After replacing the thermostat, I raised the temp setting to check that it was working. Seems to work but once temp is reached, the boiler start to shut on and off every few seconds. I let this run for a few minutes to see if it would stop, but after a while I just reduced the temp setting to shut it off.

Original t-stat was a Honeywell T451A2007. Hot water, natural gas boiler is a Hydrotherm R150. Replaced t-stat with a Honeywell RLV430a programmable.

Any help would be appreciated. I've also sent Honeywell and inquiry and will post here once I receive and answer.

I believe you do not have the required 2 Amp resistive load requirements necessary for the proper operation of the new thermostat. Or the system is driven by a contactor or relay. I believe your system might be relay driven if as you say it is a Hydrotherm.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-1855.pdf

Number 1 first page

RLV430 thermostat can be used to control an electric heating

System such as a baseboard heater, a radiant floor, a radiant ceiling,
a convector or a fan-forced heater.

The thermostat cannot be used with the following:
• a resistive load under 2 A
• a resistive load over 14.6 A
• Systems driven by a contactor or a relay (inductive load)
• Central heating systems

Does the thermostat operate zone valves? If so the power head could be bad. If the thermostat came with a resistor put it in place.

Thanks for the help hvac1000 and cornerstonehvac. Here is the reply from Honeywell:

Thank you for contacting Honeywell.

The RLV430 thermostat is not compatible with your system type as this
thermostat is compatible with resistive load applications (which do not
have a motor control).

Currently, we do not have a residential, programmable thermostat
compatible with your system type. It is advisable to speak to a licensed
HVAC contractor for an appropriate recommendation.

I'm not sure sure if my system in relay driven or not (or my resistive load is too low). Oh, it does not operate zone valves, FYI.

I'll call a pro for this one. The old thermostat is working fine, we were just looking to update to a digital/programmable one.

Thanks again.

NO problem. Good luck.