How Gravity Hot-Water Heating Systems Work

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How Gravity Hot-Water Heating Systems Work

In this video, Dan Holohan teaches us about gravity hot-water heating systems. Classic gravity heating is both the simplest and the most complicated system of all. It’s simple because it has few moving parts.

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boris_zmg

what was the normal psi of a Gravity Hot-Water Heating Systems when hot and cold?

DanHolohan

The same regardless of temperature. There was no compression tank. The pressure would be the static weight of the water. 

leonz

Hello Dan and Erin,

Dan would you be so kind as to also talk about top fed gravity hot water systems also?

I am attempting to convince our local Habitat for Humanity organization to use gravity hot water heat in the new homes they build especially the 2 story ones; they are using heat pumps and in our local weather they do not do well when it gets down to 32 degrees Fahrenheit and below.

JakeCK

leonz said:

Hello Dan and Erin, Dan would you be so kind as to also talk about top fed gravity hot water systems also? I am attempting to convince our local Habitat for Humanity organization to use gravity hot water heat in the new homes they build especially the 2 story ones; they are using heat pumps and in our local weather they do not do well when it gets down to 32 degrees Fahrenheit and below.

No one installs new gravity hot water systems. You can't even buy a boiler that is designed to function that way. They all come packaged with a circulator. And most want a primary secondary setup with at least 2 circulators. 

Edit: I'm wrong, apparently gravity HW is still commonly installed and is a great candidate for a habitat house using volunteer labor.

DanHolohan

My book, How Come?, covers it. 

Hot_water_fan

@leonz that seems like an extremely hard sell 

Alan (California Radiant) Forbes

I have a customer who has a gravity hot water system and needs to replace his boiler, but is afraid that a pumped system will create enough pressure to cause leaks. Even one of his contractors said so. Will he be able to keep his gravity system if he gets a boiler with a large enough HX, like a Peerless or Burnham?

neilc

what do you mean a large enough HX ??

pumped boilers on gravity systems are usually, should be, very small circs, to imitate the ole gravity flow, just enough to move thru the modern boiler,
the static fill would be the same, enough to reach the top rad, plus 5 psi, ie:12~15, what's already there,
the new circ just guarantees flow thru the boiler,

PC7060

@Alan (California Radiant) Forbes - my old New Yorker boiler operated well in gravity mode which save us when the coupler failed in the giant old B&G circulator.
The New Yorker had a large internal capacity that was well suited for gravity circulation. The TRV worked very well with the gravity feed to balance the temperature of the various rooms.

leonz

Hot_water_fan said:

@leonz that seems like an extremely hard sell 

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How can it be a hard selling point when the huge thermal mass
is there and there is smooth even heat???

leonz

JakeCK said:

leonz said:

Hello Dan and Erin,

Dan would you be so kind as to also talk about top fed gravity hot water systems also?

I am attempting to convince our local Habitat for Humanity organization to use gravity hot water heat in the new homes they build especially the 2 story ones; they are using heat pumps and in our local weather they do not do well when it gets down to 32 degrees Fahrenheit and below.

No one installs new gravity hot water systems. You can't even buy a boiler that is designed to function that way. They all come packaged with a circulator. And most want a primary secondary setup with at least 2 circulators. 

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How can you say that you cannot buy a boiler with large enough tappings
that can be used for either hot water or steam when they are available??

It would seem that in new modern construction a top fed system would be
ideal with the smaller homes they build.

DanHolohan

@Alan (California Radiant) Forbes, yes, a modern boiler will work with an old gravity system. And yes, @PC7060, the flow-control valves come with those stems that allow you to lift the weight off its seat and create gravity circulation during a time when the circulator can't run, for whatever reason.

Hot_water_fan

How can it be a hard selling point when the huge thermal mass 
is there and there is smooth even heat???

I’m not disagreeing with the merits of gravity systems! However, people don’t seem to want them. Americans want forced air for better or worse. I’m not passing judgement on them, people make decisions that are best for them. 

neilc

people want AC, and don't know water, or steam, like we know, water and steam

Alan (California Radiant) Forbes

I’m not disagreeing with the merits of gravity systems! However, people don’t seem to want them. Americans want forced air for better or worse. I’m not passing judgement on them, people make decisions that are best for them.

This may be true, but once you've gotten used to the comfort of a gravity system, many don't want to change. It's so elegant and the living work of a bygone era.

leonz

If I could do it all over again I would rip out the &^&*( baseboard and
put in top fed gravity hot water heat.

PC7060

DanHolohan said:

@Alan (California Radiant) Forbes, yes, a modern boiler will work with an old gravity system. And yes, @PC7060, the flow-control valves come with those stems that allow you to lift the weight off its seat and create gravity circulation during a time when the circulator can't run, for whatever reason.

@DanHolohan - yep, pretty easy change to the 1930’s vintage Thrush valve once I found the patent application drawing online.  

JakeCK

Edit: I'm wrong, a habitat house is a great candidate for a steam boiler setup for gravity hot water. 

leonz

Argue with Dan, he knows much more about this than me.

Hot_water_fan

This may be true, but once you've gotten used to the comfort of a gravity system, many don't want to change. It's so elegant and the living work of a bygone era.

Definitely a wonderfully elegant solution, but how many have ever lived with gravity? 1/1000? 1/10,000? I both appreciate the method and also acknowledge the peak has passed. 

DanHolohan

@Larry Weingarten has a gravity radiant system that runs on vodka. 

rconkling

leonz said:

Hello Dan and Erin,

Dan would you be so kind as to also talk about top fed gravity hot water systems also?

I am attempting to convince our local Habitat for Humanity organization to use gravity hot water heat in the new homes they build especially the 2 story ones; they are using heat pumps and in our local weather they do not do well when it gets down to 32 degrees Fahrenheit and below.

Sounds like they are using the wrong heat pump or not sizing it properly. Modern heat pumps, like my Daikin FIT, are rated to around 5*F I believe.

leonz

I have no idea but both my brothers tenants have the same huge units and they cannot keep up with the cold.
My middle brother is waiting on a $1,600 circuit board part because the tech damaged the unit by looking for the problem. All the more reason for the beautiful simplicity of gravity hot water heating.

JakeCK

N/m I'm wrong. Gravity HW is a great choice for habitat for humanity.

Larry Weingarten

Hi @JakeCK , I think I'm living proof that one can install gravity hot water, even now. But to be reasonable, it must be in a building with a very good shell. I can heat my 1800 square foot home with a 1-1/4" pipe. Putting lots of emitter in the walls, I can keep the house at 70F using 80F solar heated water. There was no single source for the information I needed to design this, but the old books gave me the info to make it work, It just took a bit of research. As Dan says, there is vodka in the system. It's in the solar side of things to prevent freezing. For that I just copied what Day & Night Solar did around 100 years ago. By having the good shell first, cost of the hydronic system was very competitive with what forced/scorched/burnt/hot air would have cost. It's comfortable and silent. Outdoor temps here have ranged from 18F to 109F, yet the house manages pretty well.

Yours, Larry

JakeCK

@Larry Weingarten that sounds like an awesome setup. Can it be easily replicated in a habitat home with volunteer help?

I thought large diameter pipes and larger delta T's were required for the convective currents to setup in gravity HW systems?

WMno57

I have half a gravity system. 1916 house with the original 2 1/2 inch nominal gravity pipes. Upgraded around 1950 to forced hot water. If I had to replace my boiler it would either be another 80 percent three pass with a forced combustion burner (is there a better term) or a mod-con. I don't want an atmospheric burner. So anything I do will require electric, but I have about 10 generators. My attic is unconditioned space, I don't want water up there. I have a 2 1/2 to 4" pipe threader, never used it. My back hurts just moving it around. Cutters for it are not cheap. Over the years some of my first floor radiators have been moved, and one was added on the back porch. With forced hot water they all work. If I returned to gravity circulation and rusty rough internal wall 100 YO pipes????????
@Alan (California Radiant) Forbes , My second floor originally had 4 radiators. When I bought the house, three were disconnected. The single working radiator shows signs of a repair near the top. These 100 YO systems have rust. I suspect my three disconnected radiators were also leakers. My system is easy to bleed because most air goes to that one radiator. All my top floor radiators probably have internal water line corrosion. Maybe your client's system never had a water line in the top floor radiators because of the open tank in the attic. Or maybe they are soon to fail rusty junk. But if you touch it, it will become your problem, because you touched it last. Right now, the pressure in your client's system at the top of those second floor radiators is close to zero (because static pressure) If you change to forced hot water, the pressure through out the system will be about 15 psi.

WMno57

If I were specing out a Habitat home it would be all electric. I would give them sweaters for the few cold days when the heat pump couldn't keep up. Maybe Sweater Stamps will become a new Gov program? Or maybe 70 degrees 7/24/365 is a Constitutional Right?

Larry Weingarten

Hi @JakeCK , I did a lot to reduce friction losses in the gravity system, so very little motive force would be needed to move water. Beginning with design day loss of about 27K BTU - so not much flow is needed. Then I used some soft copper, long turn fittings and de-burred every copper connection.
I don't think it would be difficult for Habitat to build as long as the design was well thought out in advance and people were trained in the basics of installing copper.

Yours, Larry

WMno57

I once had an International Cub tractor with Thermo-Syphon cooling. No water pump. 60 cubic inch flathead with 15 horspower. It worked and never overheated. If that engine were souped up to one horsepower per cubic inch, the Thermo-Syphon cooling system would not be able to keep up.
And now a cautionary tale about what can happen when arrogance and out of the box thinking lead one down the wrong path.
Dan Ustian as CEO of International's follow on company Navistar wanted to march to the tune of a different drummer. Every other Diesel engine manufacturer choose Selective Catalytic Reduction (SCR) with Diesel Exhaust Fluid (DEF) to meet the EPAs 2007 Diesel emission requirements. Not Dan. He wanted simplicity. "Truck drivers and fleet owners can't handle having to deal with two fluids (Diesel & DEF)". He continued to insist to his engineers that they design engines that will meet the new emission standards with Exhaust Gas Recirculation (EGR). Even when they told him it wouldn't work. And it didn't work.
"Navistar's EGR decision led to significant reliability issues and quality problems (which were ultimately traceable to the fundamental physical reality that recirculation of exhaust gas introduces intrinsically abrasive soot and inherently corrosive acid gases back into the engine). Truck drivers began losing trust and confidence as Navistar vehicles were breaking down frequently. Consequently, they abandoned Navistar trucks in favor of competitor's trucks."
The EPA sued Navistar, Dan lost his job, and International now buys their engines from Cummins.
https://en.wikipedia.org/wiki/Navistar_International#Failed_engine_strategy
The IH DT466 was the best medium duty Diesel engine ever made, but time marches on.
Sometimes when everyone else is doing things different than you, it's for a reason.

WMno57

I still like cool, outside of the box gizmos. I just wouldn't bet the farm on one.

leonz

Hello Larry and all that take care of gravity hot water systems.

I am wondering out loud here about a fun fact involving WD-40; If you spray the toilet bowl above the water line before you flush the toilet the water drops out a great deal faster into the vertical drain stack.

I am curious about it as the small amount of WD-40 in the total scheme of things in a beautiful gravity hot water heating system would help the radiators heat even faster, or would it be a waste of WD-40 in the system that is filled with water and the only exposed portion is the open to air expansion tank?

Larry Weingarten

Hi @leonz , That’s an interesting question. I’m thinking a glass piping test rig with colored WD 40 might show how well the idea works. 🤔

Yours, Larry