Try as I might, I couldn't find any information online about Apollo hydronic heating system or their water heater AND their issues. As anyone who's had the pleasure of living in an apartment with these systems, they use the water heater to heat both the potable (domestic) water and to heat your living space. There must be an advantage somewhere, but if you take too many Hollywood showers, you're going to take longer to heat your home. I guess the advantage is its simplicity in only needing one gas appliance, not to mention the cost saving in building and installation. These aren't too terribly high maintenance, and for the most part they will happily heat your apartment and water without a problem. However, the little maintenance they do need is crucial. Neglect this one little thing and you have a problem that will confound you and probably even a few service techs to boot. I used to install these things back in the day, and I hate working on them. It isn't that the parts themselves are too hard to fix, but the location is what makes these a pain to get to. New water heaters in general are the bane of my existence by design. You see, the government mandated about a decade ago that water heaters needed flame arresters to help prevent them from igniting flammable liquids stored next to same. These flame arresters are usually made of a dense screen material that's about a half inch thick and about eight to twelve inches around.
The problem is that these screens will plug up in short order from dust and dirt getting sucked into them from the draft. The result is that the water heater will go off on limit and if you're lucky, you'll be able to cool it down and relight it. If you aren't, you're going to spend a couple hundred dollars to replace the pilot assembly. Such is the price of progress and trust me, I hate this design with a passion. However, the fix and prevention isn't too terrible as long as you have some space to work. If not, prepare to lay down and stand on your head to fix this abomination.
Again, this repair is pretty straightforward and requires basic hand tools as well as a vacuum cleaner, air tank or air compressor with a blowgun. You could just use a vacuum, but this will be a little more tedious. This fix can expose you to moderate to severe burns. There is also risk of fire and explosion causing injury, death or property damage as you will be removing gas lines. Make sure the water heater is cool before you work on it. When in doubt, consult the services of a competent service technician to effect this fix. The cost will be minimal compared to an emergency room or funeral bill. This may be extreme, but better to be too careful. Do this and any other fixes at your own risk.
You will need the aforementioned vacuum cleaner, and air source if you can get it. You'll also need a set of wrenches from 3/8, 7/16 and possibly a 3/4 or an adjustable wrench or gripping pliers, as well as a socket set to remove the cover plate. You'll want to remove those two nuts to that plate, unscrew the thermocouple, pilot tube and main burner tube from the gas control. Carefully, work the burner out without tearing the gasket, paying attention to how it goes back together and set it aside. There will be the screen material underneath the burner, so blow that from the inside out with your vacuum or compressed air. Spend a good ten minutes on this and if you can feel air movement through it, you're golden. Take a second and clean around the water heater with the vacuum cleaner and blow the screen through one more time. Clean any more debris that works out. Repeat this process a time or two, or until there's no more debris evident. Put everything back together and check for leaks as you start it up. Use bubbles for this as they're more user friendly, but be careful not to get them all over the gas control itself. Carefully stick your hand next to the screen under the water heater; you should feel the slightest of air movement. If not, shut it down and clean it again. See, I told you this was a pain, but does anyone listen?
So, how does one prevent this fiasco? Every water heater comes with a filter that wraps around the base. This is made of a soft plastic with a fine screen and is usually black in color. The trick is to inspect this every month or two and clean it with a vacuum when it starts to plug up. Most of the time, people will either throw it away or remove it because it's too much of a pain in the rumpus to get it back on. Bite the bullet and make sure it gets back on, or replace it if it's missing. It will save you a no heat or no hot water in the long run, and the time and expense of correcting same. Maranatha!