Gravity Hot water system pressure

Timco

I have seen old gravity systems on a single story house that would not hold 12#...it would dump onto the roof right at 12# so it was filled until water dumped onto the roof and was full...

Tim

Josh (The Youngster)_2

Gravity Hot Water System Water Pressure

My family recently moved into an older home with a closed gravity hot water system. The system is a Sears Homart 600 with an Amtrol expansion tank rated to take 70 psi. The system is only used to heat our second floor because the lower floor has been converted to a heat pump. When we moved in the pressure gauge showed almost no pressure at all when the system was not heating and got up to about 10 psi when the heat had been on for a while. We had some troubles with one of our radiators heating up so we called the previous owner's heating and air guy. He fixed the problem with the radiator but the gauge is now constantly at 29 psi. Is this normal or should it be lower than this? What is the normal running pressure for a closed gravity hot water system? Thanks.

Brad White_203

Normal Operating Pressure

for a residential hot water boiler is not usually more than 25 PSI maximum, but is often half that for a two storey house.

The relief valve, by code, is set at 30 PSI so there is your safety limit.

Basically, the system cold fill pressure should be, in your case, about 12 PSI. This accounts for the fact that every foot of height above the gauge will impose 0.433 PSI. Stated another way, every PSI will raise water 2.31 feet.

Also you want to have at least 4 to 5 PSI residual head at the top of the system.

Say your height from your boiler to the top of your highest active radiator is, oh, 16 feet.

(16 x 0.433) + 5 PSI residual = 11.93 PSI, OK, call it 12.0

If your system climbs that high (29 PSI) when heated, you have a lack of expansion tank capacity. It may be flooded or just not large enough.


You may know that most if not all original gravity systems were "open" at the top. The expansion tank was a vented tank at the high point and often had a toilet-tank type float valve to top it off. It may have been vented to the attic or vented with a pipe out onto the roof. No way to over-pressurize that baby (unless the vent froze).

Take a look upstairs, maybe you have one. If not, the new tank or tanks should be of the diaphragm type. If installed high in the system they can be smaller than if they were to be installed at the bottom.

Josh (The Youngster)_2

It's a diaphragm type tank

Thanks Brad. It's a Well-x-trol model # WX102. Looks like the capacity is 4.4 gallons and it's mounted right near the boiler in the basement. Is that sufficient for the system?

I'm going to go ahead and release some of the pressure and try to get the cold fill pressure down to about 12 psi. Is there anything else I need to check to make sure that is the correct pressure for my system? Thanks again for your help.

Brad White_191

4.4 Gallons?

Not nearly large enough in my experience. If not a larger one, get two more #30's and charge them all to 12# and connect them together. That tends to work for most gravity HW systems serving medium size houses in my experience.

I say "use multiple tanks" because two #30's are less money usually than a single #60.

While you are doing this you may want to secure a new gauge to make sure it is all reading correctly. Hate to see you rely on a gauge that was installed during the first Roosevelt administration....

MARK MEYERS

NORMAL PRESSURE

TO BRAD WHITE -VERY INFORMATIVE TO THE PERSON WITH PROBLEM ON GRAVITY SYSTEM-YOU EXPLAIN TO HIM OR HIS HEATING CONTRACTOR WHATS UP WITH HIS SYSTEM, THO I PUT 15LB. ON SYSTEM TO ACCOUNT FOR THAT FAR RAD.IN THESE TALL CEILING HOUSES WHICH ALMOST ALL OF THE GRAVITY HTG. SYSTEMS WERE IN. IM SURE THAT THEY CAN WORK THIS PROBLEM OUT WITH YOUR INFORMATION

MARK MEYERS

PRESSURE-GRAV

TIM U ARE RITE ON 12LBS WOULD DUMP ON A SINGLE STORY HOUSE ON GRAVITY<THO I DON'T REMEMBER THE LAST 1 STORY GRAVITY HOUSE I WORKED ON-WE LIVE IN AN OLDER TOWN MOSTLY 2 STORIES. BUT USING 2.5FT PER POUND.--6.5 TO 7LBS WOULD BE PLENTY-IT HAS TO BE FIGURED AND CHANGED TO MEET THE NEEDS.

MARK MEYERS

PRESSURE-GRAV

TIM U ARE RITE ON 12LBS WOULD DUMP ON A SINGLE STORY HOUSE ON GRAVITY<THO I DON'T REMEMBER THE LAST 1 STORY GRAVITY HOUSE I WORKED ON-WE LIVE IN AN OLDER TOWN MOSTLY 2 STORIES. BUT USING 2.5FT PER POUND.--6.5 TO 7LBS WOULD BE PLENTY-IT HAS TO BE FIGURED AND CHANGED TO MEET THE NEEDS.

MARK MEYERS

PRESSURE-GRAV

TIM U ARE RITE ON 12LBS WOULD DUMP ON A SINGLE STORY HOUSE ON GRAVITY<THO I DON'T REMEMBER THE LAST 1 STORY GRAVITY HOUSE I WORKED ON-WE LIVE IN AN OLDER TOWN MOSTLY 2 STORIES. BUT USING 2.5FT PER POUND.--6.5 TO 7LBS WOULD BE PLENTY-IT HAS TO BE FIGURED AND CHANGED TO MEET THE NEEDS.