Just in the process of sorting out things for the new house and we’re looking to get either a logburner or use the current open fire (after having the chimney swept).
Our eldest has mild asthma managed with an inhaler.
Are there any issues known with fires and asthma?

I don’t know, but if you want an Aga Rembrandt cast iron fireplace give me a shout. I’ve just removed one from our house.

There is if you light one without opening the flue.
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…so im told.

Even with an approved wood burner you can taste smoke outdoors on still days. Shouldn’t notice anything in the house whilst its burning.

Open fire might be worse than stove, which is obviously sealed when the door is shut.*

You might find outside the house is worse at times in the cold weather.

Worth considering smokeless fuel.

*EDIT: vents are open of course…

As above shouldn’t make it any worse in the house but does reduce air quality in general. Unlikely to make a difference as just one fire but it depends if you want to be a part of that particular problem.

Better to run a smaller stove flat out rather than a bigger one damped down for a more efficient and complete burn.
An open fire doesn’t offer this and has the joy of being able to smoke the room out, just because.

Had a stove fitted last December, MsD has well-controlled asthma.
She says she has zero problems from our stove, or even our garden chiminea.

Do you live on Alaska?

If not just get a decent central hearing system and save from your trendy smog.

I have Asthma and have had a log burner for a few years without a problem, it’s central heating that kills me

Cheapest solution, try the open fire first? But common sense leads to it’s ‘open’ therefore stuff can escape into the room. However wood burning stoves also elevate particulate level in the room they’re in, so they could affect your son (seems it might not from comments above). It could be quite a lot of money to spend to find that out, unless you can expose them to a stove elsewhere.

Or you could do what’s perhaps best for everyone and do neither option. It could be that a stove may not trigger asthma but the elevated particulate level may have other health detriments to all of the family. Personally I think you’d have to be off grid, out in the sticks, with a nearby copse and this is your primary and only heat source for it to worth it. But then options like biomass boilers might trump it in that situation.