3 questions

Timco

1...If the fan just came on with the t-stat, it would take longer to heat as the furnasty would have to battle the cold initial air running through it. The delay helps it get ahead of itself. 2...Since the furn blows a few minutes until the HX is cooled anyway, a damper would do nothing, but on a boiler they are great for trapping stored heat in the water and keeping drafts of cold air from reaching the HX. 3...A few furnasties I have serviced have a temp switch or rollout switch near the HX and at a few locations to help stop a runaway furnace. The contacts in the fan control's high limit will open if it goes over the set limit, which is pretty reliable.

Tim

zeke

few questions

I have a few questions that have bothered me for some time:

1) How long should it take for the furnace fan switch to close allowing airflow to the registers. Does anyone use a time delay relay to activate the fan. Wouldn't it be more cost effective to abandon that switch and just have the fan come on when the Tstat calls.

2) What , if any, are the savings in using a motorized flue damper to inhibit thermal loss during "off" time.

3) How reliable is the high limit switch on a furnace and does anyone use a backup, since a failure of that switch can cause heat exchanger failure.

Any input or insight into these would be much appreciated.

jp_2

older ones:

1.) the blower comes on when the hx is up to running temp.

2.) an open flue is an opening in the house, if you could measure air volume going up you can determine BTU's needed to replace that warm air leaving with cold air coming into the house- Infiltration......

3.)my parents furnace had this happen, tstat said warm enough, burner kept running, hi limit apparently didn't do anything, or maybe the gas valve was stuck. then what??

zeke

Timco ,
Thanks for your reply. Do you have any links for an electric flue damper of the type you mentioned?

Timco

http://www.johnstonesupply.com/2/tabid/1783/default.aspx?product_group=1797

This is what I was thinking you were refering to...

Tim

zeke

Timco,
Thanks for the link. I will look into it but wonder whether the bearings can survive 550 deg stack temps I get.