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I recently had a brand new HVAC system installed in my attic. The furnace is natural gas. The furnace kept shutting down after 7 to 12 minutes, due to "too much air" blowing on heat sensor switches and overheating them. It would take over 2 hours to reset, then it would shut down again after 7 to 12 minutes.

After 3 service attempts to correct the problem, they finally (without telling me) cut a hole in the heater box, screwed on a vent register and simply drained the excess heat into the attic.

That stopped the shutting down problem, BUT they told me they fixed it by replacing the sensors switches and mentioned nothing about the new vent. I only discovered what they did when I opened the attic access door, and felt hot air blowing out. The attic was very hot.

My attic is full of blown-in insulation and mouse droppings - I don't like forced air blowing around up there.

Is this a standard, safe fix?

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  • Is your attic vented to the outside...? – ThreePhaseEel Jan 23 at 23:05
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    If you live in a place that gets snow you definitely do not want heat melting the snow on your roof, having the water run down and freezing on the eves creating ice damns. – Alaska Man Jan 23 at 23:21
  • This is simply astounding to me and why I try to do as much as I can myself. I don't trust anyone. If you live in a cold weather area, this is very bad. – Evil Elf Feb 23 at 12:41
  • Because the only other fix (other than re-ducting your entire house) is swapping out for the correct sized unit at their expense, which is why they shined you on. – Mazura Feb 23 at 16:11
  • Do you also have an air conditioner on this air handler, and if so have you used it yet? Analogous issues if the A/C is oversized for the ducts/registers is freezing condensation in the air handler (which causes many issues), dankness (cold, humid house), abnormal amount of condensation in the handler, possibly exceeding the capacity of the drainage system .. which is a BAD thing in an attic, and premature failure of the A/C. And cooling the attic with a rogue vent might cause condensation on the roof in summer .. that would lead to an interesting question here! – jay613 Mar 7 at 23:14
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"Too much air" would not trigger overheat switches. Too much heat would. One possible cause is an oversized furnace with not enough registers or not enough return air or both. This could cause overheating as you describe. It's a guess.

[EDITED to add 3 additional, less drastic, steps] : There are some things you should try, that I hope you or the installers tried before installing the new attic register. You could try them now by plugging up the new attic register with cardboard and duct tape.

  1. Make sure the filter is clean. If you have a high MERV filter (8 or higher) you need to change it frequently even if it doesn't look dirty. Try removing the filter entirely just for a day to see if that alone cures the problem.
  2. Make sure all the registers are open. Does that cure the problem? If it does you may just have to live with it that way.
  3. If not, leave all bedroom doors open. Doors that are tight all around, eg that contact carpet at the bottom, prevent air flow from the registers to the return. If leaving them open cures the problem, you need to shave gaps in the bottoms of the bedroom doors or install grilles to increase airflow.

If none of those things work the solution is to install a smaller furnace, or to add ducting and registers (in and/or out) to improve air flow through the furnace. My guess is your installer did that ... the easy way. They added a register, with no new ducting, that just dumps air into the attic to increase flow through the furnace. Presumably there is no return air path in your attic so in addition to heating the attic and stirring up insulation and mouse droppings they also created air flow through the attic hatch and/or any other possible path from the attic to the return register that is probably in your upstairs hallway.

It's a guess, I hope I'm wrong. Other than replacing the furnace with a smaller one, the best solution would be to add registers in the rooms being served by this unit, but that may in turn necessitate more or larger ducts both to and from the furnace.

Hopefully my guess is wrong and your problem is something else entirely. :(.

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Do you know the size of the old furnace that they replaced? They may have installed a much larger furnace, which is common in many areas, to make sure that they do not install a too small furnace. They should have done a heat loss calculation to correctly size the new furnace. I am not quite sure what you mean by "too much air blowing on the heat sensor switches" so I am guessing that you mean the furnace is shutting down due to excessive air temperature. Guessing what your problem is, is difficult since we do not know the size of the home, it's location, the heat loss and needs, or the size and model of the equipment. Much more information is needed.

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To me this sounds like a lazy way of adjusting the system for improper flow rates over the heat exchanger.

Without additional information of the model , duct sizing and both vent and return square inch opening it is not possible to say with certainty.

So with the information provided, I would say it is very inefficient and at some level unsafe.

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