So, I have recently had some issues with my boiler (hot water - not steam) system. After hiring some guys to stop by and take a look at it, they told me that the boiler pressure reducing valve, pressure relief valve (stuck partly open) and my pressure gauge are bad (all of which I believe and suspected for some while now). So, effectively I can't leave my auto-feed from the road on because it will over-pressurize the system, and I get water/steam out the pressure relief valve every time it warms up. I did empty the expansion tank after refilling the system the last time (~2 days ago) so I know that it's got somewhere to go if it needs (and it does take on water when the system warms up, but it also comes out the pressure relief valve at the same time).

Additionally, the system was set to ~220F, which they indicated was over water's boiling point and should be brought back to 180, which makes sense and we did so.

However, the system is set to 180F and it's still producing steam and I'm not sure why. We verified that the water is circulating properly and the aquastat is correct as well. The HVAC guy who was here gave me an explanation of why it could still be producing steam even though it's set at 180F but I didn't understand it and after asking him to reiterate it for the third time I gave up. He mentioned something about the backfill of water getting heated but it still didn't make sense to me unless the water wasn't circulating (which we know that it is).

I'd be interested in any explanations - I'm mostly trying to make sure I understand this system. I'm sure I left out some critical details so feel free to comment and I'll add whatever you'd like. This is an old Burnham boiler. Thanks,


  • 1
    Is this a steam or a hot water system? Mar 30, 2015 at 22:12
  • 1
    Please update and be explicit about whether this is a steam or hot water system. Otherwise it is hard to help.
    – Jeremy
    Apr 1, 2015 at 12:26

1 Answer 1


I will give this a try.

Physics says that water can not make steam, at sea level pressure under 212 degrees F.

It is also why steam pipes and automobile systems are under pressure, you can hold more heat, when water is under pressure and not have it boil. Pressure cookers cook food faster for the same reason.

The only way to make steam, from water, at 180 F, is to do it under partial vacuum.

To get water to boil at 180 F, you need to reduce the air pressure to about 7.5 psi.

7.5 psi is a column of water about 15' tall.

If a boiler tank, had a 15' tall pipe, filled with water, and caught between a drain trap and the return to the boiler, it would make steam at 180 F.

I have a physics background, and I am not a stationary engineer, well not a train engineer either, to be honest.

Hopefully someone who has actually worked on a boiler system can take this and run with it.

  • Note, never pay someone to work on a boiler that thinks the temperature of the water should be below the boiling point of water.

You are better off hiring a competent plumber to work on a storage water heater. You get the same result with fewer parts, faster, and for less money.

  • It's not clear from the original post whether he has a hot water or steam system. If it's hot water, you don't want it to boil. And if it is hot water, there is definitely 15+ feet of head between the boiler and radiators.
    – Jeremy
    Apr 1, 2015 at 12:22
  • @jzeolla - doesn't this answer the question?
    – Jeremy
    Apr 4, 2015 at 16:21

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