Best Of

You can try it with 1" but you may need to increase if you need more than 80,000 BTUh to heat your home.  That 1" or 1-1/4" is not written on stone tablets that were delivered to a mountain in a desert.

The pipe sizing is based on keeping the flow speed below 6 feet per second in order to keep the velocity noise below a noticeable level. Above 6 feet per second will become loud enough to hear. It is not that bothersome when water moves faster than that 10 FPS when you open a faucet because the noise stops when you close the tap. With a heating system that noise would be unacceptable.    The next thing is the amount of gallons per minute will flow thru the pipe and this is directly related to the FPS number.  And finally the temperature drop (∆T Delta T) of that water flowing thru the boiler or the radiator as the amount of heat that is added or removed from the water.  With the industry standard of one gallon per minute for every 10,000 BTU based on a 20° temperature change (∆T), we can come up with some rules of thumb for pipe sizing based on the amount of heat you need to transfer.  

If you change the flow rate or the ∆T, you can also adjust the pipe size. So it is possible that 1” can be made to work for you by FPS or GPM, or by changing the ∆T in order to get the system to work.  

For example you can get 100,000 BTU to go through 1” pipe if there is a 25° ∆T  in the design.  But when the water gets to the emitters (the radiant floor for example) it must move slower in order to release more heat to the floor.  The longer the water stays in the emitter the more heat it will release.  You also need to let the water stay in the boiler longer in order to change the ∆T in the boiler.  So slowing down the water may be able to move more BTU through the smaller pipe.  

The best person to make those calculations would be hot_rod if he is interested, since he does that stuff all the time, and I have stopped designing systems since I retired and might make mistakes and don't want you to rely on my numbers where dollars and cents are involved.  But It may work with 1” with the proper flow and ∆T.  It may also be just fine if you only need 80,000 BTUh on the coldest days.  Without a proper load calculation, we may be discussing a point that does not really matter. 

One final note. this transformer is rated at 24v. 15 VA and may not be strong enough for the Honeywell thermostat, the Honeywell HPSR101 relay and the RIB U1C relay. If that happens you can purchase a 20 VA transformer that will fit in that same position. Amana 24v. 20VA Transformer from SupplyHouse.com. I think you should be fine, but if the transformer fails after a few months of use, get the stronger transformer.