Water pressure, why is it needed?

[ChoppyChoppy]

Aside from raising the boiling point, why is water pressure needed in a hydronic heat system?

If I have a 2 story house, boiler in basement, why should it have ~20psi?

Being a closed loop, there's minimal resistance to pumping water up ~25ft, as return water coming down is essentially pushing it up... right?

Or in the case of my heating system, the boiler is the highest point. It's set to ~10psi.

 

[NSMaple1]

Not sure raising the boiling point is much of a reason.

Main one might be to keep air out of the system & so what air is in it, will vent right. Then there's cavitation in pumps & valves - most circs spec 4psi at the inlet.

Any system will have pressure somewhere in it, the most at the bottom Your 25' height means if there is zero at the top there will be 10psi at the bottom - 0.4psi per inch of height. If your boiler is at the top you might be able to drop that some if that's what you were getting at - 5psi at the top should be OK. But then again you need to make sure it doesn't drop out when the system is cold. So 5psi cold would maybe equal 10 or more when the system is fully hot. Depending how much water is in the system & what expansion room you have. There have been times when mine has dropped too much at the top during the summer when it went fully cold. That can make air accumulate in a high spot in a zone and airlock it.

 

[uniballer]

Most hydronic systems have an automatic water feed. It works by adding water when the pressure drops. So that won't work without pressure.

Adding fresh water also adds oxygen, and that should come out as soon as possible to reduce corrosion. The air scoop and automatic vents work OK for that, but only if the system is under pressure.

Lastly, if the system is full and cold, the water will expand when heated, so the expansion tank gives the water somewhere to go (otherwise the excess would have to be drained when hot, and added back when cold again).

You could build a hydronic system that works only on gravity pressure, but you would have to open it to the atmosphere to handle expansion (e.g. a vented tank, perhaps with overflow), and couldn't use automatic water feed or venting, etc. This wouldn't be all that different from the way the cooling system in a modern car is managed.

 

[ChoppyChoppy]

The question is more of a "things that keep me up at night" thing.

It's "industry standard" for the pressure to be roughly 12-25psi, and a taller house is run toward the higher side.

I'm curious on the actual reason and haven't been able to find an answer. As far as I can understand, it shouldn't need any pressure to function.

 

[ChoppyChoppy]

Not sure raising the boiling point is much of a reason.

Main one might be to keep air out of the system & so what air is in it, will vent right. Then there's cavitation in pumps & valves - most circs spec 4psi at the inlet.

Any system will have pressure somewhere in it, the most at the bottom Your 25' height means if there is zero at the top there will be 10psi at the bottom - 0.4psi per inch of height. If your boiler is at the top you might be able to drop that some if that's what you were getting at - 5psi at the top should be OK. But then again you need to make sure it doesn't drop out when the system is cold. So 5psi cold would maybe equal 10 or more when the system is fully hot. Depending how much water is in the system & what expansion room you have. There have been times when mine has dropped too much at the top during the summer when it went fully cold. That can make air accumulate in a high spot in a zone and airlock it.

Yeah, I realize boiling temp isn't so much an issue with ~180* water, as a boiler in this use isn't truly a boiler.

In a closed system, static head doesn't exist (.433psi per foot?).. right?

Even still, figure 8ft floors, a 2 story house might have 15ft vertical between the boiler and the top floor baseboards.
For 25psi, that roughly 58ft, which would be a really tall house!

I understand a slight positive pressure to ensure it's air free, but not understanding why elevation would change the pressure on a closed loop.

 

[ChoppyChoppy]

Most hydronic systems have an automatic water feed. It works by adding water when the pressure drops. So that won't work without pressure.

Adding fresh water also adds oxygen, and that should come out as soon as possible to reduce corrosion. The air scoop and automatic vents work OK for that, but only if the system is under pressure.

Lastly, if the system is full and cold, the water will expand when heated, so the expansion tank gives the water somewhere to go (otherwise the excess would have to be drained when hot, and added back when cold again).

You could build a hydronic system that works only on gravity pressure, but you would have to open it to the atmosphere to handle expansion (e.g. a vented tank, perhaps with overflow), and couldn't use automatic water feed or venting, etc. This wouldn't be all that different from the way the cooling system in a modern car is managed.

Fill valves really shouldn't be on aside from when filling the system. It's a recipe for a flood if a leak forms or the valve fails.

A LWCO should protect the boiler from firing when dry.

 

[ChoppyChoppy]

Figured it out.

Has to do with weight of water. Each foot of water is .433 psi.

So a heating pipe 20ft tall, that is 0psi at the top, would have about 8.5psi at the bottom.

 

[NSMaple1]

Figured it out.

Has to do with weight of water. Each foot of water is .433 psi.

So a heating pipe 20ft tall, that is 0psi at the top, would have about 8.5psi at the bottom.

I already said that.

 

[ChoppyChoppy]

I already said that.

I generally skim through blocks of text.
I'm used to sort of speed reading with paragraph separation.

25ft at .4psi an inch is almost 60psi...

 

[NSMaple1]

I generally skim through blocks of text.
I'm used to sort of speed reading with paragraph separation.

25ft at .4psi an inch is almost 60psi...

Ooops. You got me there. I typod inch for foot. Better go change that.

Edit: Guess I can't change it, must be too old. My mistake lives on forever...

 

[ChoppyChoppy]

Ooops. You got me there. I typod inch for foot. Better go change that.

Edit: Guess I can't change it, must be too old. My mistake lives on forever...

No big deal.

I guess it hadn't sunk in when I read your post and the .4psi per inch didn't make much sense.

While researching, the question came up often so I'm not the only one.

I'm not sure why it's recommended to be 18-25psi on a 2 story house if ~8 psi is adequate. I'd think less pressure is better?

 

[NSMaple1]

Depends where's exactly your measuring the pressure too.

I'd start with 4psi at the top then add 0.4 per foot of system height, for what it should be at the bottom. That would work out to measuring 12psi at the bottom of a 20' high system. I think there are some out there who run open systems. I don't know how they don't airlock at the top on occasion.