Vaporstat pressure cycle time

jim_51

Question about the normal cycle time / modulation for a gas boiler when cutting out on pressure, what is considered too short?

This is a small 150k BTU boiler where I just replaced the pressuretrol with a vaporstat set to cut-out at 12oz and cut-in at 4oz. Once everything is heated up the boiler will fire for about 2 minutes before cutting out, and then rest for about 2 minutes before cutting in. Is this excessive or normal?

I replaced all the radiator air vents with new varivalves and insulated the mains, the valves are getting hot and everything seems to heat nicely, I'm just concerned about the cycling rate and not sure if I should increase the cut-out pressure since everything seems to heat nicely at 12oz (probably can go even lower than that).

I picked up the Lost Art of Steam Heating book, but didn't see much mention of cycle times in it.

Larry C_13

What is your heatloss?

Is the boiler too large for the heat load?

Larry C

Unknown

Balance

Think about the relationship between the load and the boiler output. If the boiler output matched the load EXACTLY, the boiler pressure would rise (after the vents close, taking them out of the equation) through cold radiator time; through warm radiator time; and stop rising at HOT radiator time, as hot radiators are putting out what they are designed for, and in a perfect world, that is what the boiler can produce.

In the real world, the boiler isn't perfectly sized. It's a small percentage larger (hopefully). With a small amount of extra capacity, say 5%, the pressure would continue to rise.

Here's some math. Producing 105% of the load, the pressure would slowly rise above pressure setpoint, with 100% load. Rise slowly being the key phrase.

When the steam boiler goes off on pressure, you have 0% boiler output feeding 100% load. Pressure will drop like a rock. The boiler CAN'T maintain pressure when it's off. The off cycle will be very short.

If your on cycle is short, you are oversized by more than our example shows. The more oversized the boiler is, the shorter (and less efficient) the RUN CYCLE will be. It's the short RUN cycle that hurts the efficiency, not the short OFF cycle.

That's why sizing the boiler correctly BEFORE it is installed is so important, the only thing you can really vary to correct the cycling is the size of the boiler.

Noel

jim_51

size

I calculated 276.5 sq ft EDR, and the boiler lists 375 sq ft for steam and a D.O.E capacity of 120,000BTU. Dan's book had a calculation for BTUs something like this: 276.5 * 1.33 * 240 = 88,258BTU.

Does this seem within reason for size or way too big?

I can hear Gorton main vent puffing away every so often, but should I try replacing it in case its partially clogged? I did leave the feeder handle slightly open by mistake once, and came back to find water trickling down from the vent, but I would think it would be okay once it drained out.

matt_65

I took out the old siphon loop, it was pretty crudded up and the horizontal kind, redid some of the controls piping to make sure the vaporstat is leveled and on a vertical loop, and added a new tee (currently plugged) for a low pressure gauge.

I used black pipe for all this, is there any problem with that? It seems to be the same as what was there before. I also ended up raising the gauge and vaporstat about 8" higher than where it used to be (still connected to the water column's top tee though) since the vertical siphon loop and vaporstat were too wide and couldn't be straightened fully because it ran into the boiler's face.

Looks like I made a mistake doing the EDR, I've got 5 column thin tube radiators in most rooms, and cast-iron radiant/convectors in the bathroom and kitchen. The EDR ends up being 217.5 sq ft, and the boiler is rated for 375 sq ft.

This doesn't seem so grossly oversized, but is it possible to downfire a gas boiler by adjusting the regulator or manual cutoff valve? Or do problems with condensation and combustion usually preclude doing this?

The only other problem I can see after reading through TLAOSH is that only one riser from the boiler is being used, this is a Utica PEG150 with a tap on both sides, and the other near boiler piping seems correct enough. Is using only one tap always a problem? Or does it depend on the heat exchanger configuration?

Sorry for all the questions, we just moved into an old colonial with one-pipe and this is going to be our first winter with steam heat, I've been trying to get things tuned up and ready.