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Comment by jacobsenscott | Hacker News Reader

If you live in a place with forced air heat you can probably set the furnace fan to be on all the time. Install a hepa filter and you are good to go.

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Furnace fans often use a lot more electricity than you’d expect. You can check by looking at your electric meter with it on and off (or maybe by looking at a sticker on the furnace).

Multiple HVAC techs I've talked to have recommended against installing good/high quality filters in a forced air system. They've explained that high-MERV filters reduce airflow which causes the fan to wear out faster and may affect the performance of the cooling/heating. I'm not sure how accurate that is, researching it seems to suggest a MERV 10 to 12 is a good balance, but I've been told to use a MERV 3 filter.

You should be able to look in the manual and see the maximum MERV your forced air system is rated for.

MERV 3 is really really minimal filtering. Your furnace almost certainly is designed to take better filters than that.

It is not actually as simple as looking at the manual. You need to take ducting layout into account, since there is a lot of air flow reduction from any ducting system, and some can be much worse than others. It definitely does cause your furnace to overheat if you drop below the required airflow rate, something that can lead to short cycling and even premature failure by cracking your heat exchanger. All of this is calculated out using HVAC software when a system is commissioned (in theory and in many building codes, but not so much in practice).

As can be seen, this is an issue during the heating season, not the cooling season - which is when many people are fighting smoke. If you actually pay attention to home maintenance, you can go for the better filter in the summer, and drop down to the minimum or close to it during the winter.

Last I looked it appeared you are safe to go MERV 8 for most residential HVAC units. MERV 3 is not filtering much

The only problem is that instead of re-circulating and filtering conditioned air, a furnace fan is going to bring in external unconditioned air (and is only filtered once).

Your furnace pulls from outside? Mine doesn't. I would've thought you'd want to heat the warmer indoor air to save energy.

I haven’t had a central air system that does this. Mine have always had return vents and it just cycles internal air.

I’m actually very curious about more intelligent systems. Those that can:

- bring in outside air on occasion to freshen things up

- negotiate the best way to cool a home depending on outside air. It drives me mad when my AC kicks on and it’s actually colder outside.

As a kid I always wanted an external vent on the refrigerator that pulled in outside air when it was cold enough.

> - bring in outside air on occasion to freshen things up

All HRV's do that. "single room recuperator" from aliexpress is in every flat after renovation in Lithuania.

I’ve never seen a furnace fan that pulls in outside air, unless it was some DIY garbage that wasn’t to code, or a huge money waster from 100 years ago.

From where? Bringing in outside air is what a 'fresh air' system does like an ERV or HRV

Most older homes simple have an outdoor draft/damper for makeup air.

You’ll draw in a lot of dirty outside air with that setup if you don’t block/reduce outside air intake. It will still go through a filter, but it’s going to cause the filter to dirty very quickly.

An office or school's central air system might bring in some outside air, but regular house systems don't. They have intake grills inside the house.

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