I get a vaguely nauseous/ "not fresh air" smell coming from my HVAC. I think it's more heat associated than with the AC, which makes me think that the outside of the heater matrix might need a rinse. I redid the o rings in the summer, but was on AC for months, and only recently started to require heat. It was quite unpleasant at first, and is becoming less noticeable now. I have no reason to think the matrix is leaking. I'm not too hot on the HVAC anatomy, but does anyone know of any good ways to clean the outside of the matrix? There are various "bombs" available on the market - anyone found a good one? If i wanted to do a more physical clean is that going to be a dash-off job, or is there a pathway to getting an aerosol solvent directly into the heater box via one of the vents?

the drain from the ac might be blocked

What sort of smell is it? Sweet sickly smells are usually coolant, if its more like stale or damp smells I'd think mad-as is pointing towards the right area.

Although if the AC drain is blocked you usually get a puddle of water in the passenger footwell.

Drains are clear, I think. Certainly the "flowers" above the gearbox are, or can a blockage develop internally? I poked it with a length of plastic tube anyway, and I didn't get covered. Anyway, I think the issue is not with the a/c, it's the heater core, I think.
It doesn't smell sweet/ like coolant, although my sense of smell is rubbish, it's more of a "burning plastic" type feeling, more than a smell.
I changed out the pollen filters today, which were damp and a little black in places, after less than a year. Both fans are working full bore.
So, can anyone think of a way to clean /rinse the matrix short of stripping it out.
Prior to changing the rad the "feeling" getting bad wd often coincide with a hevac service book symbol. But in nanocom there wd never be any fault reported. Does hevac have a max temp on the heater core? I wonder if the core being "too" hot wd throw a service book error and at the same time that extra heat was burning something off?

I'd be inclined to dump a load of the cleaning stuff down each of the pollen filter holes then poke the Prog button. That will close the recirculate flaps and put it on max heat with both blowers on flat out. Can't do any harm and may sort it. Extra heat may have caused something to start to cook but I can't see it really, it'll never get hot enough to start to melt any of the plastic. Sure you haven't got a dead mouse in there?

I'm just a bit wary of putting in something that the blowers might not like, Richard.? The other option I just thought about is to put my borehole camera into the heater core temp sensor hole for inspection, and maybe then rinse out that way?

Where? The heater core temp sensor is the one clipped to the inlet pipe just before the O ring joint. I can't think of any way of getting to the actual matrix itself without major dismantling.

ah no, you're right - I was thinking of the one that plugs in lower down - which is for the evaporator, so that's not going to work

I changed the pollen filters the other day. I think what may have happened is some (spray) cavity wax that i was using to coat the windscreen wiper mechanism may have contaminated the cabin air intakes in the plenum, and to an extent the filters themselves. Changing the filters has helped, but I'll probably need to pull the filter housings, and make sure everything is cleaned out, and then make sure the drains are completely clear, for good measure