Short Cycle Mystery

harakelh · March 2019

This is a classic row house with a single pipe steam heat system. So there is:

Basement - Utica boiler with a wet return
Garden apartment - a previous owner removed this floor from the steam heat system
Top 3 floors - these have the steam heat.

The boiler cracked over 10 years ago and was replaced with a Utica system that has a modern smaller water capacity. I don't remember issues the first few years. The boiler has a McConnell & Miller PSE-802-24 low water cut-off and a McConnell & Miller Uni-Match automatic water feed. The low water cuts-off on each cycle and if the water feed is allowed to run, progressively more water fills the system. So, I have too much water yet the low water cut-off still cuts-off. I got lazy about checking the water for a couple weeks and found water coming out of a radiator on the 2nd floor.

I had an incident where the boiler appeared to be full of sludge, presumably the gunk from the wet return. The plumber said no problem and drained and refilled the boiler.

To address the short-cycling I had the wet returns replaced. Judging from the sight glass the water returns faster but not fast enough to prevent the short-cycling.

I had a follow-up problem where the water in the sight-glass was not accurate and I inferred there was some gunk blocking the sight glass. The plumber cleaned it out and replaced the sight glass.

With the automatic feed cut-off closed, I need to add water every 3-4 days. If I add enough water to fill the water near its maximum sight glass line, I get water hammer in the pipes near the boiler. You hear it through the house but it's definitely happening in the basement.

I had the vents on the risers in the basement replaced and replaced all radiator vents and ensured the radiators are tilted to the return. I have a radiator that sometimes sounds like water is boiling but there is no water hammer which I find odd. Another radiator that has a tapping sound at the start of the cycle, the taps speed up and simultaneously fade out.

Appreciate any help on this, thanks.

STEVEusaPA · March 2019

Where are you located? You need a steam pro, not necessarily a plumber. Someone who can look at the entire system.
If you are losing that much water than either your returns are still leaking, or your boiler has a crack above the water line and it's losing water (steam) out the chimney.

Jamie Hall · March 2019

Let's take the easy one first: the radiator that taps -- that's expansion. Not to worry.

Let's take the bad news second. If you have to add water every 3 to 4 days in a system that size, you have a significant leak somewhere. That needs to be found and corrected, or the boiler will have a short and unhappy life.

There are many reasons why the low water cutoff might activate on each cycle, none of them healthy. If things are operating properly, the water level should drop no more than a couple of inches from cold to full steam -- if that. The easiest one to check -- and you can do it this afternoon -- is to see what pressure the boiler is set to cut out at Should be no more than 2 psi, and preferably more like 1.7 psi. Another possibility is incorrect near boiler piping (the water hammer in the pipes near the boiler suggests this, too), particularly a missing or improperly piped equalizer. It would help to have some pictures of the near boiler piping, to see how it was actually done. Badly plugged up wet returns are another possibility, Did your plumber actually flush the wet returns, or did he just drain and refill the boiler? I'll probably think of some more...

gfrbrookline · March 2019

I have a similar problem but on a much larger Burnham V904. I go through about 2" of water in the sight glass if I turn the water supply to the LWCO off every three to four days. If I turn it on half way it over fills by about 2" every three to 4 days. We have replaced or repacked every floor valves, checked all of the vents, maxed out the main venting and replaced a leaking section of main pipe and still have the same problem. Vaporstat set to 12oz cut out 6 oz cut in.

We also have a water hammer that only appears between 3 and 4 am every morning. Three clangs every time. We do not run any set backs and the pipes in the basement are properly pitched.

When we had the boiler serviced this summer the contractor spent a good deal of time flushing my returns so I doubt they are the suspect although my previous contractor never flushed them so maybe crud has pushed down the system.

Short Cycle Mystery

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