Hooking up old 1950's in-slab radiant floor heating system

vmuser123

Someone suggested I come here and ask about my old in-slab radiant floor heating system. I live in a neighborhood where roughly 200 homes were built back in the mid 50's with an electric heated, glycol based, hydronic, in-slab radiant heating system. Over the years some systems broke, other owners took advantage of incentives or programs, energy savings, etc. and today most homes are converted over to oil-fired hydronic baseboard.


A couple months ago I said "what the hell I'll give it a try" so I blew out the old pipes (with air) and pressurized them (with air). Things got busy and I forgot that that I had pressurized the system so it sat like that (about 45psi) for about 4 months. A couple days ago when I went to disconnect the air I was expecting to find that the system had lost pressure (due to a leak in the system) however it was still sitting at ~45psi.


What is the downside to hooking the in-slab radiant heat system back up? I would want to install a cutoff valve of some sort so that if it ever broke it wouldn't keep pumping glycol+water all over the house. Other than that, are there any negatives? If it were to break can it be repaired? (I assume that means chip up the slab around the broken area and then ???)


Thanks!

hot_rod

Operating cost! The old systems like that around here had a bit of 1/2 Celotex board under, maybe around them, if any. I suspect , a good portion of your heating $$ will be going directly into planet earth, a % into the living space.

Is it steel or copper pipe? Eventually all metals deteriorate. You are pushing 70 years on that system? I would not spend a lot of $$ on rehabbing that system. If you do have a plan B in mind.

Zman

Fire it up!

Others on here have first hand experience with these old systems. I bet someone has worked in you neighborhood.
The old systems did not have much insulation so are not as efficient as modern ones. If you have flower garden near the house, it may extend your growing season a bit. Most failures were do to corrosive interaction between the in in slab piping and the concrete. What type of piping?

For best results, I would suggest a condensing high efficiency boiler. You absolutely want to use outdoor reset which varies the supply water temp as outdoor temp rises. This makes the system more efficient and comfortable. High efficiency boilers do this automatically, if you are tying into an existing conventional boiler, you will need some controls and parts to make it work.

Post pictures and more details.

The find a contractor section on this site is a great resource to help find knowledgeable folks in you area.

vmuser123

Thanks for the comments. First, this will be 100% DIY. We have an older-ish 82.6% efficiency, condensing Burnham boiler (with oil fired/heated hot water). Beckett oil burner, Honeywell controller.


What type of piping: I have no clue because it's in the slab There are steel/iron (looks like black pipe, NPT thread) supply and return lines sticking out of the slab but everyone has told me I'm crazy --back then they used rolled/coiled copper. All I can see is black pipe sticking out of the slab. (is that good/bad??)


@ Hot Rod: What does "rehabbing the system" entail? I assumed I could just attach a manifold, fill it and go... Boiler has supply and return. Slab has supply and return... Hook boiler's supply to slab supply then hook slab return to boiler's return... What am I missing? Glycol part is easy --but need to add a fill port/spiggot. (a good friend is a heating and plumbing guy, I'll just ask him)


Do I need to worry that the pipes have been semi-dry for at least 20 years? I understand the BTU differences between a glycol and non-glycol system (slab house so slant fin runs across unconditioned attic then drops down to baseboards). The house is insulated extremely well (I did it). What am I missing (other than heating dirt under the slab)?


Thanks!

hot_rod

Yeah, welded steel was used around here. I did some with my dad back in the 1960:) If it is steel coming out of the slab, probably the whole system is steel.

In some cases manifold boxes are scattered throughout the building. I found one under a fireplace hearth on one job.

By rehab I would not invest in a new boiler and components unless you could utilize them if or when the tube in the slab leaks. Radiant walls, ceilings or panel radiators are other options if that time comes.

If you have all the pieces, hook it up and see how it performs.

Unless pipes run outside for some reason I would just use plain tap water, not expensive, messy glycol.

The steel pipe system I last worked on only rusted through where the tube touched the ground below and was in a puddle from a bathtub leak. The steel tube encased in the concrete looked brand new when we cut into the slab to re do the system.

We did a thin gyp overpour on that 1952 vintage job.

jumper

Did glycol in fifties have corrosion inhibiters? Did they pickle steel pipes then? Maybe everything is okay? And by now the steel is stable so you don't need glycol anymore?

Make lemonade if there's no insulation. Use summer solar energy to heat ground in the summer. Then there's a nice reservoir of heat for the winter and the slab will stay warm.

I wouldn't bother myself but hey who knows what you will learn. Electrically heated hydronic sounds nuts to me.

vmuser123

Our neighborhood (about 225 houses) was built in the early/mid 50's as Navy housing or civilian housing for people working at the local base. As part of the incentives to buy into the neighborhood, the Navy covered the cost of electric. We (neighborhood) also have 3-phase 208Y power. There are similar neighborhoods throughout the country but usually the other 208Y neighborhoods are due to the early AC compressors requiring 3ph. 208Y was a good solution because you still have 120V and many/most things can run on 208V as well.


There are only three 208Y neighborhoods in the country (that I know of) like ours with original electric heated, glycol based radiant floor heat. We are on the east coast, the northern-most neighborhood and we are one of two, attached 2-car garages... If you want to internet-stalk bad enough you can narrow it down to a 50/50 between our house and two blocks over


I don't really know anything about in-slab heat like this. I'm fine calculating zones, btu's, etc. and putting in hydronic slant fin. Just never messed with in-slab radiant. My gut says hook it up, what do we have to lose (except those damn baseboards we hate) but wondering if I'm missing anything else...


It is my understanding that our slab is roughly 8" thick. A 6" 'normal' or 'standard' slab, with an extra 2" for the heating coils. I have no idea how they are run/orientated. The only thing I do know is that several of our neighbors have all had their systems fail in the same place. Along the common wall between the kitchen and the bathroom, behind the refrigerator. Don't know anything more than that other than if my system holds air what am I missing??

Jamie Hall

Both steel and iron will last almost indefinitely if they are encased in concrete -- unless they get damp and there is an electrolyte in the water, such as salt (even small amounts). The problem in that common wall may relate to a little cracking there -- wouldn't be surprising -- plus moisture getting in either from the bathroom of the kitchen. Makes sense.

I'd just fire up the system and see if it works -- why not?

the_donut

We have a couple 3 phase apartment buildings. Always fun to explain that to a water heater tech.

Jamie Hall

Worse. It's not always safe to assume that your three phase is 208 -- sometimes it's 240...

Gordy

Fire it up! If it held 45psi for four months it’ll hold 12 psi.

once planet Earth warms to equilibrium the efficiency of the slab goes up.

It’s the edge losses of the slab that’s the killer. Snow melt if you have close proximity sidewalks to the slab is an added bonus, and a green house effect for bulb flowers like tulips could be in bloom in the winter.

Like @hot rod says you could have plenty of still radiant alternatives if things go south. It could last another 50 years. It’s already made it this long.

ratio

A cheap IR camera, even the ones that attach to a smartphone, can tell you a lot about the buried pipes once you've got some hot water in 'em.

jumper

vmuser123 said:

.

(except those damn baseboards we hate) but wondering if I'm missing anything else...
Don't know anything more than that other than if my system holds air what am I missing??

Well you know the radiant was abandoned. Probably for a reason. Don't blame you for damning baseboards. They can work better with multiple thermostats. Helps with that off/on hot/cold issue.

vmuser123

jumper said:

Well you know the radiant was abandoned. Probably for a reason. Don't blame you for damning baseboards. They can work better with multiple thermostats. Helps with that off/on hot/cold issue.

That's really what I'm curious about too. Usually if it's too good to be true... it's too good to be true... This seems too good to be true. And also if I were to spring a leak, how the heck do I fix it? Just chip away the slab, clean it up and then.... I can weld (if this is welded steel pipe) but would need a pretty big hole in the slab to weld in a patch section.

vmuser123

Gordy said:

It’s the edge losses of the slab that’s the killer.

Can I do anything to prevent this? Dig down around the edges of the slab and insulate? Or am I SOL, just deal...

vmuser123

Gordy said:

Fire it up! If it held 45psi for four months it’ll hold 12 psi.

Couldn't edit my other post. Really only 12psi? Should I back off the 45psi? I don't want to break anything. What pressure should I be running hot water at through the in-slab system? Thanks!

Jamie Hall

On the pressure -- all you need is about 0.5 psi for every foot that the highest point in the system is above the pressure gauge, plus 5 psi. 12 psi should be ample.

And you can insulate around the edges of the slab -- it will help efficiency some -- but... not sure I'd bother...

Gordy

A hydronic system only needs enough pressure to get water to the highest elevation in the system plus a little extra. In your case 12 psi is plenty.

Gordy

Edge losses can be decreased by adding insulation to the perimeter of the slab. Involves some labor. It will deffinetly help. I’d wait to see if the operating costs are unpleasant. I doubt they will be.

Gordy

I will say see what your tubing lay out tells. Some times they held the tube in from the edge 30” to curb the edge losses. . It depends.

hot_rod

How many square feet is the home?

If in fact the slab is 8" you have about 2.5 yards of concrete for every 10 x 10 = 100 square feet of slab. Or about 10,000 lbs of slab per each 100 square feet.

You have a very high mass system. Without under slab insulation the earth also becomes a part of that flywheel.

Getting that flywheel up to speed, so to speak, in the fall will certainly make the meter spin

It will be a hard slab to regulate as load conditions change since it will not heat or cool quickly. High mass works well in large garages and commercial applications, not so much in residential applications.

Certainly living on concrete favors floor radiant, it is sometimes hard to be comfortable when the floor is at ambient temperature.

As others have indicates, give it a try if it holds pressure and all the pieces are in place.

It be interesting to see how many neighbors systems are still operating, and get their opinions. If the majority have been converted, there is probably a good reason.

Zman

When you attach to your boiler you are going to want to set this up with a mixing valve that will do outdoor reset and boiler condensation protection. The outdoor reset will adjust the supply water temps as the outdoor temps change. This will add efficiency and make the home more comfortable. The boiler condensation protection will assure that the cold return temps will not damage your boiler. The Taco I-series work well.
I mag/dirt separator is always a good idea with old pipes. Caleffi makes a nice one.

If you post some pictures of your boiler piping it would help.

JUGHNE

One thing to consider; if you fill the system and pressurize it to the 12Psi and it holds you may want to shut the water feeder to the system (if you have one) so if you get a leak under the floor you are not washing the fill from under your slab.

The more important thing is to have a low water cut off on the boiler. So if you lose water the boiler will not dry fire and at best become toast or worse explode when water is added to a hot boiler.......this LWCO is usually mandatory if your radiation is connected below the level of the boiler. And is always a good idea in any event. IMO

vmuser123

hot rod said:

How many square feet is the home?

Taxed at 1052

hot rod said:

You have a very high mass system. Without under slab insulation the earth also becomes a part of that flywheel.

Getting that flywheel up to speed, so to speak, in the fall will certainly make the meter spin

Love the username and the auto analogies. I have a bunch of BMW's from the 70's, an old Scout-II, a LR Series IIa... and a bunch of bikes (and a few Vespas). I get the earth being part of the flywheel thing.

hot rod said:

It be interesting to see how many neighbors systems are still operating, and get their opinions. If the majority have been converted, there is probably a good reason.

Neighbor behind us has his slider cracked when it's 15 degrees out... Pretty sure it's not a lightweight flywheel situation. Actually his only comment to me about his heat is that it's easy to keep even/maintain but it takes a LONG time to get up to temp and feels like longer to purge heat.


But isn't that a good thing? Especially with smart stuff, outside temp (and humidity) sensors, slab + inside temp sensors, etc.... Just build a contraption that tells the boiler to keep the slab at a set temp 24/7/365 (plus variables)?

hot rod said:

there is probably a good reason.

They all break under the common wall between the kitchen and the bathroom, behind the refrigerator. All of them. We have a pretty big neighborhood/block party every year. Every single person I've spoken to who has had (or knows of) a failure has had it in the same spot. (I'd actually be interested in doing some proactive work to prevent this)

Zman said:

When you attach to your boiler you are going to want to set this up with a mixing valve

Similar/same as drinking water mixing valve? Clearly I need to do a little more research about this whole process...

Zman said:

boiler condensation protection.

Is delta t that big of a deal with slab radiant? This may show how little I know... Can you just speed up the rate of flow through the system? Or put on a secondary pump to cycle the system 24/7 even when heat is off, creating a min temp band in the slab so delta t isn't ever crazy? (prior owners of our home ruined a chimney due to crazy delta t numbers)

Zman said:

The boiler condensation protection will assure that the cold return temps will not damage your boiler. The Taco I-series work well.
I mag/dirt separator is always a good idea with old pipes. Caleffi makes a nice one.

If you post some pictures of your boiler piping it would help.

Can't post pictures at the moment. I have so much wood/lumber stacked up against the boiler I couldn't get to it if it caught fire. I did repipe the system about 2 years ago. What do you want to know? Burnham boiler, Beckett burner, Honeywell control, Taco air scoop, don't know brand... green pressure bladder the size of a grill propane tank, two taco zone valves, house is only a one zone single loop. 82-feet of fins... yeah I know... 007 taco circ pump, 1" copper between boiler and first drop down/up to baseboards.


I did that... it's a slab home so everything runs across attic then drops down to rooms. Hot Rod might appreciate this... I have a 5-foot metal brake (for car stuff). When I ran the copper across the attic floor (above house ceiling) I bent aluminum flashing on the brake into a V. Then I ran the copper pipe as close to the drywall (just over the strapping) as I could, then put the V flashing over the pipe like this: /o\ and then I foamed around the aluminum flashing. This gets the copper exposure to the conditioned house drywall ceiling, as well as insulates it a bunch and also keeps the pipe open so it doesn't creak or bind/bang as it expands/contracts. It's a pretty slick setup and since I wrapped the 3/4 copper pipes with black tube foam then encased that stud cavity with expanding foam (on their way up/down from the attic) I cheated and did a single zone loop at 82 feet. Then I tweaked each room by pulling/cutting fins off the rooms at front of the loop and leaving more fins at the back end of loop. System is awesome/balanced... Slab is still cold as F.

Zman said:

I mag/dirt separator is always a good idea with old pipes. Caleffi makes a nice one.

What is that? (I'll Google it too)


Our boiler is in good shape. Combustion numbers are good, I drain the crap and flush the (oil) tank and line every 3 years, boiler firebox is in great shape. I do a full clean and check the numbers once a year (fall) and in spring I clean the ignition head (and check the numbers). I have three spare burners sitting in boxes in the garage. Knocks on wood... Haven't needed them yet... Blower motor will probably go eventually, everything else is good.

Zman said:

If you post some pictures of your boiler piping it would help.

Sorry, like I said, it's a bit inaccessible right now. happy to answer questions though. If not I'll try to get some pics tomorrow.

JUGHNE said:

One thing to consider; if you fill the system and pressurize it to the 12Psi and it holds you may want to shut the water feeder to the system (if you have one) so if you get a leak under the floor you are not washing the fill from under your slab.

The more important thing is to have a low water cut off on the boiler. So if you lose water the boiler will not dry fire and at best become toast or worse explode when water is added to a hot boiler.......this LWCO is usually mandatory if your radiation is connected below the level of the boiler. And is always a good idea in any event. IMO

Thanks. Both of those are really good suggestions. I don't have a LWCO currently. I'll go grab one or look at them and figure it out. I was always worried about a pipe bursting and spitting water up into the house. Never thought that I could spring a leak and have a giant hole under the house slab. Thanks. I'll monitor both pressure in the system and LWCO at the boiler.

hot_rod

It's always fun to have a fellow moorhead and tinker on board.

The main issue with high mass is controllability, especially in areas that see frequent and maybe wide temperature swings.

If you have sub freezing at night and the next day 40- 50 the slab may be too torqued up and over heat the space. Any south facing glass, passive gain adds to that over-shoot potential.

Weather responsive controls that monitor both indoor and outdoor conditions can smooth that out but not completely eliminate swings and solar gain influence.

As far as repipe and or upgrade. It wasn't uncommon to pipe directly to cast iron boilers, run 160F to those older designs. Some had manual called a (dumb) device bypass adjuster valves to temper outgoing supply to help prevent overshoot. No boiler return protection or temperature mixing and response on other basic systems. Depends on how say the installer was at the time, I suppose.

I've worked on old systems like that where the linoleum tiles would squirm around as the floor got excessively hot.

Like restoring an old vehicle, how much time and money to spend to bring it up to modern day design. Or live within the operating conditions it presents in current configuration.

A wood overlay system would be another option to still enjoy radiant but have more of a dragster instead of a Freightliner

Try it as is for a season, learn more about upgrades or adding insulation. Even with cheap fuel prices, no need to heat the great outdoors.