cph setting on t-stat

tmw

I understand why and how my system cuts in and out due to pressure on my vaporstat and temperature setting on my t-stat.



I think I'm missing something very basic here, but: How does the cph setting on the t-stat work? Why is it necessary if we already cut in and out on  pressure and temperature?  I'm just trying to figure out what this does..



thanks for indulging this rookie.

Jamie Hall

There's been a good bit of discussion

on this one -- but basically the cycles per hour setting controls the minimum interval between thermostat calls.  The objective of the exercise being to take into account how slowly (or quickly!) a given heat source will heat up itself and the surrounding space, and how slowly it will cool back down again.  Thus a steam or gravity hot water system -- a lot of thermal mass involved -- is slow both ways, and therefore we usually set it at one cycle per hour.  Whereas a hot air furnace is extremely quick (no thermal mass at all) and thus we set it at six, typically.



Other thermostats may use an anticipator instead; this does much what it sounds like, and deliberately heats up the thermostat itself (there is a miniature electric heater in there) so that it shuts off before the space is up to temperature, allowing the thermal mass of the system to keep going and not overshoot.



More sophisticated thermostats can "learn" the system they control, and vary these settings (within small ranges).