Question on pressure.

[SmokeEater]

I've read quite of few threads from the forum's contributors, but can't remember seeing a mention of pressures in piping created by circ pumps. Anyone out there have pressure gauges on their loops know what might be typical pressures in 1 inch and 1 1/4" pipe? Different circ pumps must have some effect on pressure also, but don't know?

 

[EffectaBoilerUser (USA)]

I'm a little (lot) partial to the Grundfos 15-58 FC circ. pump. This 3 speed pump (with integral flow check) seems to cover 99% of the projects I am associated with and fails ver, very rarley.

I have used it on projects which utilized 3/4, 1", 1-1/4" copper and have never had a problem.

I run most the systems at 15-20 lbs max. pressure.

Brian

 

[jebatty]

I think that Effecta might have misunderstood your question. Here is a long answer to a simple question.

A pressurized hydronic system has two types of pressure in a typical home environment. First is static pressure in the system to overcome the effect of atmospheric pressure (gravity) to lift the water to the maximum height of the pipes/heating system. Typical minimum pressure in a home system when the system is cold is 12 psi. Each 1 psi will lift water 2.3 feet at sea level. So 12 psi static pressure will lift water just about 28 feet, which is plenty for most 2 story houses with basement + extra pressure to overcome cavitation in most home circulator pumps. A system which is 12 psi cold and completely sealed against expansion would likely result in disaster and explosion in the system because water expands as it heats and water does not compress. That is the reason for an expansion tank, to hold water as the water in the system expands. And as Effecta says, most home systems with the proper amount of expansion acceptance volume will end up with static pressure of about 15 psi (a 1 story home, no basement) to I would say about 25 psi (a 2 story home with the heating system in a basement level). The size of the pipe will not affect static pressure. Static pressure in a system with 1/2" pipe will be the same as 2" pipe.

Now back to your question on pressures created by circ pumps. A circ pump moves water by creating a differential pressure, that is, pressure is reduced on the inlet side by the rotating impeller which causes water to flow into the circulator and to flow out of the circulator on the outlet side. The amount of differential pressure needed to move a specific gpm amount of water in a system is complicated, but it is related to friction in the system caused by water moving against the pipes, fittings, etc. The greater the gpm flow, the greater the friction in a particular pipe size; the smaller the pipe size, the greater the friction at a particular gpm. And the greater the friction, the more differential pressure the circ must be able to create to move the water.

Friction in the system at a specific gpm is also called friction loss or pump head. If you look at a pump curve chart you will see a relationship between gpm and pump head. Pump head in feet divided by 2.3 = psi, which will be the differential pressure which must be created by the circ pump to move the gpm's. For Effecta's 15-58, if pump head = 12 feet (friction loss), then on H the 15-58 will move about 8 gpm. And if pump head = 12 feet, that equates to 5.2 psi, which means that the differential pressure between the inlet and outlets sides of the 15-58 will be 5.2 psi, or 2.6 psi less than static pressure on the inlet side and 2.6 psi greater than static pressure on the outlet side. Therefore, pressure measured at different points in the system will not be the same as the circ is operating, varying in this example by a maximum of 2.6 psi greater than static pressure and a minimum of 2.6 psi less than static pressure depending on where the pressure is measured (if this is in error, real hydronic pros needs to chime in).

Now one more point. It is a game of chance if you just pick a circ and expect it to work as required by your system. Serious and expensive mistakes can occur which relate to the amount of btu's your system must move to meet your heat loss requirements. It is essential in any system to know your btu requirements, which will relate to delta-T and gpm's, which will relate to the size of pipes and other components, which will relate to friction loss (pump head), which will relate to picking the appropriate circulator. All of these are inter-related in fairly complex ways. One size does not fit all.

 

[SmokeEater]

I think that Effecta might have misunderstood your question. Here is a long answer to a simple question.

A pressurized hydronic system has two types of pressure in a typical home environment. First is static pressure in the system to overcome the effect of atmospheric pressure (gravity) to lift the water to the maximum height of the pipes/heating system. Typical minimum pressure in a home system when the system is cold is 12 psi. Each 1 psi will lift water 2.3 feet at sea level. So 12 psi static pressure will lift water just about 28 feet, which is plenty for most 2 story houses with basement + extra pressure to overcome cavitation in most home circulator pumps. A system which is 12 psi cold and completely sealed against expansion would likely result in disaster and explosion in the system because water expands as it heats and water does not compress. That is the reason for an expansion tank, to hold water as the water in the system expands. And as Effecta says, most home systems with the proper amount of expansion acceptance volume will end up with static pressure of about 15 psi (a 1 story home, no basement) to I would say about 25 psi (a 2 story home with the heating system in a basement level). The size of the pipe will not affect static pressure. Static pressure in a system with 1/2" pipe will be the same as 2" pipe.

Now back to your question on pressures created by circ pumps. A circ pump moves water by creating a differential pressure, that is, pressure is reduced on the inlet side by the rotating impeller which causes water to flow into the circulator and to flow out of the circulator on the outlet side. The amount of differential pressure needed to move a specific gpm amount of water in a system is complicated, but it is related to friction in the system caused by water moving against the pipes, fittings, etc. The greater the gpm flow, the greater the friction in a particular pipe size; the smaller the pipe size, the greater the friction at a particular gpm. And the greater the friction, the more differential pressure the circ must be able to create to move the water.

Your explaination is very clear to me regarding the boiler circ, and from it I think I understand how the non boiler circs would move heated water into 2nd and 3rd floor areas. The pump to send heated water to, say, 3rd floor levels would have to be sized correctly for btu delivery and for head. Example: a 006 Taco would be the wrong choice to move water in a secondary heat loop to the 3rd floor from the basement. I note that its design head is only 7 feet, or 7/2.3 psi or it can produce 3 psi differential. Furthermore, it seems to me that the pressure in the radiant loop or high temp loop could be 0 psi??

 

[jebatty]

This is fun, or confusing, isn't it. The answer may be that a 006 is a satisfactory circulator. My assumptions: Your boiler is in a basement level. You are supplying heated water to baseboards on a 3rd floor. The vertical distance and piping between your boiler and the baseboards is 30'. The zone being supplied has 30' of baseboard. The piping to the 3rd level and in the baseboards is 3/4". For simplicity I'm assuming added equivalent pipe length of 10' due to elbows and other fittings. You have a primary loop from which the zone is supplied by a zone circulator. Hot water is being supplied at 180F. The baseboard is rated at 600 btu/ft at 4 gpm, or 18,000 btu total.

In this situation I would suggest that static pressure when the system is cold probably should be above 12 psi, perhaps 14 psi, due to the third level. I assume you have sufficient expansion capacity and when the system is at 180F the pressure may be about 25 psi. You have plenty of pressure to maintain hot water to the third level.

Pump head then is based on pipe length of 30' to reach the 3rd level + 30' of baseboard + 10' of equivalent pipe length = 70' total. Required flow is 4 gpm. Pump head of 70' of 3/4" pipe is about 4.5'. Friction Loss Looking at the pump curve for a 006 at 4.5' of head the 006 will supply more than 4 gpm. You may need a circuit setter or other valve to reduce flow to 4 gpm.

This is an example. You may need to do your own research or consult with a professional for design of your actual system, or for selection of components for an existing system. I am not a professional, just a hobbyist, and I hope others spot any errors I might have made or can provide additional or more correct information.

 

[SmokeEater]

This is fun, or confusing, isn't it. The answer may be that a 006 is a satisfactory circulator. My assumptions: Your boiler is in a basement level. You are supplying heated water to baseboards on a 3rd floor. The vertical distance and piping between your boiler and the baseboards is 30'. The zone being supplied has 30' of baseboard. The piping to the 3rd level and in the baseboards is 3/4". For simplicity I'm assuming added equivalent pipe length of 10' due to elbows and other fittings. You have a primary loop from which the zone is supplied by a zone circulator. Hot water is being supplied at 180F. The baseboard is rated at 600 btu/ft at 4 gpm, or 18,000 btu total.

In this situation I would suggest that static pressure when the system is cold probably should be above 12 psi, perhaps 14 psi, due to the third level. I assume you have sufficient expansion capacity and when the system is at 180F the pressure may be about 25 psi. You have plenty of pressure to maintain hot water to the third level.

Pump head then is based on pipe length of 30' to reach the 3rd level + 30' of baseboard + 10' of equivalent pipe length = 70' total. Required flow is 4 gpm. Pump head of 70' of 3/4" pipe is about 4.5'. Friction Loss Looking at the pump curve for a 006 at 4.5' of head the 006 will supply more than 4 gpm. You may need a circuit setter or other valve to reduce flow to 4 gpm.

This is an example. You may need to do your own research or consult with a professional for design of your actual system, or for selection of components for an existing system. I am not a professional, just a hobbyist, and I hope others spot any errors I might have made or can provide additional or more correct information.

This is some fun. I fully understand your example. I forgot to mention that I was hypothesizing that the 006 was in an independent loop where the static pressure would be zero, such as that in a loop on the "heated side" of a plate HX. This loop filled and purged with the 006 then attempting to pump to the 3rd floor with all of the friction loss you have mentioned. Does this independent loop need to have a positive static pressure and an expansion tank to function?