Home boiler help (hydronic heat)

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Jan 7, 2009
Rochester, MI, US, World
Bought a new-to-me house recently, built in 1953, and has the original boiler for the single-zone radiant heating system. Not the most efficient piece around, but it works just fine. A few times in the last 2 days though, the temperature limit switch for the boiler chamber has popped. This limit switch is in series with the aquastat and has a sensing tube going into the boiler chamber itself. It has an old tridicator gauge, showing system pressure at 18 psi and water temp hovers around 155/160 degrees. Aquastat only has a high setting (no low limit adjustment), and is set to 160, so that seems to be working just fine. I noticed yesterday that the aquastat shut off the boiler when the water reached 160, and then a few seconds after, the boiler limit switch tripped and wouldn't let me reset for about 30 seconds. What seems to be up here? Is my limit switch going bad? Forgive me if I sound uninformed, because I am when it comes to boilers. System has a little bit of air, as I can hear some sloshing in the pipes, FWIW. But water IS being circulated to every room. I should add that it's been warmer here the past few days, so the boiler isn't working as hard as it was earlier in the winter. One thing that may/may not be related is that it appears that 160 is quite a low temperature setting for home heating boilers, from what my research has told me. I haven't touched it since we moved here; maybe it needs to be adjusted.
I’m not exactly reading you here as mine is a lot newer even though fairly old itself. Yes you mneed to bump up to 180. Internal corrosion is the reason . The valve you refer to, is it the TPS safety valve like the one on top of all water heaters ? If that’s the case they don’t last forever on a boiler. The minerals and corrosion take their toll even though brass. They usually start to leak slightly. New ones are $20 ish . Just shut down water intake, pop the drain at the bottom to relieve the pressure. Then unscrew old put in new with some pipe dope, done. Just turn on the water inlet again. One thing to test that always goes every so many years is the expansion tank. Tap it top and bottom . If it sounds like it’s completely full, dull thud top and bottom get a new one. $30. IIRC that being waterlogged will make the TPS valve leak too. Not Sure about that but check that first and see before buying a new safety valve.
The aquastat and the limit switch should not have the same temp or even close. The limit should be up at 200 and the aquastat at 180. Does your boiler has a TPS valve? Many boilers just have an aquastat and TPS. The aquastat should shut down the boiler at the high temp. Check the expansion tank. It's common they fail. I installed a small computer control that varies the high temp based on outdoor temp. The aquastat normally has a high temp and differential. Not a high and low.
Originally Posted By: Donald
I installed a small computer control that varies the high temp based on outdoor temp.
This sounds like an ideal Arduino project!
As mentioned above, the limit switch is designed to cut-off at a higher temperature (210* F) and its a safety device. If it trips, likely, there is something wrong with the boiler's operation. As far as the aquastat there are different kinds with high, low and differential settings. The old standard was to set the aquastat to 180*F... however that might not be necessary (most boilers are way over-sized for their application) and can lead to over-cycling (i.e., boiler only runs minutes at a time) If the boiler can maintain the heat setting in the house on the coldest of days with the aquastat set down to 160* F then it'll be more efficient. Just don't go below 140* F (gas condensing, corrosion) and ensure you don't have a domestic coil hooked up. Boiler temp vs ODT is a good way to save money as well... however you're limited with a oil-fired boiler (140* F). That's were a gas modulating/condensing boiler shines. Things can run down to 80-100* F no problem and keep a house at 70* F in those mild days. With that said, boilers can be dangerous... you should know what your doing when working on them or call a professional. Sounds like the aquastat is set to 160* and the tridicator indicates 160* as well. Either the limit switch needs adjusting or its faulty.
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When considering what to set your high limit consider you domestic hot water. I assume you have a domestic hot water coil installed in the boiler. If you do not have a tempering valve then be careful raising the high temp. The boiler is very old. I would budget for a replacement, either modern oil or propane.
Originally Posted By: Donald
The boiler is very old. I would budget for a replacement, either modern oil or propane.
Budget however do not prematurely replace. I have 1978 Well McLain oil fired boiler that was deemed likely needing replacement soon 10 years ago. It musters on requiring about $300 every other year. The time has come for me though.
Riello makes an oil burner gun that out performs the rest using a much higher pressure oil pump. You hear it come on building up the pressure for 10 or 15 seconds, when all is set, it opens up and fires.
160 is actually pretty high for radiant heat if are you referring to the type under the floor? Typically those zones are mixed with return water or if all zones use that the temp of boiler can be reduced.
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