Hydronic Subloop Won't Prime

Enginerd

Hi everyone,

After some rework that involved draining the system, one zone of my hydronic system has only 2 out of 6 radiators that get hot. I had been under the mistaken impression that all radiators in the zone were in series, so I pulled my hair out and wasted a bit of time troubleshooting this before I got into my crawlspace and traced the pipes out to understand what was warming up and what wasn't. Here is a diagram of what I found:



The real piping is even more chaotic than my diagram, but I believe this is an accurate representation of what is connected to what and what direction things are at least supposed to flow. I cannot fathom how this ever worked, but it did. There is no balancing valve and the tees are regular vanilla tees, not monoflow tees. The hot water blows straight through the tees that the subloop is connected via, with the 90* in/outs cold.

When the zone starts or stops, I can sometimes hear air in at least one of the cold radiators, but I can get nothing but water from any of the coin vents (fin/tube 3 in the diagram has no vent). All the coin vents are facing the wrong way and make a tremendous mess when you open them, because the stream blasts straight into the wall.

Unless there is some magic technique to get this bled and working, I think I have to make a change. Previous owner mentioned adding Fin/Tube 3 in the side entry, which is outside the insulated envelope of the house, and I suspect that the problem is rooted here. What would be the best way to do this? I have two candidate solutions in mind, but I'm open to suggestions.

Option A is to add a balancing valve to the shorter loop that currently flows. This involves the least plumbing in my crawlspace, so I like it, but I'm nervous that I'll end up with the opposite problem: no flow in rads 1 & 2, even with the valve at its most open position. I have not used a balancing valve before and I don't have a feel as to the adjustment authority one provides. It also means balancing the system will now require getting into the crawlspace, which is done from outside and is no fun. Are these valves generally set for life, or can I expect ongoing maintenance? Not hard to imagine needing to close the valve after every drain to get the subloop to purge, then rebalance.

Option A diagram:


Option B would be to chop out some piping and convert the zone to a series loop. Arguments in favor of this are that I can do it with the parts and tools I already have (not that great an argument, in my mind), and that I should get a larger delta-T on this loop, which is the most heavily used in the house. Arguments against are that I have to do a lot of plumbing in my crawlspace in an area that's a bit crowded with some questionable previous owner electrical work, and that I don't know if this long series loop will create a new need for balancing valves at the zone manifold. I also don't much like that this sends the hottest water to the only semi-heated space of the side entryway. There's no thermostat in there and it's really just meant to stay warm enough to melt the snow off your boots.

Option B diagram:


Have I analyzed the situation correctly? Is there some boneheaded mistake I'm making that is keeping me from purging the air from the subloop?

Thanks or reading and thanks for any advice!

hot_rod

Can you isolate various sections, purge one at a time? If not you will need a lot of flow to purge the entire system from one point.

If you want to solve two concerns, purging and eventual balance, you need to add some valves, and purge points while you are at it.

I'd be tempted to use these. While a ball valve is not the perfect valve for balance, for small regulation they work fine. The B&G circuit setter is a ball type valve, millions of them out there.

If the piping is copper, these press style can be added in minutes.

Enginerd

Thanks for the reply!

No way to isolate the subloop, but during the initial fill I did run both pumps and isolate the 3 zones one at at time. Option C that I didn't write up is I think similar to what you are describing: add a ball valve where I have the balancing valve in Option A. If I shut that valve, I think the subloop will then purge. The system was satisfactorily balanced before, so I think I could then just open it and leave it that way until the next time I have to drain down the system. Seems to me that Option A or anything not requiring a helper in the crawlspace would be more "correct" than this, but I've been accused of overengineering home repairs.

hot_rod

Do you have a good air purger at the boiler? Once you get any flow through all the loops, the air purger should take the rest of the small air out.

If you want a simple balance valve with meter included. A tee with a valve next to it would give you a purge point also.

You can do some balance at the fin tube by adjusting the dampers in areas that may over-heat.

Enginerd

That valve is exactly the one I had in mind. I like the idea of the tee and valve to provide a purge point, will definitely install that if I go this way.

The boiler has an internal auto vent, plus there's a 1" spirovent installed at the expansion tank upstream of the circulator, just before the combined return flow enters the boiler. The spirovent is quite active for the first few minutes after filling the system, but is now quiet. Picture of the setup in my other thread for a different question: https://forum.heatinghelp.com/discussion/190187/differential-bypass-based-primary-secondary

mattmia2

When you purged, did you close off the loop somewhere at the boiler, flush water in from the fast fill lever of the pressure reducing valve, and drain water out somewhere on the opposite side of the valve you used for isolation from the pressure reducing valve? You need an isolation valve at the boiler for this to work and you need to use the house water pressure to do the purging, the circualtors can't produce enough flow. This is almost certainly how that baseboard without the bleed valve was bled. Another option would be to just add a bleeder valve on that section of baseboard.

Enginerd

I did not do it that way. Thank you. I will try that tonight at report back. I do have isolation valves with purge connections at the boiler, but I had taken these to be for flushing the heat exchanger as part of maintenance.

Enginerd

Success!

...well, sort of. My isolation valves only allow for flow through the boiler in the isolated position, not through the pipe network. I followed the procedure in the manual to purge via the integral auto feed and discharge via the service port while using the circulator in manual mode to help. Opened the coin vents while everything was running and was able to get enough air out for water to start flowing and all the rads to heat up. All the blue radiators in the diagram are hot and loudly gurgling now, but the radiator vents once again produce only water. The air scoop seems to be slowly doing its job, so I'm going to give it some time.

I think adding a proper purge port is now on the to-do list, because this was a bear.

Enginerd

It took a few hours of circulation with the pump speed turned up, but all the air is out. Thanks again!

Lesson learned here: Even if you think you've already read the manual, read the manual. I had not read, or at least not comprehended, that the system was meant to use the built in auto-feed to purge, because there are instructions to crack the pressure relief open while filling to purge the air out, and this was good enough to get 2.5 zones working.

I may have to retract what I said earlier about the system being satisfactorily balanced before. The problem leg of this zone does get very low flow, as compared to the main leg. I think this was masked in the past by the old cast iron boiler's fixed high temp and pump speed, but at 130* with the pump speed knocked down, rads 3-6 are progressively colder, with 6 barely warm. I'll likely be leaving it like this for now, to get a better feel of the system, how it works, and how I want it to work before I start changing things again, but I may end up putting a balancer in there after all.

mattmia2

You can balance it to some extent with the dampers on the enclosures for the fin tube but fin tube doesn't have much output at 130 f. I would expect that to be close to the temp where it runs constantly to keep up with a mild day.

If you do put in a balancing valve, you could put it in the fin tube enclosure, you could even cut a few inches off of the element in the hotter section to do it.

Enginerd

Yes, it's been pretty mild here for the last week. I haven't gotten around to running the wire for the outdoor reset yet, but I've been manually adjusting the temperature to build a mental picture of what the reset curve should look like when I do. My understanding is that lower-is-better for temps and close to continuous is desirable.

I'm glad you said that about trimming the element. I had been trying to think of how I could fit it inside the enclosure, but had not considered the emitter to be something I could modify.