Steam Cycles Question

David_30

Stupid question but I'll ask anyway: A boiler running for an hour every four hours (i.e. 25% of the time) should use half the fuel of a boiler running 15 minutes on and 15 minutes off (50% of the time), right?

David_30

Steam cycles

My (very old) steam boiler runs in cycles of 45 minutes on and 2-2.5 hours off. The temp swings about 4-5 degrees which isnt that bad but is it ok to run this way or is it more efficient (i.e., less fuel consumption) to have shorter more frequent cycles? I've moved the anticipator on my Honeywell T87 all the way down to .5 with little impact on the cycles. Any advice?

Al Letellier_9

steamer cycles

From your description, it sounds like the pressuretrol is not working. The boiler should cycle on/off on pressure when there is a call for heat. A long cycle followed by a long shut down usually indicates problems with pressuretrols or your vents aren't closing, therefore not allowing the boiler to build pressure in the system. You could be building dangerous levels of pressure. Get a pro to look at your system. What you are describing is a very inefficient way to run a steam system


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David_30

pressuretrol

Thank you. While I search for a pro, is there an easy way for a non-pro like me to check if a pressuretrol is working?

David_30

Over-venting?

I just spoke to my plumber who reminded me that the pressuretrol was replaced last year. He thinks the system may be over-vented. I just did a specific time check on the cylces and my boiler ran for an hour and then off for 3 hours and the pressure gauge never moves. Everything is vented per recomendations here with Gortons everywhere. Is the system too well vented to reach pressure? Should I switch from a T87 manual thermostat to a digitial where I can force cycles?

Mike T., Swampeast MO

Over-Venting

"Steam" vents only vent air--once you get steam they close.

While my knowledge of steam comes only from what I've read here, the goal that is VERY often expressed is removing (venting) the air as fast and evenly as possible.

I was also under the impression that the longer the steam cycle, the better.

If you've insulated/weatherized nicely than your steam rads are most certainly severely oversized to the heat loss. Once they get up to temp in such a nice long cycle, they may well cool to nearly room temp before another cycle begins. If so, and if the boiler is well sized to the radiation you'll be producing loads of condensate as the rads again heat. Once fully heated the call can be satisifed before the boiler ever produces more than a few ounces of steam pressure.

What kind of weather for this cycle time? Near the coldest norm? If not, how much (subjectively is fine) warmer?

Do you have other problems? Excessive indoor temperature fluctuation? Uneven heat? Noises? Unusually high fuel use?

Bernard Smith

Your pressure may be too low for your pressuretrol to react. You might be better with a vaporstat. It is much more sensitive than a pressuretrol. vaporstat measures in oz. rather than pounds. Some of the steam systems were designed to only run at 1psi. You can't really over-vent because the whole objective is to get the cold air out of the system and replace it with steam as quickly as possible. Your system will probably cycle much better with a vaporstat in place instead of a pressuretrol. Your pressuretrol is only acting as a high limit safety device.

Mike T., Swampeast MO

Presume you're the same "David" with the extreme fuel bills, right?

You received good advice from that thread re:

1) Insulate/weatherize

2) Find a good steam man to carefully check out your boiler. If this was a "free" conversion from oil to natural gas I guess you now know why it was "free"...

soot_seeker_2

Half the fuel but not half the heat

David,

While half-length cycles will consume half the fuel, they will not put half the heat into your home.

The combustion efficiency will be different, as combustion is less efficient before the fire stablizes. The fuel spent bringing cold water to a boil before steam is produced will be different. The total steam production will be different. And the standby losses - the heat coming off the boiler and going up the stack when the boiler is idle - will be different.

There is an optimum cycle time for steam systems, and it varies from system to system. While longer cycles often increase efficiency, they also decrease comfort by offering large temperature swings.

Long Beach Ed

Darin(in Michigan)

T-87's are designed

for about a four degree differential to prevent short cycling the burner. The theory is the average person cant feel a four degree change unless they are that stingy old lady who cant keep her eyes off the thermometer. Also, the mercury should be shiney. If it is charcoal gray, it may be oxidized and have lost its ability to flow freely in the bulb.