact Pentair and ask them to explain it clearly. But regardless, running the heater for 5 minutes every hour uses around $700 worth of propane every month, and a brand-new heater only costs around $3500... So even if you are prolonging the heater's life by maintaining the temperature at 75 degrees, are you actually saving any money?

 

[valdisr]

But regardless, running the heater for 5 minutes every hour uses around $700 worth of propane every month, and a brand-new heater only costs around $3500... So even if you are prolonging the heater's life by maintaining the temperature at 75 degrees, are you actually saving any money?

We swim in the pool year round, more so on the weekend than the weekday, so I don't think I'd save a lot by letting it drop to 50 and then heating it up a few days later.

I'm assuming that heat loss from keeping the pool at 80 is about the same as 70. Maybe this is wrong?

I guess was really wondering was that if I keep the pool at X and the heater runs 5m an hour to keep it there, would I be better off letting it go from X to X+5 and then back down to X given that I want to maintain it at X anyways and heat loss at X+5 is the same as X.

 

[mas985]

I'm assuming that heat loss from keeping the pool at 80 is about the same as 70. Maybe this is wrong?

That would be a bad assumption. Heat loss is proportional to temperature differences between the water, the air, dew point and the sky. So the warmer the water, the faster it loses heat. With an air temperature of 60F, the heat loss at 80F water temp is twice that of 70F but that is just for convection loses. Sky losses are proportional to the 4th power of temperature difference so much worse. Also, if you don't use a cover, dew point temperatures come into play for evaporation but there too, heat loss is proportional to temperature differences.

 

[valdisr]

That would be a bad assumption. Heat loss is proportional to temperature differences between the water, the air, dew point and the sky. So the warmer the water, the faster it loses heat. With an air temperature of 60F, the heat loss at 80F water temp is twice that of 70F but that is just for convection loses. Sky losses are proportional to the 4th power of temperature difference so much worse. Also, if you don't use a cover, dew point temperatures come into play for evaporation but there too, heat loss is proportional to temperature differences.

I guess I realized it would be more but hadn’t done the analysis. I do have an auto cover that is on when not in use so that helps with evaporation loss.

I’ll try turning it back down to maintain at 75 and see if the energy usage is reduced. To be fair it’s 30F outside here now so I’m not sure how much difference I’ll see in a 5 degree water temperature change.

 

[mas985]

Is your signature correct? The pool is only 3500 gals? That should be easy to heat up on demand. An 85% 400k BTU heater will heat that amount of water about 12F per hour.

 

[valdisr]

Is your signature correct? The pool is only 3500 gals? That should be easy to heat up on demand. An 85% 400k BTU heater will heat that amount of water about 12F per hour.

It is correct. It takes about 90 minutes to raise it about 20 degrees. In theory I could just let it get below 68 and heat on demand, but I do worry about the damage to the heater. Not so much cost but the pain of having to replace it, including the downtime to replace.

It is also nice to keep it at a point where you can get in the pool within a few hours, so keeping it above 68 lines up with that as well.

 

[mas985]

OPERATING THIS HEATER CONTINUOUSLY AT WATER TEMPERATURE BELOW 68° F. WILL CAUSE HARMFUL CONDENSATION AND WILL DAMAGE THE HEATER AND VOID THE WARRANTY. Do not use the heater to protect pools or spas from freezing if the final maintenance temperature desired is below 68° F., as this will cause condensation related problems.

But note that you will not be operating CONTINUOUSLY at temperatures below 68F. Only for a short period of time until water temps get above 68F. Your final temperature is higher than this correct? They just don't want you to have a set point below 68F. This is also in the manual:

If the pool is only used occasionally, lower the pool thermostat to 68° F (20° C) and leave the heater on. This will keep the pool and the surrounding ground warm enough so that the heater should restore the pool to a comfortable temperature within about one day.

So at minimum, you could do this.

But again, these heaters are designed to minimize condensation. If it were me, I would use on demand heating. I do that for my spa even when water temps get down to 45F. Going on 15 years without any issues.

 

[valdisr]

But note that you will not be operating CONTINUOUSLY at temperatures below 68F. Only for a short period of time until water temps get above 68F. Your final temperature is higher than this correct? They just don't want you to have a set point below 68F. This is also in the manual:


So at minimum, you could do this.

But again, these heaters are designed to minimize condensation. If it were me, I would use on demand heating. I do that for my spa even when water temps get down to 45F. Going on 15 years without any issues.

I appreciate the insight. I’m going to lower the temperature from 80 to 72 and see what that does for my propane usage. If I see a difference I may end up dropping it down below that.

 

[bradgray]

To me, it seems that they're simply saying this:

• Don't use the heater to maintain the water at a temperature below 68 degrees (e.g., by keeping it just above 32 degrees to prevent freezing).
• In other words, whenever water is heated from a temperature below 68 degrees, make sure that the heater operates continuously until the water temperature reaches at least 68 degrees.

100% correct. And I like your language better.