There's nothing particularly complicated about duct heaters. We have a VEAB CV16-18-1-MQEM. Exactly which one do you have, since it makes a difference to the possible problems? Also post a wiring diagram of your installation if you have one. A diagram or photo showing where it is installed would also probably be useful.

BTW, was there any warranty with the unit and its installation? Did you ask the now retired electrician?

Posted By: richardelliotI get the impression they are just big hair driers that turn on when the inflow air temp drops below a certain level

They don't care at all about the input temperature and exactly what they do care about depends on which model it is.

Basically, make sure it is turned on, turn its room thermostat up if it has one, turn the ventilation rate up and wait a few minutes. If the casing starts to get warm, it's working; if it doesn't then there's something wrong (or needs adjusting).

The builder instructed all his team never to give out their contact details so I only ever knew the electrician's first name.

But the electrician must have given you a test certificate for the installation, and Part P notices etc and they will be signed. And if the signature is illegible, then they will have his registration number and you can get his name from his registration body, or you can appeal to them to sort out the installation.

Sale of Good and Services Act, or whatever its called nowadays, gives you rights for up to six years. If it's never worked, push somebody to fix it.

Posted By: richardelliotI've been up into the loft this evening and we have a CV 16-09-1MQU. The manual says there is a temperature sensor in the duct and I can see the cable from the unit going into the duct (the white cable).

Good; that's some progress. I think the photos will be very helpful when you do manage to reduce their size.

The MQU doesn't have a room setpoint if you're using its sensor in the duct. Is the setpoint adjuster at a sensible value (50°C or so). Presumably you have some other control circuit to decide whether the heater should be on or not; how is that set up?

Posted By: richardelliotthe way it was explained to me is that it is a pre greater for the incoming air

Ah! I've been assuming it was a post heater. So which duct is it in? And how do you know whether it is on or off?

Posted By: richardelliotThe manual says there is a temperature sensor in the duct and I can see the cable from the unit going into the duct (the white cable).

If the white cable is the sensor and the white/grey box is the heater unit then it looks a bit like the sensor is down stream of the heater. I would have expected to see the sensor up stream (before the outside air gets to the heater) otherwise the sensor is sensing the heated air not the incoming air.
Or
The dial on the unit sets the temp for the air to go into the MVHR rather than turning on the heater when the incoming air is below a set temp. I suppose either setup would work.

Do you have deg.C marked on the dial or numbers say 1-10 - or just min / max ? It could be that the adjustment range of the unit is outside the temp. of the air at the moment (air too warm) and without knowing the max/min of the temp. range available it is not possible to know if the unit is failing to come on or should not come on yet

If your electrician has checked for power to the heater and it works (presumably by bypassing the control circuitry) then it must be a control/sensor problem rather than a duff heater.

Sory, but I'm really confused. You originally said: "Last winter during 'the beast from the east' I noticed that it never turned on. It wasn't something I tested when we did the handover of the house with our builder so I'm not sure if it is ever worked (so it could be faulty or not installed correct)." But now you say: "you can hear it when it is on and I can see from my power consumption when it is working too". So which is it?

I'm surprised that you can hear it when it's on. There are no moving parts in mine. But if it is drawing the correct power (900 W maximum, I believe), then what is the problem?

I'm not clear from the manual whether the setpoint on your unit is for the incoming air or the preheated air, as PiH says. In either case I wouldn't expect the preheater to be turning on this year yet, unless you have set the threshold high. It's only purpose is to stop frozen condensation inside the heat exchanger by warming the incoming air above freezing.

The only other external control is the reset for overheat.

There are various controls and indicator LEDs inside the unit, but as you say you're not comfortable with electrics and as there are most definitely mains voltages inside the unit, then you will need to find an electricianas you asked. I hope you are successful in contacting the electrician who installed it. If not then I would suggest you contact VEAB (Sales Export Manager?) and ask them about service & installation near you.

PiH - the temeperature dial is in °C and ranges from 0°C to 30°C

It does seem to have a bit too much welly for a preheater - but with the sensor after the heater at least it will modulate - It will be silent - and in that configuration it may switch on for 5 seconds - off for 30 on a cold day to prevent the heat exchanger freezing. I think a little more information on the tests performed would be useful.

Posted By: WillInAberdeenSorry maybe I missed something but why have a pre heater on a MHRV in the UK?

All MVHR in the UK should have a pre-heater for frost protection. It's a more efficient way of providing frost protection than alternatives such as bypassing the heat exchanger or unbalancing the flows. 'Better' MVHR have them built-in. What you say is correct though that all the heat it puts in to the incoming air is immediately blown out of the exhaust (except for the ineffeciencies of the heat exchanger).

let's assume 1m/s air through 160mm dia duct = 0.02kg/s

The flow interlock is set at 1.5 m/s by default so the heater won't be on at this low a speed. I agree that it seems a bit oversized but the control circuitry seems to work well IMHO. It will never hit the overheat limit because the temperature setpoint has a maximum of 30°C, well below the cutout temperature.

There is a cutout that can be reset manually - the red button that can be seen in the photo - and there is also another internal cutout that requires an electrician, as well as the flowrate interlock that operates automatically. Hence my suggestions and questions above.

https://www.veab.com/documents/cv/montering/CV-MQ.pdf

troubleshooting, page 52ish - is this relevant?

Thanks all.

Yes snyggapa and djh I have seen that document and tried to follow it, but it was well beyond me!