Confused about heater needs

[Mark1092]

I've got a 21.6L tank with a 25W heater... The heater is constantly on but the temp doesn't seem to go above 24.5 when I need 26. I thought a 25W heater should do anything up to 25L?

I live in Eastern Australia, winters are cold, summers are hot. It's winter now, the temp in my room can potentially be quite cold (for here) eg. 15 degrees. Maybe the heater is working to breaking point? I don't think it's working too well to be honest. Had the heater for about 6 months.

Any suggestions?
Cheers.

 

[WendiDell]

I've got a 21.6L tank with a 25W heater... The heater is constantly on but the temp doesn't seem to go above 24.5 when I need 26. I thought a 25W heater should do anything up to 25L?

I live in Eastern Australia, winters are cold, summers are hot. It's winter now, the temp in my room can potentially be quite cold (for here) eg. 15 degrees. Maybe the heater is working to breaking point? I don't think it's working too well to be honest. Had the heater for about 6 months.

Any suggestions?
Cheers.

What kind of fish do you have in a 21.6L tank?
In most cases those 2 degrees won't be a problem as long as they are consistent.
If you got one of those 25 watt submersible heaters that you can't adjust the temp. Then 24.5 is most likely the warmest it will ever get.
The temp here in the winter gets down to -11.111C to -15C, I rarely turn on the heat in the house due to cost and my fish do fine.
But if you don't feel comfortable with the 24.5*, then you'll need a better heater that you can adjust to up 26*.

 

[austinsdad]

Wow! You got winter in July there?

 

[WendiDell]

Wow! You got winter in July there?

OOPS!!! I guess I should have explained it better. I meant when it is Winter in Oregon, those are the average temps..

Now that it is Summer it's still only 13.88* (57*). On occasion we have temps. that get over 37.77* (37.77*) for a week or two at a time. But not all Summer like a lot of other parts of the US.
But at or below freezing is a typical Winter and most of Spring day for us.

It's nice getting a break from the constant heat in the Summer.
But I bet the Winters where you live are a bit harsh compared to Oregon's?

 

[Sparky74]

Try to follow me here. This will be far from scientifically accurate, but I think contains a hint of logic

I have a 200watt heater and it seems to be the commonly recommended size. I have no issues keeping temp even with heat(house) turned way down.

It takes approx 8.4 BTUs to heat a gallon of water 1 degree(F). I don't think it matters how long it takes either. Of course factors like the surrounding temp matter, but for arguments sake, lets say the water being heated is at room temp.

5.7gal
25 watts=85 BTU's.
85/8.4=10.11gal

I have a 55gal:

200watts=682 BTU's
682/8.4=81.19gal

This is far from exact science ....

While you have a bigger buffer as far as percentage goes, I have 20 more gallons of buffer(of heating capacity).

Now the science gets even more exact lol. While my 55gal has more exposed glass(ie more heat loss potential), I would think a 5.7gal would have a larger percentage of water exposed to glass, therefore resulting in a greater heat loss per gallon. Does this make any sense lol? I'm totally theorizing here...

So, I'm thinking you are on the borderline?

I need a cup of coffee

 

[HUKIT]

Try to follow me here. This will be far from scientifically accurate, but I think contains a hint of logic

I have a 200watt heater and it seems to be the commonly recommended size. I have no issues keeping temp even with heat(house) turned way down.

It takes approx 8.4 BTUs to heat a gallon of water 1 degree(F). I don't think it matters how long it takes either. Of course factors like the surrounding temp matter, but for arguments sake, lets say the water being heated is at room temp.

5.7gal
25 watts=85 BTU's.
85/8.4=10.11gal

I have a 55gal:

200watts=682 BTU's
682/8.4=81.19gal

This is far from exact science ....

While you have a bigger buffer as far as percentage goes, I have 20 more gallons of buffer(of heating capacity).

Now the science gets even more exact lol. While my 55gal has more exposed glass(ie more heat loss potential), I would think a 5.7gal would have a larger percentage of water exposed to glass, therefore resulting in a greater heat loss per gallon. Does this make any sense lol? I'm totally theorizing here...

So, I'm thinking you are on the borderline?

I need a cup of coffee

Wow thats like having to do homework, I just use 4-5w per gallon.

 

[hackteck]

the 5W per gallon rule works fine

 

[BillD]

200 watts should not be the commonly recommended heater size. needed heater size is affected by ambient room temperature, in that the difference between the temp of the room and the desired tank temp will be affected. if you are only heating a couple of degrees above room temp a minimum heater size may be sufficient, while a 10 or 15 degree difference would require a larger heater. The rule of thumb is between 2 and 5 watts per gallon, depending on the manufacturer. The danger with using a heater that is too large, is that should it fail in the on position it will cook your fish.
the math above in basically correct in showing the size needed. A 25 W heater should be able to heat a 5 gallon tank. You could test the heaters capability by shrouding the tank with a blanket or something and seeing if the temp comes up. This will tell you if you need a bigger heater or another of the same size.
I would also agree that a couple of degrees shouldn't make that big of a difference, but I tend to keep my tanks on the cooler side. Cooler temps mean higher oxygen content in the water and less bacteria. As an example, none of my livebearer tanks are heated and the temp ranges from around 66F in summer to 74 in spring and fall.

 

[Sparky74]

200 watts should not be the commonly recommended heater size.

I meant as an example for for the 55gal (at least everyone I asked said 200watts was a good size).