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readings that point to efficiency

jp_2 Member Posts: 1,935
Tim, what numbers can tell you the most about the efficiency of the equipment you are testing?

if you didn't know, what numbers tell you the equipment is still cold and not up to a stable operating condition?  how much do they change?

I'm mainly wondering about inefficiences due to the equipment on cold start up?  as in short cycling?  what are your thoughts?


  • Tim McElwain
    Tim McElwain Member Posts: 4,524
    edited April 2011
    The number one answer is the customers


    If we are talking combustion efficiency then O2, Net Stack Temperature, CO  and draft are the main things I look at.

    If we are talking thermal efficiency relative to cycle then a number of factors have to be addressed. Is this a Mod/Con or standard furnace or boiler

    Do we have DHW?

    What do we have for emitters?

    Does the unit have ODR?

    What is the Delta T?

    Are we working off a system that requires setting a heating curve?

    What is the maximum boiler water temperature?

    Was a heat loss done before installation? If it is an older system has a recent heat loss been done?

    Is it necessary to be operating as cold start, perhaps maintaining some boiler water temperature would be more feasible?

    Is a setback thermostat involved?

    These are just a few of the things that have to be considered. With modulating systems are we finding we are cycling on limit or are we maintaining sufficient temp to satisfy the heat demand and modulating the burner in the lower end of firing rate. The lower the better both from operating at lower temps ensuring condensing if this is a condensing unit, and using less BTU to satisfy the heat demand. The differences in burner efficiency as measured between low and high fire are not very much as the excess air is affected, Ideally if we could stay around 25 to 30% excess air at all times this would increase efficient operation as to thermal efficiency.

    I have taken combustion tests on 90% + equipment with gas pressure as low as 2" W.C. up to almost 5" W.C. and the readings do not change more than 1/2 a %. In doing so however if thermal efficiency's were measured they would vary immensely. I think Jim Davis stated it would be around 50% to 85%. The analyzer measured efficiency during all of this would be around 87 to 89%.

    The really only true measure of efficiency if we must is thermal efficiency. A measure of the flow in CFH (forced warm air) or GPM (forced hot water) times the Delta T divided by the measurement of cubic feet of gas or gallons per hour oil. If we could measure propane we would see very little difference. What is interesting is that all shows up on the gas bill all things being equal. With the bill use only actual fuel used not surcharges etc.

    The biggest problem I see with efficiency discussions in general is we talk numbers instead of applications. The system and the appliance must work together otherwise they actually when out of sync cancel one another out.

    Great 90+ equipment poorly applied poor efficiency.

    I recently consulted on a job and got my first real experience with BTU Flow Meters (ONICON Inc) on a commercial job using BACnet IP utility monitoring system. It will be interesting to follow the progress of that install. My only other experience was with flow meters used on gas lines along with correctors to insure accurate measurement of gas on high use systems. 

    I hope this is what you are looking for. I have more complex discussion on condensing and modulating efficiency. I am also including as one of the chapters a very lengthy discussion  from here on the Wall involving Mark Eatherton and myself along with others in my upcoming Mod/Con Manual Volume II it is titled Modulating is it efficient?. 
  • jp_2
    jp_2 Member Posts: 1,935
    thanks tim

    I'll comment more later, I guess what I'm really curious about is the hit efficiency takes when short cycling, its talked about a lot here, but there seems to be no hard numbers. 

    it seems short cycling itself has no hard definition either, so that makes it more of a fuzzy subject.

    so, thats why I'm asking about the combustion side, I think that's where the answer lies in the short cycle debate. also  combustion stability/instability should be able to define short cycle, certainly any time before things get stable can be considered short.
  • Tim McElwain
    Tim McElwain Member Posts: 4,524
    jp that is what

    makes Lambda Pro rather interesting as it will correct on each cycle to ensure the proper firing gas/air mix for a more stable firing even on short cycling. My take on short cycling by the way has to do with the inability to have higher turn-down ratios on our equipment here in the USA and also the lack of equipment that can fire down in the lower BTU range to match some of the very low heat losses we are seeing on some systems.

    I liked Dave Yates article in Contractor magazine titled "The Biggest Loser" it also has a lot to do with cycling as we more closely can control flow and balance temperature in each zone in particular the smaller zones.
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