Flow Question/mystery with Circulator

DJDrew

I am struggling to make sense of a radiator in our house that does not heat well, and I am even more confused by the results of a trial using a different circulator.

Attached is a piping layout of what I was able to diagram in the hot water heating system of the main heating zone. The radiator that is circled in red is the problem radiator, it only gets warm if the system has run for over an hour (which meant it was a cold day.)

The system installer had put in a Grundfos UPS15-58FC, 3-Speed Circulator. When it was set on "High" the system did okay, though was always slow to respond, running 45-75minutes per heat call. The circled radiator would be the absolute last to heat up - if at all, while the rest of the house would heat up evenly. Set on Medium or Low speed with the Grundfos, the system would respond slower and slower.

On the suggestion that there is a lot of water content in these 1938 pipes, I swapped the circulator to an AquaMotion AM10-3F1 (3 speed circulator that supposedly mimics a Taco 0010). This is where it gets interesting:
- On "High" speed, the overall system is even more slow to respond and the radiator circled in Red never heats up.
- On "Medium" speed the system behaves similarly to when the UPS15-58 was running on high, perhaps a little slower, however the radiator at the end will eventually heat up.
- On "Low" speed everything is super slow to heat.

I am fairly confused with the fluid flow dynamics at play that the medium speed on the AM10-3F1 would work better and heat the circled radiator than the high speed setting.

Likewise, I am throughly confused why the radiator circled in red does not heat up with the rest of the house, especially when a larger radiator that is further away at the complete other end of the other line heats up without issue.

Any thoughts on what is going on flow wise? I have tried flushing the line with water to clean it and replaced the radiator valve with a new one thinking it was a bad valve. I am lost as to what I should try next...

Jamie Hall

I won't address the oddities of the pumps-- something about their flow vs. head curves is the problem there, and I don't have the curves for either one handy at the moment.

The radiator which is slow to heat up simply indicates that somewhere in the lines -- supply and return both -- between the split to the radiator Floor 2 rad 6 and Floor 1 rad 7 and that radiator there is a good deal more head loss in that radiator's feed or return than there is in the others. One would really have to examine the piping layout to see why that might be -- or it might be that for some reason the supply or return to that radiator is considerably more clogged with rust. You say that you have replaced the valve and are sure it's wide open, so it's not likely to be that.

DJDrew

Interesting thought, thank you! If we end up opening those sections of pipe to investigate, I worry the pipe or existing fittings might get damaged beyond reuse after not moving for 85 years, would you suggest replacing with new iron, or convert to copper for the last leg of the run? (I am assuming the people who originally installed this knew what they were doing and we'll find something obvious in the process.)

Jamie Hall

I'd convert to copper -- or even PEX, depending on what water temperatures you are running.

pecmsg

DJDrew said:

Interesting thought, thank you! If we end up opening those sections of pipe to investigate, I worry the pipe or existing fittings might get damaged beyond reuse after not moving for 85 years, would you suggest replacing with new iron, or convert to copper for the last leg of the run? (I am assuming the people who originally installed this knew what they were doing and we'll find something obvious in the process.)

the pipes in this house are 100+ years. I saw NO corrosion on the inside when working on the system.

DJDrew

pecmsg said:

the pipes in this house are 100+ years. I saw NO corrosion on the inside when working on the system.

That's interesting! I am not sure if there is corrosion on the inside, but I know anytime we've drained the system it's a lot of black gunky water, and anytime we bleed a radiator black gunk flys out. I put a magnetic dirt separator in the system and flush it clean occasionally... I wouldn't be surprised at this point if there is a build up of gunk in a line somewhere causing this flow issue.