I'm not really a hydroponicker, but I always thought that fans were advisable in hydro setups mainly to prevent the lamps from heating things up too much. That is, the fans are there to get rid of heat. Maybe cut down on humidity too. You evidently don't have a heat problem. So maybe you should just turn them off? Or just turn them on infrequently?

Yes the fans are needed to circulate air reducing moisture on leaves causing mould. Air will also need changing to reduce carbon in the air. Was just wondering how people do this and heat the enviroment with out blowing all the hot air out a vent and just heating the room. Might try using the fans with all box sealed up and heater on then open front twice a day to remove any build up carbon. all experimental at the moment

Carbon in the air? You mean like CO2? That's not going to harm plants. You don't blow away CO2 in greenhouses. In fact, many greenhouses add CO2 to the air. Plants draw down the amount of CO2 in the air.

Yes I got that wrong way around, with no outside air the carbon will become to low for the plant to grow so need to open the door to recirculate the air adding in more carbon. I believe this is why they have a fan input and exhaust outlet. i cant see this has to be all day so hoping to only do this change out of air twice a day. Not sure why some grow boxes have carbon filters on outlet if the plant has used up the carbon but that's another question :)

I do wonder why your box temperature is so low, if you have lights. I guess the surrounding air temperature must be quite cold. But then it's just a matter of insulation. If you increase the amount of insulation around the box, the temperature ought to rise significantly, just from the lights. Fluorescents will heat better than LEDs. When I do starts under lights in a cold garage, I just throw a blanket or two over the whole thing, and it gets pleasantly warm inside. Now, if CO2 is important, you might want to flush it every once in a while with fresh air. Twice a day seems sensible. I don't worry about CO2 with starts because they are tiny plants, and the draw-down from them will be miniscule.

They are LEDs and I could insulate as you say to bring up the temp. It was only if I put in a air inlet fan and extraction fan that the temp will drop a lot. This is a start up box so will go with the sealed insulated box, internal circulation fans and open box twice a day to change out the air. Will see how this goes. I am also setting up a 4 shelf rack to grow the lettuce on to full size under LED lighting but still constructing this at the moment

I've got a similar situation, my box is in my basement which is cold during the winter, probably in the 40s and 50s fahrenheit mostly. Last year I used a seedling heat mat under a tray, which is on an insulated piece of plywood. So from bottom up it's foam insulation, plywood, heat mat, tray. I drilled 2" holes in the top and one in the bottom, and I think that provided enough air exchange via stack effect although I haven't measured. Meanwhile, a small desk fan is placed in the grow chamber to blow air across the leaves and strengthen the stems. Ideally the whole box would be insulated, but I haven't gotten there yet.

Got my box insulated and external fan fitted, doing much better, takes a few hours to get up to temp but it gets to 21 deg and the fan kicks in and keeps it at or below 21 deg. seeing a few issues with the lettuce which I will post on a separate post. Thanks for all the advice