I am about to build an mdf oven, the size will be  L1220mm, W610mm, H635mm. 

It all depends on how much insulation and heat loss you have through the walls, I have 2 ovens, both use a 3kW cooker element with fan (centrifugal type) and PID controllers, one oven is 8ft x 4ft x 4ft external dims and the other is 7ft x 2ft x3ft, I've not got the smaller one up and running yet as I'm in the process of building it but the other one (the larger) will hold 80deg +/-0.5deg all day if you want and the element only runs for 30odd seconds every few minutes even when it's really cold in the workshop! That's with 50mm celotex to ALL faces (top/btm/sides/ends), all edges/corners sealed/packed & taped and the door made with a really good fit.

I haven't got any photos of it but the fan motor is outside with the spindle going through the side/end, through the insulation into an ally heater box, then using a centrifugal fan rather than you have in your photos. The element position is similar to yours on one and adjacent on the other due to space restraints. I run a tube (ally) into the centre space of the fan to suck air into from an area of the oven, it then gets blown over / out into another part of the oven. I also have 2 speeds on the fan but I only ever use the slower one.

The controller has 8 ramp/soak programs, 8 segments each. I have got TDS's for OOA pre-pregs that i may use but thats a bit down the line yet, for now my aim is to get a basic oven built and running, make some moulds and postcure them and get used to the controller and oven at the same time as i have no experience.

No problem... I use a 3kw fan heater but it will work with 2kw if I wanted. There is a 50mm base of Kingspan which I put the MDF box on top of. I put the same Kingspan round the sides... well mostly I have to get some more to be honest but I am going to make a new oven soon as the shape needs a change. (MDF is cheap to use and try,if you find need to make changes), 10mm MDF is actually very good at keeping heat in. I have 2 degrees difference from top to bottom.  One option I have been looking at is using a workbench for the frame as I had to get a 2.4meter long bench and the shelf seems perfect for the moulds I have. 900mm high, 900mm deep and 2.4m wide. I think it would would make a good oven with a bit of work.

This is the inside of the current oven.. I only put my MTB in there to clear some space and give a mate an idea of the size... 3kw works perfectly with this sized oven.

 

I think your MDF may continually draw heat out since it is a solid type material panel.

I have 3kw oven that is a 2" foam core with thin sheet steel skins. It's 4m long x 1.1m wide x 1.1m high.

It will heat up to 100C in about 45 minutes with small light moulds inside.




For me in Australia 3kw has to run on 3 phase power. If it were 2kw then normal household single phase is ok.

Here most households don't have 3 phase whereas industrial workshops usually do.

2Kw will work for me and it is a large area to warm up. But I dont ramp up that fast. The fastest I ramp up is 1 degree every 90 seconds. Not because that is as fast as it will go but because that is all I need it to go up by.  MDF does warm up a little to about 36 degrees C (the door of my simple box) but the kingspan I have clad the outside with keeps most if the heat inside. The next oven will be more permanent as this was a proof of concept oven that works perfectly and is cheap. I can build a more solid one using an interlocking bench for a small workshop oven in my layup room. A bigger one using 90 degree steel frame, plywood, kingspan and some more ply for the inner skin it is trying to keep the metal which has a path to the colder outside to a minimum. A full MDF or ply inner skin which overlaps is the best option especially if you add rubber seals to the door frame so there is no real gaps.