Why do non-battery devices not Ack all the time?

I have a heating relay (a Hortsmann Channel Boiler Actuator (SSR303 / ASR-ZW)) which usually works fine to turn my heating on and off. It is a mains-powered device (ie no batteries). However, now and then it fails to respond to Indigo's request to switch on.....the log shows

send "CH Relay" mode change to heat failed

and a quick look on the Indigo UI shows it as not responding with an Ack.

I thought that mains-powered ZWave devices would not "time-out" like this, as they have no batteries to conserve. Am I incorrect?

I believe that you're correct, but I don't think that's what's going on.

I had a couple of old "Home Manageables" dimmer modules that would periodically no ack on me. I'd wake up in the morning and one string of over-cabinet lights would be on. Had to unplug and replug the device to get it to respond. Since replacing those two units with Aeon Smart Dimmer 6 modules, I have had zero problems. In fact, the only problem I've had recently is where the HVAC guy unplugged a multi sensor in the attic and failed to plug it back in.

This device has to be kept awake by sending it a command every hour otherwise it drops into fail-safe mode i.e. off.
You need a script that says something like: if it is off, turn it off else turn it on. Run this on a schedule every 55 minutes.

For more details, see this post: http://forums.indigodomo.com/viewtopic.php?f=58&t=10911&p=73900&hilit=ssr303#p73900

Thanks guys. I am already doing that every 10 mins or so, but I had forgotten that I had, at some point in testing, disabled the check throughout the night......I've removed that so will see later if that sorts it.

Thanks again.