Adding insulation causes furnace to run more often?

[mkirsch1]

The attic had old R13 fiberglass batts between the joists and a couple of inches of blown-in cellulose on top of that. Definitely not enough for Western NY winters. So, the tax refund went toward 20 rolls of R30 attic blanket (on sale at Home Depot, cheaper than R25), which I just finished installing yesterday evening. Now the main area of the house has R50, just over what the Owens-Corning and EnergyStar websites recommend for my area.

When I moved in, I re-balanced the system by playing with registers and dampers and got it to where the furnace would run once an hour for

20-25 minutes, and the temperatures in the rooms were much more even. It was pretty bad before that.

Now that I have the insulation, the furnace runs 10-12 minutes, then is off for 15-20 minutes, and then runs again for 10-12 minutes. I'm wondering if the insulation was a good thing or a bad thing now... Shouldn't the furnace run LESS often?

[Joseph Meehan]

Not less often, but it should have the gas or or electricity on less total time.

Rebalancing likely changed the cycle time. Sounds like it may be better the way it is and more efficient. When it runs for say 12 minutes it may have the heat source on for say 5 minutes running for say 22 minutes the heat source may be on for 15 minutes. The time that the blower is running and it is not using fuel, will even out the heat and recover the heat still in the furnace when it shuts down and move it in to where it will do more good.

Of course it is possible that you did not do all that good at balancing have reduced the total air handling capacity, that could damage the air handler and cause reduced efficiency. You also may have set the balance so the area with the thermostat is getting a smaller part of the heat so it calls for more heat, resulting in warmer other areas, but more cost.

The time the heat stays on in each cycle depends on the temperature outside and how much heat is lost. You should be loosing less heat. Of course if you made some sort of error that could increase your heat load. The only way I can think you could have done that would be to have removed what was there and just used the new. That "blown-in cellulose" tends to seal cracks and leaks. I hope you kept it. The other factor is an adjustment at the thermostat call the anticipator.

[dnoyeB]

Yes, it should run less often if the outside conditions are the same. This is given that you were losing significant heat through your roof. It could be that your major heat loss is through your windows, or some other area.

If the insulation made a difference I would expect your attic to be colder than it was before.

[mkirsch1]

Thanks for the reply.

I did indeed leave the cellulose, and unrolled the attic blanket over the top of everything. That's where I got the "roughly R50" figure. Way too much work in hauling all the old stuff out, and pretty darned foolish if you ask me.

My main concern is the drastic change in how the furnace is running. If having the furnace cycle so frequently is normal, then that's not a problem.

Right now, I'm timing how long the fan runs, but I'm going to start listening for the burner instead. If that's only running for the first

5-7 minutes or so, then I have indeed made a difference. When the furnace was running for 25 minutes, the burner would run for the first 20-22.

[barry]

Could be different weather from the time you took the previous "measurements." Or your recollection of what was happening then could be a tad off.

Insulation a bad thing? Really? I don't think so, wouldn't even consider the possibility. Anyhow, insulation is only part of the picture, especially if it's only above.

Loss through walls and via leaks can be significant too, and affect your timings, depending on ambient temps and wind-speed.

IOW, get the losses down, get the heating system working properly and efficiently, then find other projects.

J

[hallerb]

any ducts running thru attic? if so you might have damaged or crushed them:(

[hallerb]

whens the last time you changed the filter:)

a clogged dirty filter can cause this exact trouble

[lwasserm]

Assuming you have a forced hot air system here. Most furnaces turn the fan off a few minutes _after_ the burner goes off. So it is possible that the fan may be running longer, but fuel usage could be lower than before the insulation was installed.

[mkirsch1]

Yep, it's forced air, gas-fired. Ducting in the basement, so no chance of crushing that. The furnace filter was changed just before I took posession of the house. FHA wouldn't approve the loan otherwise. It's only been 2 months so I can't believe the filter is that clogged yet.

Besides, the change was SUDDEN, as in I went up in the crawl space with

12 rolls, and came down 90 minutes later to the furnace on this new cycle. It was actually colder outside, and windier, before I installed the insulation and the furnace didn't run this often.

More observation, it appears that the burner runs about 6-7 minutes, and the fan 4-5 minutes afterwards. Time between furnace runs has actually increased to 25-30 minutes.

[trader4]

I find it hard to believe that adding insulation over R13 and a couple of inchs of additonal blown-in insulation in the attic would make a drastic and noticeable difference in the duty cycling of the furnace. Is it possible you changed something else, like created a draft somehow that's making it;s way to the thermostat? Left an access panel open?