Advice on relay rating for UF heating

nick123 · Jul 31, 2010

Hi all - I've a 2 room 5 zone DMX controlled under-floor heating system to connect, and am looking for advice on relay ratings. The zones are 1450W x 2, 700W, 400W, 560W. One driver being considered has 8 x 8A relays, which I don't know if are synchronised with zero crossing, and I'm wondering about any issues with inrush current. As there's spare capacity on the relay board I had considered that two relays could be used on each of the 1450W zones to share the load if the control software triggers both at the same time, though we should probably cover the case where the software is faulty and doesn't actually switch both in tandem. So I'm wondering if we should be going for relay boards (or triacs/IGBT as an alternative) rated 10, 16 or even 25A to be on the safe side or if 8A should be fine? Advice gratefully received!

davelerave · Jul 31, 2010

afaik you can measure the 'cold resistance' of the heaters and see if the calculated wattage(Vsquared/r) corresponds to the rated wattage

i think that should confirm if there's any inrush

i'd guess prob not

and you'd be using AC-1 rated contacts
-the supplier or someone familiar should be able to confirm any inrush-if not the 8amp poles should be good anyhow for the max load being switched(1450w)

if you're referring to 'paralleling' contacts (load sharing/arcing)-it doesn't serve any useful purpose although they used to be wired in series to reduce 'arcing'
(this method has been superseded by the use of 'snubbers')

nick123 · Aug 1, 2010

Thanks Dave. The cold resistance for the 1450W zones is as expected at around 37 ohms. I don't know if the relay contacts and traces on the PCB are rated based on RMS voltage though, and assuming 320V peak, the current would be nearly 1 amp over the rated 8 amps at that moment.

davelerave · Aug 1, 2010

i don't think the peak voltage needs to be considered- only the rms when sizing the 'contact rating'

in theory you're prob ok -but it could be safer to oversize contacts in the long run

someone with more UFH experience might know better