Gas burner turndown test

STEVE PAUL_3

Pardon my ignorance, but can someone explain to me what a burner turndown test is? And why it is done?

Mark Eatherton

If it's what I think it is...

it applies to CSD1 gas trains, and essentially, you are creating a condition whereby the pilot will lose partial flame strength, and there is a certain threshold that the gas train is required to lock out and shut down requiring manual reset.



With CSD1, any time there is a loss of flame signal during a sustained call for heat (other than hitting high limit) it is required to drop out and lock out.



HTH



ME

Charlie from wmass

On Buderus boilers

You need to test the combustion at full burn and at 30% or so burn rating. Is this what you are talking about?

Tim McElwain

There are two tests that

are required when testing for safe light-off on equipment by technicians in the field:

1. Is the pilot safety turn down test. It requires the pilot to be reduced to a point that will just hold in any safeties, a minimum flame if you will, you then attempt to light the main burner on residential systems it must light safely in 4 seconds or less. This is typically required in the field when someone has replaced or retrofitted a pilot on a boiler, furnace or water heater. It has to be done no matter what the ignition system, millivolts, microamps etc.



2. A main burner turn-down test is one that is done less frequently but still should be done when changing the firing set up on equipment. To help you understand this a burners lowest input cannot go lower than 1/3 of input, and that is not BTU input but input as far as gas pressure to the burner. Lets say we have a burner that has 3.5" W.C. for outlet pressure then that system say on a step opening gas valve would usually have a pressure of around 1" W.C. on the low end and should light safely. When changing a system from a full light-off system to a two stage system it means that a safe light-off must occur on that burner when fired at the first stage input. If once the retrofit is done it does not then the staging cannot be done. It is always best when determining a change of firing rate in the field to check with the manufacturer before spending the time and money to do the change and see if they approve and have tested with that setup.



On modulating systems the factory setup for low fire must meet these criteria in the testing before the equipment can be sold. This is perhaps one of the things that affects the turn down ratio on burners and probably why we do not see burners firing down around 10,000 or 20,000 BTU's. When you drop down very low in input many servo regulators will start a surging of the flame (up and down) as the regulator searches for a higher input (higher downstream demand).



On the commercial side this is why we have CSD requirements on gas trains such as low pressure and high pressure switches etc.



Hope this helps, perhaps someone who is involved in the laboratory set up of this can refine my explanation better.