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# System considerations for hydronic system spanning 40 ft height

Member Posts: 2
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!

• Member Posts: 23,273
Your thinking on gravity is correct... in part. What got left out is the static pressure. As I'm sure you know, normally a static (cold, pumps off) pressure of around 12 to 15 psig is maintained at the lowest point of the system. This is adequate, assuming that this is also the lowest pressure point in the system when the pumps are running ("pumping away") to ensure that the lowest pressure at the highest point in the system is greater than psig.

however. That pressure is only good up to 33 feet.

What you will need to do is to set the cold pumps off static pressure in your system higher; the usual rule of thumb is to try for a minimum of 5 psig at the highest point, which, for a 40 foot elevation difference will require 22 psig.

Since that is only 8 psig below the pressure relief valve, I would strongly advise that in addition you increase -- like double -- the capacity of the expansion tank on the system, to limit the pressure rise. You may want to consult with the manufacturers of the expansion tank; you are looking for a pressure rise of no more than 3 psig from cold to hot.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 22,107
Can you locate the boiler on the upper level? Some boilers allow you to run 45 psi, you would need to switch to a 45 psi relief valve
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 15,512
Also make sure to pump away from the boiler and EX tank. Locating the boiler on the top floor would be ideal. But you should be able to make it work with the boiler in the basement. Make sure to pressurize the ex tank with ait to the correct actual system pressure you will run.

Some boilers can run more than the standard 30psi relief valve setting.
• Member Posts: 81
You need to compensate for the static weight of water. .433 x 37 + 4 psi = 20.02. That will lift the water to 37' + 4 psi extra. If you short the system of pressure, you could end up sub-atmospheric at your highest point.. Trust me, not a good place to be. Air will find its way in, typically throught key vents that arent tight.
I am sure the outdoor wood burner folks will argue with me. Typically the burner is lower than your second floor baseboard, which makes any point higher than the wood boiler, sub atmospheric. So how do you do it? The wood burner gets isolated and you fill the house system with the PRV on the boiler. You have to get all the air out. Once you are certain air is removed, you turn on the wood burner to the boiler system. Remember, the wood burner is an open system so you are going to shoot some water out to the WB. and it may even overflow on the ground. Once it all settles, turn the circs on and it will run with part of the system sub-atmospheric as long as: You don't have any leaks in the tubing or vents. If you get a slug of air in the system, you may have to refill the boiler side again.
I hope this helps with your undertaking.
• Member Posts: 2
Ok thanks! Seems like running a higher system pressure could cause a lot of unintended consequences and create some x-factors I'd like to avoid. If the zones were all on home runs and each stayed on a single level, and I could put the boiler and pump in the "middle" (vertically) of the height, I'd be dealing with a more normal system pressure right? Some runs would go down 18 ft and some would go up 18 ft. Or does that still equal the same total pressure? For some reason I'm having trouble conceptualizing this.
• Member Posts: 81
All we are trying to do is overcome the static weight of water and add a little for a buffer. If you get it to 18' you just cut your pressure requirement considerably. You could use your PRV at 12 psi, the way they are shipped. Its a hard concept that guys on this site don't always grasp. Circulators to not pump water like a water pump does. They circulate water by creating a slight differential between the supply and the return sides. They do NOT lift water. That is the job of the boiler fill valve.
Hope this helps with conceptualizing it.
Steve
• Member Posts: 23,273
Putting the boiler higher -- in the middle or on the top floor -- will indeed reduce the static pressure at the boiler. It will NOT reduce the static pressure difference between the top and the bottom. For 40 feet of water that will be a bit over 22 psi. You can't get around that.

Putting the boiler higher will allow you to run it -- at its location -- at a more "normal" pressure, that's true.

The pressure at the bottom will always be 22 psi or so more than at the top.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 22,107
Another option to prevent the pump delta from popping a relief valve, if you are pumping into a mod con for example, is move the expansion tank connection.

The delta P from the circ can be additive or subtractive depending on the location of the expansion tank/ PONPC.
With the tank connected 1/2 way around the loop, the pressure will decrease from static fill, from that point to the circ below.

So in this example 22 static at boiler, 5 psi at top. Tank connects at top. Circ develops 8 psi (∆P). So at the boiler with pump running you would see about 13 psi, instead of 30 psi if the circ and tank were at the boiler.

As per @Jamie Hall suggestion, upsize the tank to that specific application.
Size a tank with the Wessel sizer

50 gallons of water heated from 50- 180°F. fill at 22 psi, max. pressure 27 psi

N-90 tank for 7.7 gallons required acceptance.

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 1,122
edited March 29

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.

• Member Posts: 2,245
Cadillac is expansion tank under vacuum at top of system. Besides minimizing pressure it enhances deaeration. Locate circulator well below top of system.