Pump size

[curtis]

My uncle has a outdoor wood boiler about 200ft from his house. He has 1" pex ran underground and then through a heat exchanger in the house for a baseboard heating system. The owb is a open system and he has a grundfos UPS26-99FC pump running the owb water from the boiler to the house. Does that seem like it's the correct pump or do you need more information about the system?

 

[heaterman]

My uncle has a outdoor wood boiler about 200ft from his house. He has 1" pex ran underground and then through a heat exchanger in the house for a baseboard heating system. The owb is a open system and he has a grundfos UPS26-99FC pump running the owb water from the boiler to the house. Does that seem like it's the correct pump or do you need more information about the system?

Do you know what size the heat exchanger is?

.......Using just the pex + a half dozen ells and a couple ball valves for estimating, the 99 will move about 8.5 - 9 GPM at approximately 24 feet of head on speed 3. Add the flow resistance of the heat exchanger and you could be in the 6-7GPM range at nearly 30 feet of head.

The pump will not live a long and happy life at those numbers. With an open system like that, the pressure available at the suction inlet of the impeller is not enough to keep it from cavitating. Especially if water temps creep up to 180* or higher.

Pump cavitation does two things to wet rotor circs like we use these days.
1. It can literally hammer the impeller to pieces.
2. It allows formation of air bubbles which can work their way back into the "can" of the pump and cause the motor to overheat/burnout.

 

[jebatty]

Given heaterman's analysis, who sold/told your uncle to use 1" pex for a 200' run and to use that circ? It seems to me that dealer's and mfr's of all boilers, especially outdoor wood boilers (due to length of pipe runs), need to be held accountable for their negligence and incompetence.

 

[Coal Reaper]

seams most OWB installs use 1" PEX regardless of the length of run and operate the circ 24/7. pump only has to last 5-10 years to keep up with the OWB, right?

 

[jebatty]

An example: One OWB mfr advertises 120,000 btuh output and suitable for install up to 400 feet from the residence. To move 120,000 btuh, assuming a 20F temperature drop between supply and return, 12 gpm of water must flow. 1" pex at 12 gpm has a pressure drop of 7.63 psi per 100 feet. If the boiler was installed 150' from the house, total pex run is 300', total pressure drop is 22.89 psi just in the pex run and not including other plumbing and resulting pressure drop of that plumbing, the heat emitters, heat exchangers, elbows, etc. The pex pressure drop = about 52' of pump head. You would be hard-pressed to achieve this with any circulator except a high pressure commercial circulator, and then you also would need a pressurized system with sufficient pressure to avoid pump cavitation and damage..

The point I am making is that moving btu's with water is not simple pipe and circulator. All are inter-related. The first step is determining heat loss, then btuh needed to meet heat loss, then the size of the boiler to provide the needed btu's, then gpm of water needed to be moved to carry the btu's, then a combination of selecting pipe size and circulator based on pressure drop, distance, and water volume to be moved, and then also NPSH required to allow the circulator to operate efficiently, without cavitation and damage.

Using 1" PEX regardless of the length of run and operate a circ 24/7 is rather meaningless in dealing with any of these questions. It's like saying "I need a truck to haul stuff," then buying a pickup without knowing the load capacity or the size of the engine, and then trying to use that pickup to haul a 20 ton load over the mountains on constant coast-to-coast runs.

 

[curtis]

Ya I have no idea who told him what size pex to run to the house. I will find out what size heat exchanger he has inside and let you guys know. He did say that he always ran the pump on the highest speed it had. I know also that the way it was installed it ran 24/7. He burns a lot of wood and I told him that the boiler he has is not efficient to begin with but by burning seasoned wood and having the right setup( pumps, line size ect.) It could be made to run a little more efficient. He had the mind set a lot of owb people around my area ( including me before I joined the site) have, green wood is for boilers because it lasts longer in the boiler. I'm slowly changing his perspective of that and will hopefully fully convert him when I get my garn running.

 

[curtis]

Here us the heat exchanger that is inside for the baseboard heaters