New Guy in training

Newbee

Given:
Forced air heat
Runs for a minute then burner shuts down and fan stays on.
What is the items to first check?
Boss asked and I should know

Mark Hunt

Check the


air filter.

Burner is cycling on limit which indicates poor air flow.

Mark H

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colbydoglvr

check the air filter first its probably clogged. the burner is shutting down on high limit. also check the firing rate and make sure it is not over fired.

Newbee

Now he says the filter is new! What next?

toearly_2

Gas or Oil


> Given: Forced air heat Runs for a

> minute then burner shuts down and fan stays on.

> What is the items to first check? Boss asked

> and I should know

Weezbo

fire eye (cad cell is likely sooted ,clean it.

lchmb

burner issue

Depending on the type of unit, and the control on the fan and limit. It could be that the burner is going out on safety or it could be the fan and limit is stuck towards high limit and is short cycling......Is this supposed to be gas or oil?

Kal Row

lots of things...

Most likely first:

Low airflow, (filter, blockage, bad motor or capacitor, blower-blades dirty) – hi limit trip

Really bad thermostat anticipator setting, or failure of anticipator (a lot on digital-tstat), or a semi-shorted burner relay, causing excessive current draw – throwing off the anticipator – all it takes is a soldering splash on an exposed relay coil

Blocked/overloaded flue (causes spillage, may cause auto-pilot blowout, etc) same goes for not enough combustion air on some systems – happens a lot with natural and forced draft devices sharing same flue – eg: another furnace died and was replaced with a forced draft unit, and now an older natural draft unit, sharing flue, goes nuts

Secondary flame sense - on some controls if the pilot sense says ok – it will give the secondary main burner flame sense 1 minute - really dumb if you ask me – see this on HotSurfaceIgnition systems or big commercial gas staging – even worse sometimes, had one HSI, that did not turn on the igniter – but DID turn on the gas!, and ignored the flame sense, the whole block reeked from gas – cuda-gon-booom!! – have the dead control sitting on my desk – it’s from a pool heater, and they will have to pay big bucks to get it out of my hands – think a thunderstorm caused it to fail that way – InMyUnhumbleOpinoin, outdoor devices, should have an electrically separate shielded secondary flame sense shutoff/alarm system – like they really give a rats-@ what I think

Low Gas Pressure – often overlooked problem that gives all sorts of grief, use U-tube, get at least 6 inches with all other major gas loads on the line on

heat-ex burn through failure – this of course is the life threatening one. Use CO tester in plenum - disable system immediately if CO is present – and consider yourself lucky, it failed near the high limit!!!

Kal Row

and like weesbo said it it's oil, diry cad cell or bad stack sw

Dave Bush

Sure, THIS filter is new.

What did the one before it look like. He probably changed it a half hour before calling for service.

Is there an ac coil on top of this thing that might be clogged with cat hair? Is this a gas-fired "90 Plus" furnace with a secondary heat exchanger? (I didn't catch what fuel this was...)

colbydoglvr

maybe there is a football sized hole in the exchanger, the old filter caught fire and poped the unit off on high limit. the fire in the blower compartment caused contamination in the chamber and got the cadcell dirty causing the unit to trip on reset. unfortunitly it is an old 8184g. the customer is hitting the red button right now for the 46th time and you are going to get a call at 2am saying the fire department is there and fortunetly for you the problem is now longer an issue because on the 84th push of the button it decided to light. customer ran upstairs scared as the dickens, unfortunitly he forgot to open the front door of the house as he was running away from the furnace that started bouncing around the basement when it finaly lit. customer is now on front lawn recieving mouth to mouth, fire dept called you at 2am to find out if you had any ideas on how to pull the humidifier out of customers butt... call boss tell him you solved problem and ask him to change humidifier media befor you re install it... lmfao sorry i got bored...

Dave Bush

That scenario.

Well, it could hapen.:=}

Newbee

This is a gas system, with a.c.

Unknown

Newbee

I need more information. Is this an intermittent pilot or is it Hot Surface Ignition? Be more specific as to control numbers etc. That way we can really help you out.

John T_2

Customer is playing with the stat

Newbee

Solved it

Found to be tripped high limit switch & Off level Stat

Thanks for the info. guys

Kal Row

go back and check the anticipator setting...

on those types of tstats - it's real easy to mess with it = happens a lot!!! - people see a slider with numbers - and say "hmm i wonder what this does, maybe higher number means warmer temps..." little does that customer realize that the higher number will make it kick off faster