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correct, where you connect the expansion tank into a system, establishes the PONPC

Physically the tank can be remote mounted

@RascalOrnery anything with a boiler needs a high limit it is a safety control that saves the system (hopefully) when all else fails.

Well you must not do much oil with hydronics then.
With the exception of a properly sized oil boiler running on design day, the rest of the time it is oversized.

Add any kind of zoning and especially with outdoor reset, and it’s extremely oversized the rest of the year. The result is constant short cycling.
Therefore, a buffer tank is always recommended.
Even worse if you’re using it for snow melt. Because you should not even think about running super cold water return with glycol threw a boiler.

I’m only interested in giving the best advice. But feel free to do things incorrectly as much as you like.

call it 100 sq feet of snow melt? A basic class 1 is about 100 btu/ sq ft, so the snowmelt, when running could use 100.000 btu/hr

What size boiler?

The more btu you can afford the snowmelt , the quicker it melts, 100 btu/ ft is on the low end. If you get a big dump, you may need to break out the shovel🤭

Ok - more info:

Yes! I did in fact adjust the boiler's controls when the old coil was taken out of service. That's when the Resideo electronic unit went in. It's set purely for heating, no temperature maintaining. Oil use sure dropped a bit. The local oil guy did a service, and that helped too. Unit hadn't been cleaned in 4 years.

The walkway is short - but annoying enough to deal with that it justifies getting melted. I'm aware of the need for a heat exchanger and antifreeze. Since it's being formed out, I figured the cost of at least prepping it for heat now, is low enough to justify just putting the pipes in. It's a 3 ft wide, 20ft (if that) long, with a bit of a square spot by the patio. Obviously not running all the time, just for snow. I think I have a Watts book on design guidelines here somewhere.

I expect to do the family room first (virtually no electric wiring under it, so easy access, and easy to get insulation under the piping.) then changeover the rest of the house. The boiler is oversized, and even with a smaller nozzle, tends to run shorter cycles than what I think is ideal. (it fires maybe 3-5 minutes while running and pumps the rest of the time). This got a bit worse after I started insulating all the basement pipes. Nothing had insulation on it. The prior homeowner tried to 'help' the kitchen's lack of heat by putting a run of baseboard under the floor. 🙄

FWIW, I'm just outside of Catawissa, PA. The previous owner of the property was apparently somewhat well known, and quite a character, from the stories I've heard…

@nasadowsk said: "Domestic water was a coil, but now is a heat pump hot water heater."

I believe this to mean that the oil fired boiler was/is equipped with tankless coil. Many of my customers have had heaters like this and have switched the DHW to gas. electric or other DHW source. Out of all those customers, only a small percentage of the plumbers that installed the new DHW system knew to fix the tankless coil control on the oil fired boiler system to stop making priority hot water for a coil that is not being used. YES I'm saying the the oil fired boiler thinks it is still making DHW and will waste fuel trying to make hot water that will never be used.

If this is the case with your oil fired Crown boiler, then I can save you about 5% to 10% on your fuel usage by making some minor adjustments on the oil burner control that is currently operating your oil burner and circulator pump(s). Ask me how!

I use anthracite coal for radiant, it's a great match. I wouldn't want to pay to melt sidewalks with oil though. Sheesh. I pay the equivalent to 88 cents a gallon when comparing btu to BTU. And it would be expensive even for me. It's like three to five times the heat requirement compared to your house per square foot. I can see a very small pad maybe, there are also different classes of snow melt systems, those which keep it dry take a lot of BTUs. Great questions, keep them coming.

Radiant can be more efficient since your system temps can be a lot lower. You may have to run through a mixing valve if you still want to run the baseboards at full temp, the fact you don't have a tank less coil could greatly enable you to keep system temps lower.

@nasadowsk , where in semi-rural PA are you located? We might know someone………..

If radiant is a no-go for whatever reason, my second choice would be cast-iron baseboard. It transmits heat by radiation as well as convection, increasing comfort.

sounds like a good Sargent!