Fin Tube - 2nd Tube

ryanwc

I just noticed that one of the fin tube emitters in my house is different from the others. It has a finned tube (obviously), but above that, it has an un-finned tube.

What's going on there? Is this an attempt to create a one-pipe solution by allowing some water to bypass the finned part?

I did find a photo online of what I'm talking about, but I'm going to post that link in a reply to this post, in case there is a filter that doesn't like external links.

ryanwc

Here is a site with a picture of a similar fin tube design:
https://grabcad.com/library/baseboard-heater-with-heating-element-finned-tube-1

Jamie Hall

Where -- and how -- is the supply and return connected?

EBEBRATT-Ed

It's probably just the return pipe. With most baseboard the supply water goes in one end and out the other end.

Because of the building construction or other obsticales sometimes the return and supply ar put on the same end and the return pipe is run back inside the baseboard enclosure.

Perfectly fine

ryanwc

That makes sense. Thanks. It's installed from sidewall to sidewall of a narrow space with knee-wall attic on the other side of those walls. Only one of the knee-wall attics has an access door, I'm not clear when the work was done, but it may make sense that they had access to one side and not the other, and ran the pipe that way. With furniture placement, including a 40g fish tank that's full, it would be difficult to remove the covers to confirm.

(Edited - I figured out that I can see a lot with the covers on. There are no pipes going down into the floor, and a pipe entering the wall only at one end. The weird thing is that it enters at the end where there is NO access to the space enclosed behind a knee wall. I wonder how much heat is just being lost to that space, and why it was done that way.)

One thing I'm doing is trying to puzzle out the flow of the hot water distribution in the 2nd floor. It looks to me like the smallish room where this particular fin-tube is installed was original to the bungalow, while the much larger bedroom was either added or expanded about 20 years ago. I have some of the architectural drawings, but they don't show the path of the hot water.

I can trace the new copper pipes that serve the 2nd floor zone as they run along the basement ceiling (the older portions of the house have larger old pipe, is that galvanized, or lead?) My heat camera shows me a few stripes where a pipe must run under the flooring of the 2nd floor. But the trail goes cold, so to speak. Meaning, the telltale lines of heat across the floor stop without it being obvious how the circuit completes.

Ultimately, the question is what to do about a system based on a Peerless Purefire 140 (140k btu capacity), with a 2nd floor zone with 45 feet of fin tube baseboard, which would seem to be 22,950 btus at 170, and 14,400 at 140.

The Purefire 140 modulates at 4:1, so I think it's lowest setting would give 35k btu. It short cycles routinely when it's serving only the 2nd floor, and delta T in this zone is very low.

I'm wondering whether I could add a cast iron emitter or two to bring capacity closer to that of the Purefire. If that made sense, it would seem best to add them to the "cold end" of the loop, since they'd continue to emit with some efficiency at colder temps than the fintube. If I added them to the warm loop, they'd emit nicely, but the fintube would emit even less.

There is other weirdness to the distribution system. For instance, the floor plan front and back is symmetrical. The back is the site of the one-ended fin tube I had asked about. But in front, there is no emitter there. Instead, the pipe runs somewhere under the floor or beyond the drywall, from kneewall attic to kneewall attic, entirely outside the heated space. (The pipes are insulated there, at least.)

The HVAC company (found via this website) that fixed some but not all of my problems last winter will be here in a week for a routine maintenance visit. I'm trying to understand my system better so I'm ready to ask questions.

ryanwc

Thanks. I'm able to look through the louvre and I'm pretty sure you're right. There's a full 50-gallon fish tank preventing me from easily removing the cover to be positive.

There's something surprising to me. The two ends of this emitter both touch knee walls. My understanding is that there is space behind both. Only one has an 'attic' door to let me see what's there. The supply & return piping is on the opposite end, meaning it seems to head into inaccessible space, that has no door into it. But maybe the knee wall didn't exist before the renovation. It was all just unconditioned attic space.

Partly what I'm trying to do is to figure out the configuration of distribution pipes for the 2nd floor of our house (zone 2). I have a Peerless Purefire 140 Mod/Con (140k BTUs), with a 4:1 turndown ratio. If I understand, that means it can produce as little as 35,000 BTU/hr.

My Zone 2 radiators consist of 45 feet of fin-tube, which I believe gives me an emitter BTU total of 22,950 at 170 degrees, or 14,400 at 140 degrees. It short cycles, especially at this time of year, since the Mod/Con is configured to have lower supply temps when outdoor temps aren't too cold. Delta-T is always narrow for this zone.

I'm trying to figure out how to add emitter capacity. My first goal is to figure out how the water flows. Below is a sketch of what I know.

• Double red lines are emitters,
  
• Single red lines are places where my Flir attachment tells me there is a hot pipe under the floor,
  
• The red x is where I believe the supply and return enter the 2nd floor, because that's above the spot where the pipes head up from the basement,
  
• and the red arrow is the supply/return of the particular radiator I first asked about.
  

So my new question is Do you think that there are two loops in this zone, one heading to the emitter at bottom, the other tracing a path around the rest of the floor?

Otherwise, I can't figure why that red line would run from closet to bathroom. And, it would make sense of why the supply and return are at the same end of the emitter at bottom.

ryanwc

I'm wondering about adding a cast iron radiator somewhere. I thought it would make most sense at the "cold end" of the loop, since cast iron could still emit at a lower temp. (Is that a reasonable idea?) But was surprised to discover I may have two loops.

I have a routine maintenance visit from the HVAC company next week (chosen using the Find a Contractor link here at Heating Help!). They did some repairs last year, chronicled in a thread here, fixing a problem possibly created by a previous contractor. I'm trying to get a better understanding so I can line up some good questions when they come out.

hot_rod

Many of the mod cons have control parameter to help lessen short cycling, read your manual, look for a ramp delay, output limit, or other control options. Different brands have different names for those adjustments.

EBEBRATT-Ed

@ryanwc

it's hard to say. But whatever you do you should not mix cast iron on the same loop with baseboard. They don't heat the same so that doesn't work too well.

I would check into doing what hot rod suggested. If that doesn't work you could add a buffer tank.

ryanwc

Thanks!

Just as you suggested, right there in the manual:

"Ramp Delay Step Modulation - useful for systems with very small zones which can cause a boiler to short cycle."

Also, "Anti-Cyling - a minimum off-time to prevent short cycling." Presumably this helps by ensuring supply water has already given up most of its heat before the boiler begins again. This setting is in minutes, but seems also to have a TDiff override function - if the difference between supply and target is greater than X, it turns anti-cycling off, and X can be 20-40 degrees (or you can turn the TDiff override off. I'm going to have to think about how that works a bit.

Excellent. Really appreciate it. I'll test how Ramp Delay Step Modulation works tomorrow, and try to internalize the Anti-Cycling/TDiff stuff so I can figure out how to make that work.

ryanwc

Ramp Step Delay was already on.

Increasing Pump Post Purge from 4 minutes to 9 and Anti-Cycling time from 5 minutes to 10 pushed my flame cycle from 3:30 to 4:40. So I moved both to 15 minutes.

At the lower setting, there was also one "instant cycle" - the ignition provided enough energy that it immediately pushed supply over the target, and shut the system down again. . At the longer delay, the return temps were low enough that ignition didn't push supply that high, and the cycle could get itself started.

I also changed Boost, which pushes the target supply temp higher if the call for heat continues for a certain amount of time. It was at 7/90. I changed it to 10/60.

Next is to reduce my overnight setback, and synch the first and 2nd floors better, so that the 2nd floor is mostly drawing at times when the 1st is also, and their combined draw is closer to boiler capacity.

But I don't really want the house at 69 all night. I'd rather sleep in a cooler house. Sigh.

ryanwc

>it's hard to say. But whatever you do you should not mix cast iron on the same loop with baseboard. They don't heat the same so that doesn't work too well.

Generally, I get the idea. But consider the drawing above, showing that most of this is in one single large room. I was thinking a cast iron with greater capacity could go in one or both of the carved-out spaces near the top of the drawing, just below the knee-wall attic. These spaces are under slanted roof, making them not particularly useful living space. Heat coming from a radiator there would radiate towards the back of the couch and the back of a chair, and slowly heat the air. But it wouldn't directly change the experience of anyone in the room. I'm not sure why that radiator taking a little longer to come to heat, and then remaining hot a little longer, would matter much.

And in a another thread here about mixing cast iron and fin tube. someone said "It can only work with continuous running and outdoor reset." I have outdoor reset and would like to move closer to continuous running.

But I admit I don't know. I do want to understand why it may not work for me.

Or is there another option for increasing the emitter capacity of that loop? Higher capacity copper-tube in a radiator?

mattmia2

ryanwc said:

Ramp Step Delay was already on.

Increasing Pump Post Purge from 4 minutes to 9 and Anti-Cycling time from 5 minutes to 10 pushed my flame cycle from 3:30 to 4:40. So I moved both to 15 minutes.

I was thinking starting start at 20 or 30 minutes and shortening it if the cooling water in the loop wasn't enough output.

If there is a setting to limit the firing rate, set that to just a little over the heat loss of the whole house to start with(assuming if there is dhw from this boiler that it gets it's own call with it's own limit). The boiler only knows there is a heat call, not how big the system that is calling is. It will try to fire at a rate to get the supply water temp up to set point as fast as possible, so with your low mass loop that doesn't radiate much heat, it will fire at a high rate and heat all that water quickly. You want it to fire at a lower rate for a longer time.

EBEBRATT-Ed

@ryanwc

CI versus fintube. They both work that's not the issue and if the heat runs constantly not an issue but cI takes longer to heat up when it does heat up being a larger mass it cools slowly.

Fin tube heats up quicker and gives off heat and cools down quicker.

ryanwc

Matt, Ramp Delay Step Modulation does that. It modulates the firing rate to 20% of input, and then it would increase if supply isn't approaching the target fast enough.

Strangely, when the call for heat is from Zone 2, the meter often reads Input - 1%, even though the boiler theoretically can't do that, and only has 4:1 turndown.

Also, the highest allowed value for Anti-Cycling is 15 minutes.

I just did the emitter capacity calculations. Our old fashioned radiators on the first floor/zone 1 seem to have a capacity of 59,000 btu at 170 degrees. The 2nd floor/Zone 2 is 26,100. The second floor has maybe 80% of the square footage of the first floor, and loses heat through a roof with an R value of 18. Air sealing is good all around.

mattmia2

You should start out with a heat loss calc of the building if you don't already have one. That will let you better understand why it is behaving the way it does.


You could add a mixing valve or other strategies to decrease the swt of the ci zone. If it is oversized and it likely is by a lot, it will overshoot once it gets hot with all that heat in the mass of the radiators or it will have to run very short cycles with the anticipator in the thermostat having the shut down the zone virtually before any heating occurs.

ryanwc

>Fin tube heats up quicker and gives off heat and cools down quicker.

I can easily see how that could be problematic in a setting where different radiators were in different rooms that were regularly closed off from each other, with the thermostat in one. Our space is all interconnected, the thermostat is right at the door that connects them. The doors are pocket doors that are almost never closed. A radiator having a slower, longer output might mean the two other rooms were cooling longer after the boiler stops, while the big room continued to gain some heat from the iron. But I don't think that would be a huge problem, given my understanding of how these three rooms interrelate. It might be fine that my office and the bathroom sometimes got a tiny bit cooler than the big bedroom/wife's office.

Is there a better way to add output to this circuit?

How do operations change if I add a buffer tank? It stores heat because I'm producing it faster than I can get it into the house. And then I have to get the tank heat to the house, since I don't want it in the basement. Currently, on a 5 minute boiler run, the pumps run for 15 minutes to get the water cooled to kind of near the start point, which seems to imply that radiator output is about 1/3rd the lowest boiler output. If I had a buffer tank storing a lot of additional heat from a 10 minute boiler run (the recommended cycle), the pumps would have to run a lot longer to get rid of that heat, right? The boiler controls don't allow for pumps running longer than 15 minutes. Do I get a different controller for the pumps, that somehow relates to tank temps?

mattmia2

ryanwc said:

>Fin tube heats up quicker and gives off heat and cools down quicker.

Is there a better way to add output to this circuit?

You could replace the fin tube with something that has more mass and is designed for a lower swt like cast iron baseboard or panel radiators or runtal style radiators.

All about buffer tanks:
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf

There are numerous control strategies. One is to run the boiler off an aquastat in the buffer tank and have the thermostats just control valves/circulators for the zones circulating from the tank.

BTW if this is a mod con boiler, which i think it is, you will get much better efficiency if you can run it at return water temps under 130 or so.

ryanwc

Thanks for the buffer tank reference. Been reading up. It's a lot to internalize, but I feel like I'm starting to understand how it works, why and how it might apply in my situation. I'm still not clear on the controller issue, but I'm clearer on controlling principles. I just need to keep reading.

It seems like the imbalance in emitter output between zones 1 and 2 is an issue that would stand in the way of full optimization even with a tank, and that running at low water temps may require more emitters upstairs. I'm guessing that something like a stainless steel radiator/convector has similar incompatibility issues with a zone of fin tube baseboard -- that it's a large thermal mass that takes a while to heat up. Is that accurate? I don't really want to spend thousands on new emitters, and was hoping I could bump capacity rather than replacing it. Maybe that's a pipe dream.

On another forum, someone mentioned the idea of low flow/high flow zone valves rather than on/off valves, to take some of the overproduction of heat when only the small zone is calling. Has anyone had any experience of that?