Electric ufh not reaching desired temperature

Bundle of questions and some help - I've had Electric underfloor heating in my bathroom for a couple of years now. My floor is up to working temp (22 degrees) in about 1.5 hours from cold (around 8 degrees).

Hot/Cold spots could indicate voids in the SLC and/or cement. Or could indicate large irregular gaps in the heating element spacing.
How long did it take to lay? To lay mine took the following timeline - 1/2 day to lay SLC before insulation; Wait 36 hours to dry, 1 day to lay insulation, mat, thermostat sensor and next coat of SLC; wait 36 hours before cementing tiles down (roughly 25Kg cement to 2.25 Sq Mtr tiles, so quite a thick layer); wait overnight before grouting. Mat had to be duct taped down especially where the heating cord was taken off the mat to navigate corners etc. The SLC onto the mat is important to stop voids around the heating element - SLC flows better than floor tile cement. Wait one week before powering up floor. Detailed and Recorded Test and Measurements (resistance of heating loop and insulation resistance) of the mat before laying, once mat had been laid, once the floor had been tiled and finally before powering up. Photos taken at every step for warranty purposes.

Firstly insulation is important, even the proper stuff in 6/10/16 thicknesses; unfortunately you've missed that out so will have to live with that.

Where is the control box? For use in a bathroom It should not be in the bathroom. Is the control box set to use the air temp, floor temp or both? It must be set to use floor temp. Check the controller is not set to Holiday mode - that can affect max temp.

You have the heated areas of the floor clear of mats, cupboards or anything else you keep in the bathroom? A stool on legs is OK.

Are you adjusting the floor Maximum temp and not the working temp? There is separate settings for that (the max setting is to protect the mat.) The controller if electronic can be complex to setup to work properly - took me several attemps to get it right; once set it's pretty good except the clock is not battery backed to not loose time if the power is cut off.

Do you have the test and measurements results of the mat installation at every step?

Is the thermostat centrally between two strands of the heating cord? i.e. about 40-50mm from each strand.

HTH's.

I fitted out a large 2 bed flat with electric UFH. Was impressed. In prime rooms 150w per metre was used. Worked well with Heatmiser thermostat/timers. Best to fit two corrugated tubes for the floor temperature sensors. If the original fails with it holding fast unable to be retrieved, then use the second tube. Better than ripping up a floor. The plastic tube is buttons saving a lot of future hassle.

Insulation under the matting is essential. Also, general insulation, like windows, walls, ceilings etc. if you have double glazing, secondary glazing may work, making it triple - much cheaper and less hassle than ripping up a floor.

If the heating is under-performing, then try general insulation before ripping up a whole floor(s). If the kitchen UFH is under-performing, then try an electric kickspace heater to supplement the UFH. The UFH thermostat may be man enough to switch the UFH and kickspace. HeatMiser stats can switch up to 16A. So in the depths of winter, or first thing in the morning, a switch over the worktop can switch in and out the kickspace. Off in Spring and Autumn, on in Winter. Much cheaper than ripping up the kitchen floor. The kitchen will not overheat as the stat controls both.