Fin Tube Element In Tub Apron Some Choices Questions

PGB1 · November 2021

Hello All!

In one bathroom, there is a 3 column, seven section radiator with estimated 31 square feet EDR. The zone that it's part of has a 3/4" Type L copper loop & the radiator is tapped off via a mono flow tee to 1/2" copper tubing. The supply temperature at the radiator is 170-F (typically). There are balancing valves to limit the flow to that radiator.

It heats the room well. But the cast iron bathtub is a big iceberg. Warming the tub would be ideal.

The plan is to:

A) Install a piece of fin tube radiation in the tub's apron, putting the piping in series with the supply to the cast iron radiator (before the radiator is the only choice due to crowded joist conditions below). Flow would be: To apron fin tube, out of fin tube to radiator, out of radiator to zone's loop. I'd stand it on the floor in the apron by using two-piece stand off hangers clamping the element's tubing, with the floor bracket screwed to wide, wood "feet". The copper stub-ups from the basement would also help hold it upright. For venting, the branch to the radiator has a force bleed cock and we also have a Spiro Vent. (Or I can put a coin vent at the tub's wet end, where there is access.)

Radiant barrier will be installed on the floor of the apron, the non-apron side (against an unheated indoor wall) and the dry end (an outside wall). I can't get under the tub, so radiant barrier will be on the bottom of the floor sheathing in the basement. (As long as there is a 19 mm or larger gap on one side, radiant barrier is amazing. Installing perforated aluminum barrier in our 2nd floor bungalow ceilings before drywall and in the attic made a tremendous difference in the heating fuel usage. The rooms also stay much cooler in summer.)

But, my questions are what would you all choose to do for the following:
For the fin tube, I have available some 60+ year old 1" copper with 4" steel fins (a take-out from this house). I can bush it down to 1/2".
I also can purchase 3/4 or 1/2" new fin tube from the local supply house.
Which would be your choice? Is there a great btu difference between steel fins & aluminum?

Is the apron fin tube better off with or without the cover? Since there won't be an appreciable amount of new air entering the apron, I was not sure if the cover would help more of the trapped air circulate across the element.

Tanks for sharing your knowledge & suggestions. I very much appreciate your help.
Paul.

Alan (California Radiant) Forbes · November 2021

Too bad you can't have the radiant bathtub on its own zone. Otherwise, it may not be heating long enough to raise the temperature of the cast iron bathtub.

EBEBRATT-Ed · November 2021

I don't know how your getting air flow across the fin tube, but if you don't have air flow it won't heat much

Solid_Fuel_Man · November 2021

I'd put several runs of just the copper with the fins in there. Yes, the enclosure encourages convection, but the limited space in a tub apron would allow for more than one run of just the fin tube element. I'd run two or 3 if you can fit it.

Will it work? Maybe.... but supplying it first, the the rad is ideal, and I'd try it in my own home! I would put a baseboard T "vent elbow" and a coin vent on the highest point.

Do it, and let us know how it works.

hot_rod · November 2021

I'd use bare copper, maybe some of the aluminum heat transfer plates to increase radiant surface area. I doubt the fins will add much. Convection depens on cooler air below and warm air rising through the fins. Without the enclosure I doubt much will happen.

Ideally the tube would be in contact with the tub, then you get conduction transfer, the strongest of all.

Or put the tube on the floor, even pex tube, and hand mix a bucket of gypcrete or any flowing concrete product to pour in. That would connect tube to pour and pour to tub

PGB1 · November 2021

Thank You everyone for your helpful comments & suggestions. I very much appreciate your sharing of knowledge.

I considered putting balancing tee-cock arrangement so I can direct more water to either the radiator or the tub heat. After reading everyone's comments & suggestions, I think this will be worth the effort.

Ideally, the bathroom would be on its own zone, as Alan mentioned. Actually, that is on the "Someday" list, along with another bathroom. When that project happens (in the future), I'll change to under-floor with transfer plates, tempering valves and separate pumps.

Solid_Fuel_Man suggested several runs. The tub is an older art deco style with a very wide apron, so two sections of Slant Fin Base/Line 2000 will fit back-to back with the enclosures on. I'd probably pipe them in series so there's no chance of unequal flow through one of them.

Hot Rod mentioned tubing in contact with the tub. This sounds like the "sure thing" for heat transfer. So far I haven't figured out access to attach the plates to the tub wall. Pex-A in plates would be the method I'd use.

Ebebrat mentioned air flow. What do you all think about cutting in a pair of cold air return grills in the sub floor below the tub (unconditioned basement)? Maybe a couple of 14 x 6, one near each end of the apron, or even a full length one under the whole apron.

Thanks Again Everyone!
Paul

hot_rod · November 2021

Hot goes to cold, so some openings will allow the heat from the room to go into that space. Will it be enough? Hard to know, really

If the tub is on an outside wall or has some infiltration, air leaks, you may end up with a draft or cooler bathroom, usually felt at the floor level

A small kick space heater would certainly add enough BTUs, and a small amount of noise☺️

PGB1 · November 2021

Thanks Bob for your mention of drafts. I hadn't thought about that. This house is Air Leak City, so perhaps my cold air return in the sub floor isn't such a hot idea after all.

Thanks, too, for mentioning the kickspace heater. Many years ago I put a Grainger house brand one in the kitchen & it's been great. Only occasional preventative maintenance (cleaning) is required.

I'll have to explore your kickspace heater idea when I'm home later. If I place it under the sink cabinet, it'll aim right at the tub apron, hopefully taking away the chill of the cast iron (which is our main goal of heating the apron).

Thanks Again for helping everyone. Perhaps I can get some time today to explore the options further.
Paul

Alan (California Radiant) Forbes · November 2021

Now that I think of it, the subject of heating a cast iron bathtub has come up many times over the years. And as many times as it has been discussed to death, I don't think I've ever heard of it being done with success. Am I wrong?

hot_rod · November 2021

Alan (California Radiant) Forbes said:
Now that I think of it, the subject of heating a cast iron bathtub has come up many times over the years. And as many times as it has been discussed to death, I don't think I've ever heard of it being done with success. Am I wrong?

We used construction adhesive to glue Entran rubber tube to a few cast iron tub, back in the day. The very first Entran was very supple and easy to work with, you could tie it into a knot like a rope.
The tub probably lasted longer then the tube

If a tub is on a concrete slab with radiant heat, and you put some pour stone under the tube, you would get a good conduction transfer. Just warming the space below would take some chill off.

PGB1 · November 2021

Thanks Bob for your idea. I appreciate your help.

The tub's on a wood subfloor (over a basement). If (and it's a big IF at this point) I can get access, I wonder if embedding tubing in lightweight perlite plaster on top of foam sheet would work for conductive transfer.

We are remodeling the room, so if the tub has to be removed as part of the project, I think I'll attach pex-a to the apron walls with aluminum transfer plates on it. That'll be a guarantee of good heat transfer.
Hopefully, the tub won't have to come out (cast iron versus old guy).

It appears that 100 or so years ago, when the tub was installed, the tub is supported by the sub floor planks, not across joists (parallel to tub's length). So removing the subfloor from below to get into the apron for attaching tubing to the apron walls would require removing the tub.

If I install fin tube, I was concerned about air flow in the relatively "dead-air space" of the apron. But then I though about an electric oven. That's dead air space & the oven walls and contents get hot. An incandescent light bulb & a double pane window also came to mind. The air spaces are dead, but heat is transferred.

Keeping in mind the goal of taking the chill off the iron, not heating the room & since access from the wet end of the apron is relatively easy due to a removable closet wall, perhaps I'll try a piece of fin tube without the cover. If that fails, I'll try with the cover. Draining, filling & force bleeding the branch of the zone's easy, so it's not a big project to try, try again.

mattmia2 · November 2021

Cast iron baseboard would work a lot better if you can get it in there.

hot_rod · November 2021

yes your idea of getting some tube below then a mortar mix to connect the tube to the cast tube would be the best, maybe easiest without removing the tub. If you have access to either end or below you should be able to fish a loop pf pex around the tub.

PGB1 · November 2021

Thanks Bob.
I haven't looked yet, but it occurred to me that there is most likely a cross brace from tub to apron. This might preclude fitting fin tube in the apron. The tube-in-mortar approach will be Plan B. Putting light weight mud bed & fishing a circle of PEX might actually be a fun challenge.

hot_rod · November 2021

This type of cement can be mixed to pour. Get the tube underneath, seal any openings with spray foam cans or fiberglass stuffed around the drain cutout, and pour in an inch or so. Enough to encase the tube and touch the bottom of the tub. That will basically turn the tub into a large cast iron radiator.

I'd fill the tub just to be sure it doesn't lift up. I have had that happen when you spray foam under a fiberglass tub. The expansion type foams will lift it up if it is not weighted down. I doubt a cast tub would lift, a steel one might

ChicagoCooperator · November 2021

My building was built with one of the bedroom convectors open to the end of the tub (steam) and it really warms up the tub and the entire bathroom - it's an exceptionally short tub - 4' and it adds a lot of heat to the bathroom.

PGB1 · November 2021

Thanks Bob & ChicagoCooperator for taking time to post your suggestions.

I'd have never thought about the foam lifting the tub. That's something that would send one into panic mode quickly. Filling the tub should help, but I've seen that stuff push things apart. I suppose the window-door low expansion type wouldn't lift the tub, but it's open cell, so I'd guess the conduction is less.

Your reply about the convector being open to the end of the tub, ChicagoCooperator, has me pondering about installing a fan forced kick space heater at the closet-accessible end of the apron (the tub's wet end).
I wanted the residual off-cycle heat of the cast iron (as per a sectional cast radiator), thus the fin tube, but shooting warm air should also accomplish this.

As soon as possible, I'll test the fin tube that I already have from a tear out. Should it fail to provide what we want, swapping to embedded tube or a kick space heater is rather easy on this loop.

The old radiation element has very heavy wall copper tube with 3-1/4" square black aluminum fins. The tubing surprised me, as I'm used to seeing paper-thin tubing on fin tube sections. This copper tubing's wall measured thicker than Type L at 2.35 mm, versus 1.27 mm for the Type L that I have.The O. D. almost matches current tubing at 28.7 mm. The solder joints at fittings are odd, too. They are a dull, bluish grey & the fittings are cast, red brass. I wonder what that old radiation's tubing type is.

PGB1 · November 2021

Bob, I forgot to thank you for posting the screen shots about the anchoring cement. I'd never seen the product before. It looks handy & I can think of a bunch of uses for it around here, such as this project if I go the embedded tubing route.

For anchoring, I always liked to use the old standard "install it forever" method of sulfur. When you go back years later & pry the item up, the concrete breaks before the sulfur lets go. It seals cracks & patches in underground, concrete wiring ducts, too. The stuff was great for pad-mount transformer bolts & things that wind tried to lift. There's no waiting before bolting tight. Just dump-and-go.

About 40+ years ago an old guy taught me this trick. I was on a remodel in a restaurant and he was setting 1/2-13, vertical threaded rods to hold counter stool posts. About a minute after he poured the sulfur, he challenged me to tear the rod out. I couldn't budge it with a pickle fork pry bar under a nut.

I'd have to guess if I used sulfur to embed the tubing under my bath tub, I'd not only get shot by SWMBO because of the smell, but I'd go broke buying enough sulfur.

Thanks Again for the information about the anchoring cement. Soon it & I will become well acquainted.

Paul

PGB1 · December 2021

Finally, I got the chance to install the fin tube in the apron & seal things up.
The result is better than I expected.

The Set Up:
There are 46 inches of 1" (sort of 1") piping with 39" of fins. The fins are 3-1/8" black aluminum of a heavy gauge metal. The tubing is an odd size that is slightly over 1". I annealed the ends of the tubing by heating & quenching (copper anneals opposite of steel) and slowly shrunk it with a flare block so that modern 1" fitting would work. I think the original fittings were factory applied. They're too large to fit a modern 1" fitting.

The ends of the fin tube turn down 90 degrees with 1" elbows, then reduce to 1/2" immediately.
The only way to accomplish the task had the return end of the fin tube at the dry end of the bathtub. Any vent would become inaccessible, so I piped for force purge.
The floor under the tub is covered with radiant barrier and it's also draped down the non-apron side & end walls.

I expected that the apron wold be too hot, so while piping, I made it so I can cut in piping & valves to allow reversal of flow to have the fin tube supplied with radiator return water, as Solid Fuel man suggested. (I'm doing a gut-and-replace for the room, so the radiator isn't installed yet.) When I set our heating system up, I installed balancing valves for each device in the house, as they are all supplied form monoflow tees. This will allow me to reduce water flow to the room if needed.

I tried having the fins touch the apron, as Bob mentioned, but is was quite noisy because the apron amplified the ticking-from-expansion sound. I moved the fin tube back from the apron 1/8". I think I'll try crumbling some soft aluminum radiant barrier to contact both the fins and the apron. It may stop the noise & will give me the conduction Bob mentioned.

In action, with 166-F water entering the tubing, when the room is 65-F, the hottest I've measured the apron was 92-F at the top of the apron. I'm glad to find that it was not uncomfortably hot. The apron and end walls get warm. The bottom is warm along about half the length, bisected longitudinally. The half of the tub floor opposite the apron stays cool. My delta-T between supply & return is a surprising 9-F. I expected much less.

These are not real-world tests partly because is is not yet cold outside. It's been in the 20's & low 30's outside when I tested, so the heating zone only runs for 10 or 15 minutes per cycle. I set up our heating system with a design temp of (-) 5-F (and missed). At about (-) 10 is when the boiler runs non-stop. I'll be very curious to test temperatures then and in the (+) 10 to (+) 15-F temperatures we mostly get in January.

I think the aluminum radiant barrier helps a lot. When I measure the temperature of the bottom of the sub floor from the basement, the part under the tub (with radiant barrier above) is basement temperature- even with the heat on. The sub-floor under the rest of the room (no radiant barrier) is the temperature of the room above.

Out of curiosity, I'll try what Bob suggested and have the fins touch the apron by crumbling up radiant barrier and filling the gap.

Thanks Again to everyone for helping me and educating me. I very much appreciate what I learned here about this project.

Paul

ChicagoCooperator · December 2021

Cool!

PGB1 · December 2021

"Cool!" Warm?

The heating zone ran for about an hour yesterday, so I got a better idea of the output:
The bottom about 3- 4" of the apron were just slightly above room temperature, as expected. That's about the middle of the fin tube. The temperature rose along the height of the apron until it reached 105-F at the top of the apron. Nice place to sit when coming in from the cold! (Mitten drier, too- although nothing beats a radiator for that.)

The bottom of the tub heated rather evenly all the way across. The temperature range was 94 to 95.6 -F. The 3 non-apron walls averaged 94-F.

After the zone call ended, the tub cooled off rather slowly. It took about an hour for the cast iron to reach room temperature. I expected it would cool off faster because the iron is thin, so there's not much mass and a very large surface to radiate.

One part that I'm really pleased with is the help the radiant barrier gives.
The temperature of the bottom of the subfloor (in the basement) was basement temperature under the tub and room temperature for the rest of the room. I plan to install radiant barrier under the subfloor for the rest of the room and, if I'm inspired, the other bathrooms & rest of house over the basement. (Part of our house is over a crawl space and the radiant barrier there works great.)

JohnNY · December 2021

This has probably been said somewhere else in this thread but in hydronics we heat with some combination of radiation, convection, and conduction. Your best hope in transferring heat to that tub is through conduction. Bare copper touching the tub or PEX heating the structolite below it would be my two methods to that end.

Fin Tube Element In Tub Apron Some Choices Questions

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