Optimal Run Time for Boiler

Jeff_H

Putting aside the amount of time that it will take to get the temperature on my thermostat to the desired temperature, what is the optimal run time for a boiler? I have noticed that it take at least 15 mintes for all of my radiators to heat up, so it seems like running the boiler for less than that amount of time is not a good idea. If I run it for a little longer, it will generate enough heat for some time before the temperature will drop. But I realize that having the boiler run too long is not a great idea. I recall reading somewhere that 20 minutes is prefereable (in which case I set my timers to keep the boiler on for that period of time).

KC_Jones

These types of questions get asked a lot on here. It's honestly difficult to answer. Each house is unique, but the basic idea is to get the steam to the rads as fast as you can and then vent them slowly. You mention 15 minutes, but what is that the time of? To get steam to the inlet of all the rads? To get the first section hot on all the rads? Generally you time how long to heat the mains and after that you are heating the house. If that is taking you 15 minutes that seems excessive. You also mention a timer? Are you planning on changing your control to a timer based scenario?

Fred

Every installation is different. There are so many variables and there is nothing magic about a 20 minute heating cycle. Outside temps/air infiltration/insulation, even thermostat location play a role in "what is right". For me, it is however long it takes for the boiler to satisfy the thermostat without over-running the temp setting and/or short cycling.
On a 25 to 30 degree day, I may run 12 to 15 minutes a cycle. On a 0 to minus 10 Degree day my boiler may run 35 to 40 minutes.

SWEI

KC_Jones said:

the basic idea is to get the steam to the rads as fast as you can and then vent them slowly.

Translation (from a controls standpoint): Throw as many BTU's as you have at it until it nears setpoint. Then reduce to the required maintenance flow.

Two stage burners, anyone?

Mark N

In the Heating Museum here on Heating Help in the Old Steam Section is an article titled "It's All in the VENTING". There is a section labeled "Six Points For Best One Pipe Steam Plant Performance". Point number 6 states "Check the on off cycle, periods shorter than 20 minutes generally waste fuel , in addition to giving poor distribution of steam to the different radiators. Long on periods longer than about 40 minutes may give bumpy heating and discomfort".

KC_Jones

SWEI said:

KC_Jones said:

the basic idea is to get the steam to the rads as fast as you can and then vent them slowly.

Translation (from a controls standpoint): Throw as many BTU's as you have at it until it nears setpoint. Then reduce to the required maintenance flow.

Two stage burners, anyone?

Do you think it has been mentioned enough on here for the industry to hear us?

nicholas bonham-carter

Jeff, what sort of timer is this?
If it takes 15 minutes for the radiators to get steam, then that seems too long. How is your main venting?--NBC

PMJ

MarkS said:

The industry seems either slow to respond or completely disinterested in change and innovation for non condensing boilers. Two stage burners, modulating burners, better controls (think vaporstats, ODR), gas on a MegaSteam, etc. There's probably more R&D for residential steam being done by guys on this forum than in the industry at large. It's a shame too, they're really doing themselves and their customers a disservice.

They look at a residential steam marketplace that has no new installations going in. You won't see one cent spent on R&D under those circumstances. At the same time the installed base is so large that replacement boilers are still needed so the manufacturers are happy to to the same old thing till it dies out completely.

We have reached a place generally where high up front cost is resisted no matter how much lower the ongoing annual maintenance cost is for generations to come of the system. The only real need purchases I have made for my system in 22 years is a ball valve and I replaced the MCDMiller Low Water auto fill valve this year. All the rest was for my experimentation and fun. The piping is now 90 years old and the boiler 60 years. Residential steam is going the same way as the slate roof. Both now have a reputation for high annual maintenance cost when the fact is that both have dramatically lower annual costs - including amortizing the installation cost. We need to admit that collectively as a nation we just don't have the cash to do it right up front anymore.

Sorry to hijack this thread. As far as burner run times - let me just point out that preheat time is directly related to how long the boiler has been off. Shorter cycle times have shorter preheat times - dramatically so. And shorter cycle times allow taking advantage of natural vacuum. Bottom line I think any system that can fill rads with just an ounce or 2 of pressure will run very efficiently over a wide range of cycling setups. The choice is really about what each owner feels is most comfortable.

PMJ

Agreed.

I might modify that statement to raise that pressure limit to 8 ounces. If it doesn't ever reach the pressure limitation, the number of boiler cycles are simply dependent on the comfort level desired by the occupant weighed against the wear and tear on the equipment.

You're right. 2 is all I need but 8 is still very low and would be plenty efficient.

ChrisJ

What boiler do you have? Can you supply some pictures?

On my boiler that holds 10 gallons of water, if everything is ice cold it takes about 20 minutes to get steam to everything. On a nice cold night, say 0f it takes around 2 minutes.

On my neighbors 1920s boiler that holds 40 gallons or so, it takes a very long time if the boiler is ice cold, upwards of 40 minutes I'd say. On a cold day around 0f it probably still takes 5-10 minutes depending.

Your concern should be how long it takes to get steam to the radiators (all should be about equal) once the boiler starts boiling. This time should be 30 seconds to 2 minutes under most conditions. If it's not, you need to change venting.

Pictures of the boiler, piping, steam mains and radiators would help greatly.

ChrisJ

Hatterasguy said:

ChrisJ said:

This time should be 30 seconds to 2 minutes under most conditions. If it's not, you need to change venting.

This is highly dependent on CPH and boiler percentage. If you run 1 CPH and require a 25% boiler percentage for the current ambient, the piping will be cooling for 45 minutes prior to the next start. That 2 minute time is going to increase.

If you run 3 CPH and you require a 50% boiler percentage for the current ambient, the piping will be cooling for only 10 minutes. Now, your times are realistic.

I recently increased to 1.33 CPH for these exact reasons based upon comments by PMJ. In the future, I might consider 2 CPH in these bitter cold ambients.

You didn't include the first part of the quote which is very important. "Your concern should be how long it takes to get steam to the radiators (all should be about equal) once the boiler starts boiling. "

For insulated piping I don't think 2 minutes is unreasonable even if the piping is ice cold.

Jeff_H

Thank you all for you thoughts. It was the statement in "It's All in the VENTING" that got me thinking about this. I have a Burnham V-75 and have three Gorton #2s as main vents. The radiators are of various sizes and I am using a variety of vents (I started off with VariValues but started switching them to Vent Rite #1s and Maid-O-Midsts as the VariValves began to make noise and spit water - unfortunately the replacements make various degrees of noise - I added the Gorton #2 last year when I began to have this problem). When the radiators are cold, it definitely takes more than 9 minutes (the largest "Minimum Run Time" that I can set on my thermostat) to heat them (it's harder to tell when they are warm, but I'm guessing that the timing may be similar). Because my thermostat (RCS TZ45A) always thinks that it has hit the set point as soon as the heat kicks on. But I can program the thermostat to make the boiler run for longer than that (by increasing the set point for a set period of time, then dropping it back down).

PMJ

The time from fire to steam first reaching rads(preheat) is exactly related to time since the burner last turned off for each system. I'm going to assume that by" ice cold" Chris means room temp as most of us have boilers and mains in heated basements. If the entire system - boiler and all - is at room temperature it will have the longest preheat time - of the order of 20 minutes is what I hear the most often - including Chris above. As the time off lengthens (fewer cycles per hour) the preheat time lengthens all the way up to the worst case of 20 minutes or whatever. For each system these numbers are fixed and very repeatable. They can be timed once and recorded and hard coded into a timing device. That is how I run the preheat side of things - I measure how long I have been off and fire a preheat accordingly with no sensors at all. Off one hour preheat x min, off 2 hours preheat y minutes and so on.

jumper

>>Couldn't even get the 20 ounce gauge to lift off the peg.

At one time it worried me. Not anymore. It heats the six apartments (three stories) just fine.<<

That's the ideal situation.Minimal pressure drop so boiler is controlled by thermostat or timing based on outdoor temperature.

jumper

I like Hatterasguy's quantitative approach.While mains,risers,and pipes are warming up boiler won't turn off due to pressuretrol or thermostat.An intelligent timer knows this and doesn't kill the burner either.So those times shouldn't matter.It's a fortuitous situation when your house is comfortable when the burner is cycled by a pressuretrol.

Jamie Hall

jumper said:

I like Hatterasguy's quantitative approach.While mains,risers,and pipes are warming up boiler won't turn off due to pressuretrol or thermostat.An intelligent timer knows this and doesn't kill the burner either.So those times shouldn't matter.It's a fortuitous situation when your house is comfortable when the burner is cycled by a pressuretrol.

KISS? The place I care for is comfortable... 7200 square feet, rather dubious insulation and some alarming infiltration... one centrally located thermostat. One nice big Weil-McClain (with a beautiful drop header). One vapourstat, which on a long run (say outside temps below 0 plus a wind) may cycle off once or twice at 6 ounces.

Boiler well matched to radiation, and radiation well matched to losses.

Thermostat set for one cycle per hour.

For reference and as a sort of rule of thumb, I figure a time from cold start (and trust me, that basement is cold!) to heat at point whatever on a main at about 15 feet of main per minute after steam appears in the header -- 5 minutes to the farthest radiator in the building.

Jeff_H

I did some experimenting and here is what I found.

1. When I just set the thermostat to a certain amount, the boiler will run for the thermostat's minimum run time of nine minutes and shut off (because it reaches the desired temperature during that time). It will then turn back on about 20 minutes later. Thus the boiler will turn on about twice an hour.

2. When I program the thermostat so that the boiler runs for 15 minutes at a time, the boiler will run for 15 minutes, then stay off for about 40-45 minutes. Thus the boiler will turn on about once each hour.

Both approaches keep my home warm. #2 causes my radiator vents to make more noise than #1, which is problematic at nighttime, but #1 causes my boiler run twice as much and takes a little longer to get the radiators warm when the radiators are cold.

Should I be concerned with #1 causing the boiler to run twice as much as #2?

Jamie Hall

The difference in number of starts won't hurt the boiler. However, you may find that the greater number of starts takes a little more fuel. On the other hand, it may not -- it depends on so many variables.

I'd go with whichever makes you feel more comfortable!