UFH brain teaser. think i know but......

[telectrix]

existing UFH (2 mats each 1.8kW) installed c 2015. on a single stat by a,n,other. customer had someone else replace stat. made a right mess of it and stat fried. I then get called in to fit new stat. removed the dual back box that previous guy had smacked out with a hammer so as to fit the new stat. (was FCU and stat on dual back box, but new stat would not fit). the 2 underfloor sensors were wired in parallel, so that's how I connected to new stat. customer complains that the floor does not warm up properly. on a visit, i found 2 smallish rugs on the tile floor. now, my thoughts are that maybe 1 of the sensors is under a rug and so switches off prematurely due to the rug retaining the heat. this then cuts both mats off. I'm thinking to wire the sensors in series so that both need to activate and go open circuit for the stat to switch off. anyone agree/disagree? current measurement when system is powered up from cold shows 15A, i.e 3.6kW. not ideal on a 13A FCU but that's what i have to work with.

 

[Wilko]

Hi Tel - from memory the ufh temp sensor probes have a base resistance of about 10K Ohms at room temp. Putting 2 in parallel or series in a stat not designed for it is a recipe for weird outcomes, I reckon.

 

[MDJ]

existing UFH (2 mats each 1.8kW) installed c 2015. on a single stat by a,n,other. customer had someone else replace stat. made a right mess of it and stat fried. I then get called in to fit new stat. removed the dual back box that previous guy had smacked out with a hammer so as to fit the new stat. (was FCU and stat on dual back box, but new stat would not fit). the 2 underfloor sensors were wired in parallel, so that's how I connected to new stat. customer complains that the floor does not warm up properly. on a visit, i found 2 smallish rugs on the tile floor. now, my thoughts are that maybe 1 of the sensors is under a rug and so switches off prematurely due to the rug retaining the heat. this then cuts both mats off. I'm thinking to wire the sensors in series so that both need to activate and go open circuit for the stat to switch off. anyone agree/disagree? current measurement when system is powered up from cold shows 15A, i.e 3.6kW. not ideal on a 13A FCU but that's what i have to work with.

Can you draw the circuit on the back of your beer mat? lol

 

[MDJ]

existing UFH (2 mats each 1.8kW) installed c 2015. on a single stat by a,n,other. customer had someone else replace stat. made a right mess of it and stat fried. I then get called in to fit new stat. removed the dual back box that previous guy had smacked out with a hammer so as to fit the new stat. (was FCU and stat on dual back box, but new stat would not fit). the 2 underfloor sensors were wired in parallel, so that's how I connected to new stat. customer complains that the floor does not warm up properly. on a visit, i found 2 smallish rugs on the tile floor. now, my thoughts are that maybe 1 of the sensors is under a rug and so switches off prematurely due to the rug retaining the heat. this then cuts both mats off. I'm thinking to wire the sensors in series so that both need to activate and go open circuit for the stat to switch off. anyone agree/disagree? current measurement when system is powered up from cold shows 15A, i.e 3.6kW. not ideal on a 13A FCU but that's what i have to work with.

Tel cannot you fit a dual box and use two stats?

 

[telectrix]

thinking back on wot i wrote. 2 sensors in parallel, on cold, both would be closed circuit to allow the mats to heat. if 1 then switches out due to temp., then the other one will still be closed and so keep the mats heating. if i had installed it in the first place, it would have 2 x FCU and 2 x stats, all i can think is that the rugs may be causing the sensors to reach switching temperature due to hot spots.

 

[Octopus]

Could you not add a 2 controller?

When the customer claims they don't heat up ......... what temperature are they set to heat too - could it be that they are simply set too low?

 

[telectrix]

Could you not add a 2 controller?

When the customer claims they don't heat up ......... what temperature are they set to heat too - could it be that they are simply set too low?

set at max. 28 deg. think part of it is customer expects it to heat the room ( kitchen approx. 24sq.m. tiled floor).

 

[Octopus]

set at max. 28 deg. think part of it is customer expects it to heat the room ( kitchen approx. 24sq.m. tiled floor).

ouch ..........a its unlikely to heat the room and B ..............the running costs

 

[telectrix]

ouch ..........a its unlikely to heat the room and B ..............the running costs

i know. 3.6kW equates to about 50p/hour, less off time due to stat.

 

[suffolkspark]

Defo dont want the floor sensors both connected into 1 programmer because the resistances will be different to what it is expecting to see, just connect 1 of them and set the stat to be controlled by air temperature only so it just uses the probe as a limit

 

[telectrix]

Defo dont want the floor sensors both connected into 1 programmer because the resistances will be different to what it is expecting to see, just connect 1 of them and set the stat to be controlled by air temperature only so it just uses the probe as a limit

can you elaborate on that. as i see you advocate dissing 1 sensor , so how does the stat work on air temp as it will then be controlled by the sensor that's still connected.

 

[davesparks]

thinking back on wot i wrote. 2 sensors in parallel, on cold, both would be closed circuit to allow the mats to heat. if 1 then switches out due to temp., then the other one will still be closed

The temperature sensors aren’t thermostats, they are thermistors, so they won’t work like this.
Putting them in parallel or series they will behave like any other resistors and the controller will get some odd readings.

 

[telectrix]

The temperature sensors aren’t thermostats, they are thermistors, so they won’t work like this.
Putting them in parallel or series they will behave like any other resistors and the controller will get some odd readings.

so what's your solution? just diss 1 sensor?

 

[Toneyz]

heating. if i had installed it in the first place, it would have 2 x FCU and 2 x stats, all i can think is that the rugs may be causing the sensors to reach switching temperature due to hot spots.

I think this would be the way to correct it how it was supposed to be wired. Is the UFH on its own radial or on the ring?

 

[davesparks]

so what's your solution? just diss 1 sensor?

If both mats are identical then I would just use the one sensor.

Is the stat set to air and floor temperature or just floor temperature? (Assuming it has this option)

The floor sensor would normally just be for limiting max floor temperature when the controller is set to air and floor temperature, so if the two mats are the same power they should heat up at the same rate and one sensor be ok.

 

[suffolkspark]

As Dave says basically lol.

Id leave the second probe coiled up in the back though incase one ever fails

 

[Toneyz]

As Dave says basically lol.

Id leave the second probe coiled up in the back though incase one ever fails

Can the stat take the current of both the mats?

 

[suffolkspark]

15.65A so its right on the limit they usually say 16A max...

But just read op again and its on a 13A fcu?? This needs more than a quick fix tel...

 

[suffolkspark]

Double post

 

[telectrix]

I think this would be the way to correct it how it was supposed to be wired. Is the UFH on its own radial or on the ring?

on the ring afaik.

 

[telectrix]

15.65A so its right on the limit they usually say 16A max...

But just read op again and its on a 13A fcu?? This needs more than a quick fix tel...

nowt i can do about that. all tiled in and no way of separating the circuits. however, it's never blown the 13A fuse in the FCU.

 

[telectrix]

i'll try dissing 1 underfloor sensor and see how it goes. these sensors are not a straightforward open/closed. so davesparks post has made me think.

 

[suffolkspark]

Correct.... If you check them on ohms youll see....

Theres a setting on the screen somewhere where you can select air temp youll want it on that anyway

 

[telectrix]

if i can, i'll select air temp.customer is loaded so sod the leccy bill. it's a warmup tempo stat.

 

[Pauls0212]

I went to one a while ago and the 13a fcu was pulling 20a, i was called out because it was getting warm and discoloured. why do people not think about things before they install. But back to the problem ... if the stat accepts temp as well as probe , use 1 sensor and air temp.