Burner Cycle Time

John_34

My burner runs for about 10 minutes until the boiler reaches high limit then kicks off for about 4 minutes, then back on for another 10. This continues until the thermostat is satisfied, of all the while the circ pump continues to run. Does this sound optimal? If not, what kind of cycle times should I aim for?

dave connors

bypass

do you have a valve on the bypass? if so you might want to close it off a little to lenghten your cycle time

John_34

Yes, there is a valve. How long a cycle time should I shoot for?

dave connors

questions

how big are the mains, both supply and return. how big is the bypass line?

John_34

Both mains and the bypass are 1-1/4\"

Weezbo

*~/:) hmmm....

to me a bypass is not the same size as the headder. all it does is Temper The cold temp return at the boiler to something along 140 ¡ãF.or when used at a distance from the boiler this bypass i call a station Bypass, its general idea is to keep the water temps at the station at a certain temp to supply the boiler water temp...rather than cold water that has laid in the pipe dormant losing its last burst of heat to the surrounding environment...a station bypass also mixes the cold water that returns on a call for heat back to> the boiler......,if it were the same size as the header what would induce flow anywhere other than the header ?

dave connors

bypass is too big,

trying closing the valve located on the bypass about 50%.

John_34

The bypass valve is closed about half way........

I can close it more, but what should I shoot where as far as cycle times go?

dconnors

bypass

I would probably close the bypass until i achieved a 20 deg. delta t on the mains.

John_34

I can do that, but how long should it take for the system to get up to operating temperature? I mean from a cold start to when the boiler temp reaches 140, how much time should that take?

John_34

The boiler manual

says:
"Use a system bypass if the boiler is to be operated in a system which has a large volume or excessive radiation where low boiler water temperatures my be encountered (i.e. converted gravity circulation system, etc.) The bypass should be the same size as the supply and return lines with valves to regulate water flow for maintenance of higher boiler water temperature." The piping diagram in the manual shows the bypass line, supply and return lines all at 1-1/2".

Cutting back the supply valve upstream from the circulator slows flow out of the boiler (increasing boiler temp), and increases the amount of the return water that bypasses the boiler and re-enters the supply line downstream from the supply valve.

Is this not an okay way to do it?

Also, how long should it take the system (return water) to reach operating temperature of, say 140deg? This is a high-mass system with all cast-iron rads and is a conversion from a gravity-circ.

hr

Hard to get a manual valve set exactly



A manual bypass is a "dumb" devise. Problem being the setting you decide on now may not be correct as the heating season really hits. A manual bypass valve has no way to react and change to temperature. Better than nothing, but not much so

To truly protect you would want a thermostatic valve or an injection system with temperature sensors to respond.

How close is the boiler sized to the load on a design day? Ideally at design day it would run non stop if sized exactly right.

As far as a good run/ off cycle the best answer would be non stop burner operation!

A modulating burner would adjust to the heat demand and ramp the burner accordingly. New boilers are getting real close to that.

PM engineer mag did a survey on boiler cycling a few years back. Pretty wide range of what installers felt was "reasonable"

Keep an eye on the return tempoerature to the boiler under load. Extended run cycles with the return below dew point of the fuel, say 130-140°F is bad for the boiler and flue. A thermostatic mixer at the boiler would handle this nicely. It would keep the boiler in it's sweet spot and bleed heat into the building as the boiler maintained return temperature.

hot rod

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John_34

Okay, I'm confused,

In one of the pages on this site, Dan says:
"Q: Does the bypass serve any other purpose?
A: It allows the boiler to come up to high-limit temperature and shut off. Without the bypass, the large volume of water moving through the boiler often keeps the temperature low and prevent the boiler from reaching high-limit. This does a good job of increasing the fuel bill."

But from what you are saying, if I stay with the manual valve, I want to adjust it for a 20deg drop, and DO NOT want to cycle on/off due to high limit. Am I understanding you correctly? Also, how long should it take for the boiler to reach 140deg upon startup?

Thanks hot rod!

Boilerpro_3

That always confused me too

And I do this kind of work for a living! I don't see how having the boiler cycle off the aquastat will save money. And if it is sized properly does this on the design day, you will not be able to heat the space!

On a properly sized boiler installation in a graivity conversion where the radiation must run at 180F on the design day (pretty rare) using P/S piping with thermostatic mixing valve on boiler return, I see about a 20 minute burner on time as the thermostat cycles the burner, for typical winter weather (here in northern Illinois thats about 25F). In this time the system water temp(not boiler temp) goes up about 20F.

Boilerpro

John_34

To really do this right,

What do I need to change or add? Right now I have two zones, 1st floor zone is using original 2" gravity piping to deliver H2O via circulator. 2nd floor zone is using 3/4" copper branch circuits, also with it's own circ pump. I have a manual bypass line with ball valves, the line is the same size as the supply/returns (1-1/4") as described in my boilers manual. Where can I learn about primary/secondary and thermostatic mixing valves? Will I need to add another circ pump? If so, what controls the new pump?

hr

Two goals

you want to protect the boiler from thermal shock, most importantly. And ideally you would like nice long burner run cycles while maintaining return protection

Somewhere you will have to compromise. The heat load of the building is ever changing. If you were to adjust and install a bypass now in mild weather the dynamics will be different in full load heating conditions.

Lots of variables come into play. Is the boiler sized to the radiation? Is the radiation sized to the design load? Has the building or insulation changed since the install and calcs?

You may not gat a perfect long, efficient burn, in all load conditions regardless of the bypass, need to figure out all the variables.

The 10 minute minimun burn seems to be the number most of the respondents used when asked about cycling in the article. You should have good return temperatures in that time frame. Still think I would be more cocerned with the return temperature than the on / off cycle. Doilers will work with shorter cycles, drives me nuts, but plenty run their entire life short cycling. Low mass boilers do this commonly in low load conditions.

Sounds like you have a handle on the concept, but not quite sure how to fix it to you system. BP does a lot of these sucessfully, I'd use his method.

Here is a link to a B&G article on this subject.

http://www.bellgossett.com/press/thermalshock.htm

hot rod

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hr

More info from Burnham

http://www.pmengineer.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2732,90176,00.html

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