View Full Version : Hot Water Heat in Tall House

Joe Steam
11-29-2010, 01:11 PM
I have a four story house with the hot water boiler in the basement. I figure it is about 50' from the boiler to the top radiator. It is a new boiler and I am afraid the contractor who installed it does not know too much about pressure settings.

Now, someone that knows more than me please correct my assumptions or give me advice:
Have heard I should have about 5 psig on the top radiator, so this gives me a setting of about 27 psi at the boiler. Although the boiler is rated at a higher pressure, it has a 30 psig relief valve. I am scared to set the boiler so close to the relief.

Double checking the boiler rated water pressure, should I change the relief valve and operate the boiler at the higher pressure? And then fill the diaphragm tank (empty) with air at my operating pressure?

I noticed this because the top floors are chilly!!

11-29-2010, 02:40 PM
I don't think it is a pressure issue, and (I'm not absolutely sure), I don't think you'll need that much pressure. The pressure is to ensure in the boiler you maintain enough pressure to keep it from flashing to steam. The water outlet won't (or shouldn't) be hot enough where a small pressure difference could allow that to happen outside of the boiler. It could easily be air in the system or head for the pump that is insufficient to get it to flow there. You may also need to adjust the balance of the zones to obtain the desired flow.

Tom Sawyer
11-29-2010, 02:53 PM
15 lbs pressure ( factory setting ) is more than enough pressure for 5 stories. If it's not heating then you have other issues like a circulator that is too small. Piping that is too small or some other restrictions in the loop.

11-29-2010, 03:04 PM
You do not need 5 psig at the top of the system. All you need is enough to be able to vent the air from the system. Also in most four story homes it wouldn't be 50 ft. to the top of the highest radiator. You need .43 psig to raise a water column 1 ft.


Joe Steam
11-30-2010, 04:49 AM
This is an old house with high ceilings, and it is about 50 feet or at least 45. So the pressure of water at 50 feet is a bit more than 21 psig. It is interesting because the boiler installer used a standard water PRV for boiler fill, set at 12 psig. When I bled the upper radiators, I lost pressure up there so nothing came out. So I re-adjusted the PRV to a higher setting. Hence my questions.....

11-30-2010, 07:35 AM
Is this steam heat or circulated hot water?

Joe Steam
11-30-2010, 08:16 AM
As mentioned, hot water boiler

11-30-2010, 10:59 AM
As mentioned, hot water boiler

Sorry, I was thrown off by your handle "joe steam"

I would get with Armstrong or Taco for a recommendation about a pump with the right head pressure and flow rate. I would be very careful about changing pressure settings or reliefs on a boiler. As they say...."they got rules about that"

11-30-2010, 12:24 PM
The boiler will have a maximum allowable working pressure (MAWP) stamped into the boiler itself and also listed on the nameplate. The safety valve can be any pressure rating that does not exceed this MAWP rating on the boiler. For example, if the boiler has a MAWP of 50 psi then you may use a 30 psi safety valve or any other rating up to a 50 psi safety valve.

The static pressure, the pressure when at room temperature and no pumps or firing running needs to be sufficient to raise the water to the highest level and then add a few psi for insurance. For a fifty foot high radiator the pressure needs to be no less than 21.65 psi and a few psi for insurance, or in the real world about 26 psi. The pressure when the system is hot will be determined by the temperature difference, the amount of water in the system and the size of the expansion tank.

These are the FACTS. I worked in this field for more than thirty years.

Joe Steam
11-30-2010, 12:45 PM
Thanks Furd, this is what I thought. Wondering if the 26 psig working pressure is too close to my 30 psig relief, and if the MAWP is high enough in my boiler, I should change the relief.
OR just operate at 26 and forget about it...

11-30-2010, 05:34 PM
Working pressure should always be at least 10% less than the set pressure of the safety valve on a hot water system. This would mean a maximum of 27 psi with a 30 psi safety valve. Since the minimum pressure for your system is 26 psi and you know it will be higher when hot it makes sense to put in a safety valve with a higher pressure setting as long as it is still no more than the MAWP stamped on the boiler.

12-01-2010, 08:08 AM
Your pressure gauge on the boiler has two scales, one is PSI, and the other is "Feet of altitude". The water "altitude" MUST be at least 6' higher than the distance from the boiler up to the highest radiator. The 6' is to give some residual pressure in case of water loss or other vagaries of the system. As an estimate you need 12 psi for every 27' feet of rise so 50' plus the residual, would need about 24 psi, and your 30 psi relief valve could handle that. A "good" boiler installer WOULD have known that 15 psi was inadequate for your system.