Single story house : System and Exp. Tank static pressure?

If 12 PSI is the nominal requirement for a boiler systems static pressure in a typical two story house (and both feed water pressure regulators and expansion tanks generally come factory pre-set to 12 PSI), what is the nominal static pressure "ideal" for a single story house?

I have a single story home with the boiler in the basement level. Would 10 PSI be a better choice for static pressure in my case?

Start at page 5 for your answer

http://www.bellgossett.com/literature/files/1195.pdf

I see there that a typical two story home calls for about 8.23 PSI, so they set them at 12 PSI by default.

For a two story home 19 ft/ 2.31 PSI/ft = 8.23 PSI

If a single story home is 8 ft. shorter, would that mean:

11 ft/ 2.31 PSI/ft = 4.8 PSI

And since in the first case a cushion of 12 - 8.23 = 3.77 PSI over the minimum requirement is considered adequate, would that make the final answer for a single story home:

4.8 + 3.77 = ~8.6 PSI

I.E, would about 8.6 PSI (call it 9 PSI) of static pressure be totally sufficient for a single story home?

Is so, then it would seem that reducing the pressure on both the water feed regulator and the expansion tank to about 9 PSI would be the proper thing to do for a single story house. Is this correct?

There is no harm in running the common 12 psi.

markviii, what advantages or pitfalls (if any) are to be had for dropping down to say 9-10 PSI for a single story house? Is it safer to just leave it at 12 PSI as you suggest?

The only reason to have any pressure at all is so the system does not suck in air. The taller the building the higher the pressure must be. As long as the pressure does not go above the relief valve pressure (30ish) it makes little difference or no difference if it's 8.2 or 26.2 Personally I have never seen a system with less than 12. The pressure will change with temperature. If it's 12 at room temp it could easily be 20 at 180*. No worries!

I've read (I believe in the book titled "Pumping Away") where if the static pressure of the system is higher than that which is necessary the effective expansion tank volume is diminished, and the expansion tank requirements of the system must be increased to compensate. Is that true?

I'm sure that nearly all boiler heated single story homes are set for the 12 PSI default, so obviously it cant be that big of a concern, but 10 PSI is a bit farther away from the pressure relief point of 30 PSI, so there is just a tad more safety inherent in the single story 10 PSI system, no?

Rise in pressure also raises the boiling point.

In my non-professional opinion, it is more important that the expansion tank and system have the same static pressure than if the actual number is 10 or 12 psi. If your diaphram tank has 8 psi in it (many do right off the shelf, always check before use) and the system is initially filled with 12 psi of water, the diaphram tank will have already accepted some water before the system is heated. In that case, the expansion capacity of the tank will be reduced.

My system has two Extrol model 60 diaphram tanks installed with 12 psi each, the feed-water regulator is also set at 12 psi, and the system usually runs 18-20 psi at operating temperature. When one or both of these tanks fail, I will just install a single commercial floor-mounted tank.

-Rob

Anther way to set system pressure is via the rear view mirror

well sort of

IF -- and there is the "caviar" Fish egg Disclaimer -- If you can find a port to test system pressure at the highest point - and In Ancient systems it was where you applied the plug to fix an atmospheric pressure gravity flow system to a "modern" pressurized system -- If you have this point --

3 psi at the highest point is satisfactory

Why did we do it like that ???/ Because old systems are full of old pipe -- runs and fittings are often thin from rot as "new" air was always available at these open tanks and the near pipes grew weak - so 3 there made the system function correctly and didn't severely impinge on old delicate parts.

now your system is more modern =-= but its simply a point of testimonial as to the value you found by reading the BG link

Its all good 9 12 15 - as long as its enough - not enough is bad but it can be a lot too much before its gets bad again

You say potato

I say potato

I could give you the answer But its better for U to understand why the answer is correct and how to arrive at it - so that's why I annoy you with links and stories and a little BS

boiling point in a residential heating system really should not come into play -- UNLESS the system used to be two pipe steam and the radiation (converted over to hot water) is now so severely undersized that the system will not heat the dwelling without running excessive temperatures. OH the stories of the Nacked City Heating Company Lets go back to the study of cup sizes at Hooters with Whistlenut.

markviii wrote:In my non-professional opinion, it is more important that the expansion tank and system have the same static pressure than if the actual number is 10 or 12 psi. If your diaphram tank has 8 psi in it (many do right off the shelf, always check before use) and the system is initially filled with 12 psi of water, the diaphram tank will have already accepted some water before the system is heated. In that case, the expansion capacity of the tank will be reduced.

My system has two Extrol model 60 diaphram tanks installed with 12 psi each, the feed-water regulator is also set at 12 psi, and the system usually runs 18-20 psi at operating temperature. When one or both of these tanks fail, I will just install a single commercial floor-mounted tank.

-Rob

Would a single #90 expansion tank be roughly the equivalent of two #60's? Perhaps having two #60's instead of a single #90 is a good idea, since if the bladder breaks in one of them and it gets fully flooded (and effectively removed from the system), then at least you have the other one still functioning. That makes two a great idea vs. one.

lsayre wrote:I've read (I believe in the book titled "Pumping Away") where if the static pressure of the system is higher than that which is necessary the effective expansion tank volume is diminished, and the expansion tank requirements of the system must be increased to compensate. Is that true?

I'm sure that nearly all boiler heated single story homes are set for the 12 PSI default, so obviously it cant be that big of a concern, but 10 PSI is a bit farther away from the pressure relief point of 30 PSI, so there is just a tad more safety inherent in the single story 10 PSI system, no?

Yes, increasing the static pressure reduces the effective size of an expansion tank. There are two design goals in selecting a expansion tank pressure, (1) have some PRV margin when the boiler is at the high temperature design point and (2) have some margin for any automatic vents located at the highest point in your piping system. The graph below shows how the required pressure to meet both those goals varies with height. My design margin is 5 psi for both. Height is the difference in feet between the point where the expansion tank attaches to the air scoop and the highest pipe, or radiator top, in your system. You can use the factory pre-set value but for needed heights less than 16 feet it results in a larger tank requirement. Given that tanks come in specific sizes it may or may not matter. What is important is the expansion tank initial set pressure and the water auto fill pressure be the same.

Excellent stuff there Yanche! I think I will bleed my expansion tank down to 10 PSI, and adjust the feed water pressure regulator down to 10 PSI also. Then when the system is at 180 degrees I believe it should hover at roughly 17.5 PSI.