Hi All,

I hope someone here can help me. First some background on my system.

I DIY installed my MHRV about 13 years ago using a Vent Axia Air Minder plus ventilation unit. The pipes are all solid (apart from direct connections to HRV unit) and are either within the insulated layer of the building or wrapped with 40mm fibreglass foil wrap and taped up to avoid cold air pockets. There are a couple of condensate drains tied to the overflow of the house to carry away condensate that may build up in the extract pipework as it leaves the insulated layer to the uninsulated layer. In theory the unit is perfectly sized. In practice I believe it to be oversized as a draft can be felt at times when standing near the inlets. It's also quite noisy and I never need boost mode. It's permanently on low vent mode.

I have to admit I've somewhat neglected the system as it's always just worked. I wash and clean the filters at least once a year but they are always filthy. I should be doing it 4 times per year. I've only previously opened the unit up and cleaned the heat exchanger once about 5 or 6 years ago. At that time there was no mold present except for a small bit around the condensate drain; the unit was very clean otherwise.

We dry our clothes in the house and due to the effectiveness of the MHRV a clothes horse dries in less than a day. Over the past year or two we have noticed it is taking a couple of days to dry clothes. Up until recently (the last few months) we have had zero mold in the house; now we have noticed a small amount developing in our main bathroom.

Over the past few weeks the unit has been getting quite noisy to the point I've switched it off a couple of times at night. This evening I split the unit to find the extract fan appears to have a failed / failing bearing. Of more of a concern was the dirt and mold on the H.E. The extract blower is caked in what appears to be dust and mold also. The inside of the unit is largely dry except for a small damp spot at the condensate drain. The unit is exceptionally dirty also. Fortunately the air inlet fan and ducting is pristine so the filtration appears to be working there.

Obviously I'm really concerned with the mold issue and would like to get to the bottom of it. I can't tell 100% just yet but I believe the HE may have blocked with dust (even though it is filtered), reduced the air flow through it allowing the damp extract air to mix with the dust and facilitate mold growth. Is this a likely scenario?

Comments and opinions welcome.

Do you have temperature/humidity meters in the house? If not I suggest you get a few, they're very cheap. Once you have some readings, you'll be much better placed to diagnose the situation. If you can monitor the levels at the MVHR unit ducts, so much the better.

You say you wash the filters at least once a year, but when did you last replace them? Do so at least every few years.

You clearly need to replace a failing fan, or the whole unit if that makes more economic or practical sense.

If the extract fan is not working properly then all the air that is being pumped in by the intake fan must be escaping through cracks etc as well as through the ductwork, so you may have problems elsewhere as well. The humidity will likely have increased, as demonstrated by the mould and the increased drying times. I would suggest opening windows regularly and not drying clothes inside until you have repaired or replaced the MVHR unit. i.e. treat the house as a 'normal' unventilated house.

Thanks for all the replies so far.

I contacted Vent Axia about a replacement fan however the model of MHRV unit I have has been discontinued for over 10 years. They have no spares available.

I decided to strip the motor to see if it could be repaired. Long story short I have the old bearings removed (which have considerable play in them) and will order replacements for about £2 each. I think I'll do the fresh air fan when I'm at it.

I also took to cleaning the unit. There was mold on the HE unit but not nearly as bad as I'd thought. What I assumed was mold on the fan turns out to be polystyrene being cut / melted away with the fan movement with some mold growth on top. As usual I jumped the gun without a proper investigation. One thing to note is the fan is quite rusted. Should I clean and paint this or will that upset the balance?

The exhaust air duct into the heat exchanger has some fluff on it but it's not bad and crucially it's bone dry. After cleaning the HE there is only a small amount of residual mold on the outside of the fins at the cold exhaust side. I cleaned with a mild caustic solution (G101 from Autosmart) and left the solution on for 10 minutes or so. Hopefully all growth will have been killed off now.

Some pictures for you all attached.

Fan

Warm Exhaust Duct

Fresh Air To House Grill

Cleaned HE

Cleaned Fan Holder

IMO you are lucky to be able to dismantle the motor to change the bearings. A cheap fix. Too often these days items are manufactured with no possibility of dismantling without destruction.

For the rust you could (gently) clean off the rust and then spray with a can of spray paint suitable for the fan material. Spray just enough to cover the material. I would not brush the paint on.

Whilst Vent Axia may not have spares (at their inflated prices) often manufactures will use off the shelf industry standard units rather than design and manufacture their own (makes sense from a cost/profit view point) and the components will often have a part number or 2 that can be tracked down on the internet and ordered separately. Otherwise fans (and other mechanical bits) often come in generic sizes so searching for fan type by size and wattage can produce results.

You might have seen on another thread, I replaced my Helios unit with one from Komfovent and I am very pleased with it. I'd replaced one motor at about £1K and another was on the way out. I'm also sure the new unit is recovering way more heat. The unit tells me it is consuming 55W and recovering 840W right now. Today, It says it's recovered 12kWh for 1.2kWh or energy used (as at 10pm).

Measure, measure, measure. Data is absolutely invaluable. Find a way to record the temperature and humidity of the exhaust air - this really is vital to know what is going on.

I'm convinced that I have been overventilating my house for years. I now measure the CO2 in the exhaust air (plus a couple of rooms) and that has resulted in me reducing the ventilation rate massively. I should have fitted a smaller unit, but didn't really start gathering the data until after I'd replaced it. However, when one of the kids come home, I need to increase the ventilation to maintain the right CO2 level.

Re your mould, my suspicion would be that the Heat Exchanger is not working correctly and so the RH is not being reduced.

As DJH says, get some sensors to measure the RH and Temperature in the house. I bought these recently and read the data using HomeAssistant https://www.amazon.co.uk/dp/B08XQD8MFZ

Final thought, when replacing the unit, I fitted an inline filter box on the inlet side and that significantly reduces the amount of crap that gets as far as the MVHR unit!