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Greetings all! Bit of a physics thought experiment I guess, but I don't want to make any assumptions... I'm planning a 40 ft tall addition build with hydronic heating (and I will attempt cooling). Hydronic will be in the insulated on-grade slab, plus ceilings in the upper levels. Each level is small, at 25x19ft interior floor footprint, with 1/4 taken up by stairs/pneumatic elevator that won't have hydronic. So the lowest point of the system will be at almost 0 ft, and the highest point will be at about 37 ft height. Theoretically, when the system is full of water there shouldn't be any impact from gravity ( I think...) since the downward force will equal the upward force almost exactly- aside from minute differences from density caused by temp differences. The question is whether that is true in practice, and whether I would need to make any special system design considerations for the height, when compared with a system that is "flat". Thanks in advance!

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You don't have to fight with this and try to overcome gravity, you make gravity work for you.

A top fed gravity hot water system like top fed steam is the Cadillac of heating systems and does not require a circulator or bladder expansion tank, the only requirement is a small steel open to air expansion tank.

My home had an open to air expansion tank that was saddle mounted in our laundry room ceiling and had 5 gallons of water in it and a water level gauge tube in the end of the tank.

Why are you not considering making it a top fed gravity system for this place and take advantage of the fact that heat rises and cooler water sinks and does not require a circulator?????????????????

The single riser pipe from the boiler steam chest could be routed up the staircase opening to the open to air expansion tank and the hot water would be directed from the tank to the third floor with an OS fitting and the cool water return to the cool water return stand pipe from the third floor to the second floor and then from the second to the first then to the boilers 2 sump tapping's to start heating it again for its trip upward to the third floor.

In Mr. Holohans excellent book "CLASSIC HYDRONICS" in Chapter 4 provides the reader with an excellent side view drawing of a bottom fed gravity hot water system.

The top fed gravity hot water system elminates air bubbles as all the hot water rises to the top in the single hot water riser to the open to air expansion tank and then is fed to each floor starting at the top floor and then the second floor and then back to the first floor using a common drop pipe with OS fittings.

The great thing about this method is that there is no circulator nor the need to bleed air out of the system BECAUSE THERE IS NONE.

With a top fed steam system all you would need is three small radiators connected in series top to bottom using a single steam riser pipe and one drop pipe feeding the top radiator as they would be connected in line with each other if my memory of the flow of the dry steam is right and there is no need for return piping or a circulator.