The photo below is a Sta-Rite Maxi-E-Therm pool and spa heater. Whoever designed this heater really put a lot of thought into the design and did not just copy everybody else. This heater is my favorite residential gas heater for several reasons:
1)Note the round combustion chamber - its harder for rats to get inside the enclosed combustion chamber,and its very unlikely for debris to get in exhaust due to its simple but effective design.
2)It has one of the highest efficiency ratings in its class and price, mainly from the way the gas is mixed with the air before being introduced into the combustion chamber - notice how the gas valve is directly connected to the circular blower which pumps the gas/air mixture to combustion chamber.
3)Reliability - its one the most reliable and trouble free and easy to maintain gas heaters I have ever worked on - and I have over 20 years experience with heaters.
4)Because of its round shape, size and weight, it can be easily installed almost anywhere.
5)The outside is made of a composite material which does not rust.
6)This pool and spa heater has no pilot-it uses a ceramic ignitor to ignite burners-extremely reliable! (photo of ceramic ignitor at link below)
For commercial or high use applications such as an apartment or condominium I would suggest the Hayward ASME commercial heater shown at bottom of page.
If you are having trouble with your heater, below is a link to common heater problems:
Shown below is the same heater in a square box and different color - same on the inside but available in more sizes(BTU's)...another advantage of the square box is that if you have rats that like to chew on your heater wiring,the square box makes it easier to secure any openings...on the round heater it is next to impossible to secure all openings (mesh kits are available now to close openings) but some moth balls placed around the vent holes on bottom (making sure not to over-do it and block ALL the vent holes) can keep rats away (apparently rats dislike the smell of anything clean and sanitary)
Heater above and below is Pentair ETI 400 Heater. Mostly intended for commercial applications,it boasts 94% thermal efficiency and pure Titanium exchanger for corrosion free service. On photo below you an see two blowers - one for intake,one for exhaust....because the heater puts so much of the heat into water,the venting can be plastic. 3 year warranty including labor
Below is a heater from the Raypak heaters brand I think these are generally a good design except changing/cleaning the pilot can be a real pain on these heaters and:
If you are in California or any other area where Lo Nox heaters are required,these heaters have been converted to Lo Nox standard which I believe is not as good as having a heater designed from the outset to be Lo Nox such as the models above..in my opinion,heaters that have been converted are more temper-mental as far as pressure settings,cleaning of burners and orifices,etc. and are more likely to need maintenance to burn properly.
On the other hand,the second photo below shows commercial grade Raypak heater
This is the prototype for the Professional series commercial ASME heater made especially for commercial applications requiring a metal header but you may want to pay the premium price when you consider the warranty - three years parts and labor and additional 2 years parts on the heat exchanger BEYOND the original 3. I have installed quite a few of these - like them so far UPDATE 3/17/15: I would hold off on this heater for now...Raypak has had to replace many of these heaters under warranty...personally on my installations two have had to be replaced due to to heat escaping firebox and a third has possible problems...out of 7 or 8 I have installed not good!
Shown above and below is a Hayward H-series pool and spa heater. It is low profile and designed from the outset to be a Low Nox.
I especially like the 150,000 BTU version of this heater for spas (shown above) because it will fit in many tight spots formally occupied by 125,000 BTU or smaller heaters.
The only negative thing I can say is switching the water connections from the right to the left side requires moving 9 components and about 50 screws which is a pain in the buttocks.
The version below is the commercial model with brass manifold,which I think is best bet for applications requiring ASME heaters right now.
These heaters come standard with cupra-nickel heat exchangers for a cheap price - sweet!
I have installed about 5 or 6 of these never a callback yet!(Update I have had to replace circuit boards on one heater I installed..but they have re-designed the boards)
You may notice an absence of Jandy/Zodiak heaters on this page: they have a new heater called the JXi which i believe to be somewhat a copy of the heater at top of page-it may be good have not tried it...but the four heaters proceding it - the Lite lo nox,the HI-E2,the LT,and the LX were absolute garbage and a standing monument to the fact "made in the USA" does not always mean quality