chr What fbird is saying is that if you have no temp sensor hole on your manifold you can buy a replacement thermostat housing that has a themp sensor hole in it.

An alternative I have used several times is to plumb a sensor port into the "other hole" where the constant hot water heads to the heater. Just screw a nipple into the manifold, then a 45, then a nipple, then a Tee, then an NPT pipe to Hose connector. Continue the hose to the heater. Screw your sensor into the side TEE outlet.

I do it all in 1/2" brass pipe, looks cool.