Does a Cycleguard on Two Pipe System make sense?

KarlW

@Jamie Hall

In another post this morning, you said the following (I didn't want to hijack the post so I created this):

Now. [Chrisj] 's point is exactly correct, and is a very often misunderstood — if not completely overlooked — characteristic of one pipe steam systems. They must cycle if they are dependent on venting variations (which they almost all are) or TRVents for control. The cycling must have a long enough "off" phase for the pressure to drop to zero — usually a minute or so off is adequate. Ten minutes on is long enough.

Much has been said over the years about the nuisance of the Cyclegard low water cutout cycling It's not a bug, it's a feature. That cycling, which is there to check for foaming, also serves to provide the needed cycling for one pipe steam control.

I have two pipe steam with a Cycleguard. It has TRVs with smart valves on most radiators and a system that turns my steam boiler on when any of these radiators call for heat, along with a pressure shutoff set for 14" h2o. Only on brutally cold days like this past weekend does the boiler run for 10 minutes to get a Cycleguard cut off.

Your minute or so "off" phase is spot on for me - I average between 50-60 seconds after a pressure cut-off shut back down to zero. But, on non-frigid days (Western NY winters hover in 10s-30s) my boiler generally operates on the pressure shut-off and I'm looking to add a time-delay relay to keep the boiler off to save fuel instead of pressurizing a system that already has radiators full of steam - look for a post on this later.

My question is, with a two-pipe system, am I "better" off with a low-water cutoff that doesn't shut my system off?

That said, I likely won't do it as it would involve a major change of my boiler's controls and not match its wiring diagram anymore.

Jamie Hall

No — a two pipe system does not need to cycle to maintain control. Control is entirely with the inlet valve (or orifice) settings on the radiators. If the system does cycle — say from the low water cutoff — there's no harm, so I wouldn't go changing the LWCO.

There is a long standing — like decades — debate on the question of a delay after a system shuts off on pressure, and there is an equally long standing lack of any really good definitive research on the overall benefits and detriments. I'll look forward to your posts on this! I have my own opinions — but so do many others.

KarlW

Thanks!

The only reason I would entertain changing it is if I can figure out how to add a time delay to the pressure cut-off and the LWCO shut also activated the time delay - meaning on the coldest days, my boiler is off for a full 5 minutes (or whatever I set the time delay for) and it can't keep up in a 10-minute on, 5-minute off cycle.

Right now with my pressure control, it operates on a 1.5-minute on, 1-minute off cycle, killing the valve and costing me more in wasted fuel to pressurize my pipes without filling radiators.