Aquastat not breaking on high set point

Hi there. I have read some great postings here and I have learned a lot from everyone's questions and answers.

My mother-in-law has a WM oil-fired boiler, forced hot water and tankless domestic hot water. About a year ago, she started hearing banging coming from the heating system sometimes. Her hot water at the taps is always way too hot. I was at her house once when it happened and I went down to the boiler room. The temp/pressure gage on the boiler read over 200 deg. F! I assume the banging was from the water boiling in the system.. The Honeywell L8124C aquastat HI set point is set to about 150 deg F.
She called her heating guy, and he increased the pressure in the system.... Huh? What about the aquastat NOT breaking at the proper temp? I told her to ask him that and he dismissed that as nonsense.
A few months later, the problem was still happening, so she called him back. He changed the fill valve/regulator, check valve, etc. saying that the noise was coming from there... Again, not even acknowledging the HI set point issue.

I was at her house yesterday and the same thing: banging! I went into the boiler room and the temp was over 200 deg F and the pressure was at about 0! I flipped the manual lever on the fill valve a little, and it clicked and the boiler started to fill and the pressure went up to about 15 psi, just I believe it is supposed to be...

So, obviously the fill valve may need replacing since it seemed to have stuck closed. But, does it sound like the aquastat may need replacing? I assume there is no way the aquastat could be operating properly if it does not break at the HI set point, right?

Thank you all for any advice you can provide.

The 200 degrees may not be a problem, as 150 is low for most heating systems - somewhere between 180-200 is farily normal from what I've seen. Now, if your domestic hot water is too hot, it probably isn't installed with a tempering valve, which is likely not per the manufacturers instructions. WIth a tankless system, the water sitting in the coil when no flow is called for will reach the same temperature as the system water (which could be near boiling). A tempering valve will moderate that to an acceptable level, but if things sit for awhile, the heat transfer may flow into the cold supply, and the first bit cannot be tempered very well.

As to the pressure loss, that fill valve may not be an 'autofill' variety. You have a leak in the system, as you should not have to add water. You may also have another problem, as most boilers won't fire if the pressure is that low. If the boiler does not have one, you might want to add a low-pressure cutoff switch to the controls. It likely has one and it is not working.

Constantly adding fresh water to a boiler system adds oxygen, and that can lead to premature corrosion and failure, so find out where it is leaking and fix it.

Note, if the expansion tank is shot, during a heating cycle, the thing can reach overpressure and vent hot water. If none is replenished when things cool again, this cycle will eventually cause the pressure to drop.

Thanks for the info Jim.

To answer your points:
When I said the temp gauge was over 200 deg F, I meant it was well over it, like around 210-215. The boiler at my house is setup very similarly, and the HI point it set at 190. When the boiler temp gets up to 190, the burner shuts off. Unlike at my Mother-in-laws where the HI point is set at 150 (probably turned way down by her heating guy in an attempt to get the temp down) and the water still gets up to 200+ deg F.

Yes, there is a tempering valve installed. I didn't bother to try adjusting that yet because I thought I should first address the aquastat not cutting off at the HI set point.

The fill valve is the auto-fill variety, it just seemed to be stuck.

The boiler definitely does not have a low-pressure cutoff switch. The only control on the boiler is the aquastat.

I don't think it reached overpressure and vented, at least not recently because there was no sign of water on the floor.

I plan to replace the aquastat to at least stabilize control of the water temperature and then see what I need to do from there.

Thanks for your help.

More info on boiler

Hi again. I have more information about this heating system now.
The more I look into it, the more problems I find.
Sorry for the length of this post, but there is a lot of information I wanted to convey.

I was at the house yesterday to change the boiler fill shut off. The old one looks like it was re-used when the heating guy replaced the pressure regulator and check valve. It is the sweat variety, but the one of the sweat ends threads into the body of the valve. There was a crack in the valve body and you could see the threads in the crack, and it was dripping slowly.. So I replaced the valve and that part is fine now. But...

When the system gets a call for heat, the burner fires, and the temp gets up to about 175 deg. The burner then shuts off, but, the temperature keeps rising, up to 220-225 ! The HI set point on the aquastat is 150. Could the temperature/pressure gauge be the problem? Do the gauges ever fail? If I was going to replace the aquastat anyway, would it be a good idea to use the replacement for the L8124C - the L7224U that has an integrated display for the current boiler temp?

The heating piping is larger than I am used to (like the copper 1" mains and 3/4" branches at my house and my parents house) - the mains coming out of the boiler are copper, but then they transition to 1 1/4 threaded pipe - steel maybe? It's not a large house, just an averge size split level with baseboard heaters. Maybe the piping was left over from the old heating system?

Also, the pressure rose from about 15 psi to about 25 psi during the cycle. I was going to let off some pressure, so I tried to manually open the pressure relief valve. (First I shut off the boiler fill valve). No go, it was completely stuck, would not budge at all. I didn't want to try to use tools on it, as the lever seems to be aluminum and I didn't want to bend it. Is this normal for a pressure relief valve? Since it seems to be frozen when trying to operate it manually, does that mean it would not open in an overpressure condition?
And by the way the system does have a diaphragm expansion tank.

So, in order to relieve some pressure, I went for the boiler drain valve. I put a pan under it and opened it a little to let some water out. It was very black and full of sediment. When the pressure went down a little, I tried to close the boiler drain valve. I closed it, but it still kept dripping. I guess the sediment was preventing a good seal in the valve. I tried to open the drain more to flush it out. The handle on the valve spun. It looks like the handle was replaced recently, because it was shiny maybe aluminum, and the rest of boiler drain valve was older. My guess is the heating guy accidentally snapped the handle off the valve and just replaced the handle. I was able to bend the tabs on the valve handle down a little and get a slightly better fit on the valve stem and close it a little more. However, it was still dripping a little, so I put a cap on the bib connection, for now. I think the valve should be replaced. Probably if the pressure relief valve needs to be replaced, it would be a good time to do both. Does this sound reasonable?

Thanks again for all your suggestions.

In an older boiler, there is a huge thermal mass, so having the water temp continue to rise after the thing is shut off may not be a problem. The circulator pump usually continues to run after the burner is shut off to prevent localized heating. Your gauges may not be accuract, either.

As to the pressure rise, it sounds like the expansion tank is shot. See if it is full of water or leaks when you take the air valve cap off. Letting water out to reduce the pressure is a problem in the long run, since when things cool off, the autofill valve will add fresh water. WIth fresh water, you get trapped oxygen and more potential for corrosion in the boiler.

Replace the T&P valve. If the shaft is stuck, it won't open on its own, and is a significant safety hazard.

There can be local heating in some of the smaller boilers that causes flash (bubbles and collapsing bubbles) that makes noise. You need to keep the pressure up to reduce the flash/boiling.

The temperaure sensors on aquastats have a fair amount of mass and poor conductivity, so are slow to respond. That sometimes allows overheating.

I had one where the temperature setting error kept growing and the aquastat had to be replaced. Until I replaced it, I just applied a little "Kentucky windage" (set the temperature lower) to keep it operating.