I have just added another digital input to a UNO WiFi Rev 2 (DI8).
I am tying to a set of dry (N.O.) contacts on a small 110 volt coil relay.
The input is programmed as INPUT PULLUP which uses the UNO internal
resistor.
Whenever the relay energizes and provides a closed contact to the
digital input WiFi communications is interrupted. So somehow along the
very short wiring path to the UNO or the from the energized relay coil
some type of noise is getting induced.
If the contacts are OPEN there are no communication issues.
Can I go a different input assignment and use an external resistor
possibly in conjunction with a cap to reduce the noise?
I have a small scope and thought about seeing if I could see it with that.
I can try a different relay but will have to scrounge around to
find another one that is 110v.
What are resistor values to try if I go with an external pullup?
thanks.
How close is the relay to the UNO?
Post an annotated schematic showing exactly how you have wired it, be sure to include all power, ground, power sources etc. What are you switching with 110V side. Have you considered using a zero cross solid state AC switch?
3-4 inches....correction..... its more like 6-8"...so I am thinking proximity is not an issue...
I stuck a hand sketch in this post.
I am not familiar with the zero cross solid state AC swtich but will google it.
Not when you are dealing with 100 VAC and relays.
Is there anything else connected to the "common ground bus"?
How does the Uno get it's power?
This was on my favorite china suppler and sells for less ten $5.00 US. I am not familiar with that one but have used several similar.
The ground bus is used for the Seeed space temps, gas sensor, DS18B20s,
and various digital inputs from other boiler related relays.
This is the first time I have had an issue with a contact closure causing wifi
interference.
I did order a small IDEC relay that I plan on substituting with the current unit.
After that make a twisted pair wire setup.
The UNO power is from a power unit that plugs into the (RCA style) power jack on the
Wifi Rev2.
that's all I know right now.....
Well if everything else fails to stop the problem that solid state relay will not
break the bank to see if it cures the issue.
I appreciate that lead. 
Went to a much smaller microminiature relay, made up a tight twisted pair to the input, and
rerouted the wire. So far after hooking the input back up and with it energized and providing a closed contact everything is still functional - no more wifi interference.
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