Adding radiant floor to old gravity feed system.

heritageproperties

I have a big old building in St. Louis with residential apartments above a warehouse/office space. There is a big 350k btu gas retrofit coal boiler heating what was a gravity feed system, but now has a couple pumps on the two return pipes (I know, Dan, pump away). There are two supply runs at 2" and 2.5" that feed ~50 cast iron rads around the building (mostly on the second floor). While there are two supply pipes and two return pipes, there aren't really two separate loops or anything, they all just connect back along the returns.

So, here's what I want to do. I have an atrium and a bathroom that are under heavy renovation right now, and I'd love to put in radiant floors instead of the radiators that are there right now. What I am concerned about is that if I put in a small circulator to run the floors, that will throw off the pressure in the system, and create short circuits, starving other rads of hot water.

Any thoughts from you clever folks?

Paul Pollets

If there were pumps, I'd remove the radiators from the RFH zone and install an Oventrop UniBox. Unfortunately, It won't work without a pump on the system. I don't know of any radiant loop system that would.

Zman

I'm certain there is a way to tie it in by adding a mixing valve and a circulator if you have access at the boiler room. You may be able to tie it in other places depending on how it is piped.
Do you have a drawing or pictures?

heritageproperties

I own the building. There are ~40 rads. I can take a picture of the schematic I made of the system, but it's big and hairy. I can put a valve and piping anywhere I need, there is unfinished warehouse space below the living area.

heritageproperties

Would the mixing valve control short circuit on the RFH pumped "zone"?

Ironman

What's the btu load of the proposed radiant floors? In all likelihood, you will probably create micro zones that will short cycle the boiler to death.

If your boiler's that old, you really need to look at something modern. You're probably wasting up to 50% of your heat up the chimney.

Zman

I think we would have to see a drawing. It should be possible to pipe it so it will work

hot_rod

You might want to use a small gas or electric boiler dedicated to the radiant, if it is just a small square footage. or even electric cable or mat system.

To tie into that current system would require primary secondary piping and would present a small m micro load on that large boiler.

heritageproperties

The floor wouldn't fire the boiler just itself...

Ironman

heritageproperties said:

The floor wouldn't fire the boiler just itself...

Then you would have very unsatisfactory performance from the floor - particularly if it's a slab or over-pour.

Radiant floors run at much cooler water temp and run much longer cycles. If the floor can't call the boiler when it needs heat, then how can it heat.

Hot Rod has given you some real solid advice about using another appliance for the floor.

You really need to start with a heat loss calculation to see if a radiant floor can produce enough heat to overcome the heat loss of the areas you wanna do. The heat loss calc is the foundation for EVERYTHING in designing a radiant floor. Without that, and a proper design, you're shooting at the wind.

There's a whole lot more to radiant than just laying tubing in the floor.

heritageproperties

I guess I'm not trying to do the floor "properly". There are 7500 sqft running on a single thermostat on a different floor. The air temp isn't so much the issue, I just want the floor and tub/shower to be hot by conduction as much as radiation. I really just want to know if there's a way to balance the smaller diameter / longer run flooring heat with the big mess of cast iron rads on piping that varies between 2.5" and 3/4".

Gordy

My opinion if this is bath floor warming. Do electric mats. The good ones have thermostats with programmable schedules. Keep the standing iron for space heating. The floor gets warmed only on the scheduling. Separate system.

Zman

Maybe a bit more info is needed.
Does the main zone call for heat most of the time?
How gig is the bath/atrium area you are trying to do radiant in?

heritageproperties

I am attaching a schematic of the hydronic system in our building, which I have marked up with some brief notes. The issues for all three zones is, or will be, essentially the same. Small diameter pipes connected to baseboard heat or, potentially, under-floor radiant. I do get that electric mats are going to be easier, but we already have the issue in a couple other spots, and it would be great to come up with a fix to circulate those zones without throwing pressures askew everywhere else.

Thanks again folks.

Zman

You have super low flow rates. I can see how the balance was thrown off.
I assume you do not have a path back to the boiler room.
What is the square footage of the new radiant section?

heritageproperties

I wouldn't take those nominal rates as gospel, but they are likely pretty low. I actually just guessed at pump sizes, so it might be higher. The bathroom would be ~100sqft, atrium, which is currently the single baseboard heater in the middle has ~250 sqft.

heritageproperties

What do you mean I dont have a path back to the boiler room? I could run a new return line and hook it back in just before the pumps. The schematic is slightly incorrect in that there are 2 return ports in the boiler, but Taco doesn't believe such a thing exists anymore.

Zman

Some good points were made about short cycling the boiler and the infloor having different reaction times.
I am guessing the boiler is hot most of the time. If you had a path to take pex tubing back to the boiler room, you could tie this in as a separate zone and really simplify this thing.
From a system balance point of view, I would be comfortable tying a small setup like this into your existing setup a circulator, mixing valve and closely spaced tees.

heritageproperties

Again, I don't think I want to enable any of these 'zones' to call for heat individually. I have a single Tstat now, and it + boiler keep the building warm for an affordable amount.

My main aim is to keep the floor and tub in the bathroom vaguely warm when the boiler is firing/hot. Right now, the baseboard heaters are barely warm b/c they don't have sufficient flow. I'm really just looking for a way to keep their flow balanced with the flows to the big old iron rads w/o creating a short circuit, and do the same for any floor radiant zones.

Can you go into a bit more detail on tying in said small setup? Do you mean to add a circulator in the boiler room, or tie in close tees near the existing pumps?

Its a bit tricky to pull fresh, hot water out early on, but there is that single rad that cuts off the 2.5" line. I could deprecate that radiator and use the 3/4 connection to attach that line for those zones. How does the mixing valve come into play?

Zman

Primary secondary is a way to allow 2 loops with different flow characteristics to live in harmony. In the loop you are having trouble with, they probably tried to use a circ to boost the flow and created some weird sub loops by accident.

Radiant loops usually want to run at lower temps than traditional radiators, mixing valves lower the water temps on the radiant side.
Attached is a common configuration.

heritageproperties

I did remove a couple pumps, and restored the original diameters of the gravity piping. System efficiency was notably better, heat more even. I guess I've got some sort of mashup between 2 pipe gravity originally, and what now resembles primary-secondary in a couple spots?

TeggyBrandon

Radiant floor heating consists on delivering heat through the existing floor surface, as opposed to using air ducts or baseboard heaters. The upfront cost of radiant floor heating is generally higher than that of other systems, since it is necessary to install hot water piping or electrical wiring under the floor. However, but this is compensated by improved comfort and energy efficiency.

According to the US Department of Energy, space heating is the highest energy expense in American households, and represents around 45% of energy bills on average. Therefore, any improvements to heating performance and efficiency can yield an attractive return on investment.

leonz

heritageproperties said:

I am attaching a schematic of the hydronic system in our building, which I have marked up with some brief notes. The issues for all three zones is, or will be, essentially the same. Small diameter pipes connected to baseboard heat or, potentially, under-floor radiant. I do get that electric mats are going to be easier, but we already have the issue in a couple other spots, and it would be great to come up with a fix to circulate those zones without throwing pressures askew everywhere else.

Thanks again folks.

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Edited today:

Are you totally against returning to a gravity hot water system??

If you have a big enough steel expansion tank vented to the basement drain you will have plenty of water weight to move the hot water heat everywhere and use gravity fed panel radiators in the much smaller spaces like bathrooms to avoid tearing up the floor.
You will not need floor heat of any kind as everything from the floor up will become warmer and the heat even all over the building.

I think if you find someone locally that works on steam they can help you solve your issues as you have stated the heat delivered has really increased by reinstalling the same size pipe and removing the circulators and they can solve the rest of it for you quickly as I am sure a small panel radiator can be used somehow in those smaller spaces with your gravity heat.


My thoughts on keeping it simple if at all possible

heritageproperties

Honestly, I'm in favor of returning to the original gravity, but mildly concerned about response time to calls for heat. The strategy for the building getting too hot on this boiler was opening windows...

My issue with the couple zones is that they were never piped for gravity. Sadly, those zones are reasonably important to get heat. They do OK-ish as is, but could be better. The floor in the bathroom is already gone, and I wish the heat came out between the outside wall and the shower-to-be. Perhaps there is a better solution than underfloor?

leonz

I am going to assume the replacement boiler is in the center of the basement as that would be the logical place for it as the dead men knew what they were doing to paraphrase Dan Holohan.

Their removing the steel expansion tank on the upper floor where ever it was was huge mistake.


If you have TRV's on the panel radiators in the bathroom etc. that would work well as only so much water will pass through them and you can use hot water towel warmers too.

Having a storage tank or two in the basement for more return water would let you have much more thermal mass and also require much less fuel to heat the total water volume thereby using a lower temperature water.

Each of the radiators should have valves on them,and that controls the flow of hot water into each radiator and the the exit speed back to the drain line as only so much water will go into each boiler.

I cannot picture a good plumber installing a gravity hot water system
without valves on the radiators.

I hope the radiator valves still work as it sounds as if they left them all the way open and that of course made the building hotter.

I am a novice at this but you have a very simple system that takes advantage of basic physics which the new plumbers screwed up and now you need to do some work to fix it right.

I am not sure how big a steel expansion tank to replace the one they ripped out but you need a new one, type growl, hiss, spit here.

Adding a large insulated hot water storage tank to the basement return loops will provide much more thermal mass and reduce the fuel used as less fuel will be needed to heat the total volume of water.

If those radiator valves are stuck open that explains a lot.

You need someone that has steam experience to help you as I said before as the last plumbers that worked on the system did you no favors by keeping it as it was intended meaning "SIMPLE" .

These are the thoughts of a novice that owns 4 of Dans books talking about the Deadmen" with much reverence on a cool Wednesday morning.

leonz

If you could take some pictures of the basement plumbing runs, the radiators and the valves on the radiators that would tell us a lot as I am sure that it will not be a monster of a job to fix to bring back the gravity system..

heritageproperties

All the rads have TRVs.

The old expansion tank is intact, but does seem to get water logged over a season or two. I'm trimming it with some B&G airtrol kit for easier recharging and better air management.

All the original piping is intact, or I have restored it with PEX that matches the original diameters. There are 2 circulators that have been added to the returns to the boiler. I'm reasonably sure that the pumps were added in the last 20 years to account for magnetite accumulation in the pipes over the last 90+ years. I wish plumbing them into the supply pipes looked easier, but 2.5" iron pipe is nothing I've been excited to mess with.

The parts of the system that aren't part of the original plumbing are the zones I'm concerned with here. The challenge is finding a way to feed those zones with a pump, and how to attach said pump to the system without throwing the rest of the system out of whack. I am pondering Zman's suggestion of a mixing valve on some sort of sub-loop with a pump triggered by tstats in those areas. If it calls for heat when the boiler isn't firing, tough stuff, it just has to use residual heat in the loop.

leonz

Once you get that airtrol valve in there you will be in fat city and the system will perform much much better and you wont need the circulators. They obviously did not listen to the deadmen when they put the circulators in there.
Your system will not go out of wack with a very small circulator and mixing valve to feed those zones after you get the steel expansion tank fixed as it will have the proper air charge in it by then.
The returns could be tied into a small header pipe to the sump line of the boiler.

I think once you get the airtrol valve in the steel tank and bleed it properly to obtain the proper air water volume you can eliminate the circulators as they are part of the problem as all they did was move air bubbles around and they ended up being purged and flooding the steel expansion tank making it water logged and no longer effective causing the no heat or low heat conditions.


(Edited today)

PLEASE TAKE NOTE:

The bladder expansion tank and any automatic air vents have to be removed so the system works properly after you finish the repair of the existing steel expansion tank.

If at all possible please purchase a true triple gauge Marsh Instruments
(marshalltown triple gauge-I really like my Marshalltown 4 inch triple gauge) and install it in the steam chest so you can properly fill the system with water as the triple gauge will let you fill the system with water while you are venting the radiators on each floor starting with the basement and working upward by making sure each radiator is vented open before you start filling the system and closing the vent on each radiator as they become filled with water and then travel to the upper floor until they are all filled.

After the system is filled the pressure indicated in the cold system is also where the manually adjusted altitude gauge is secured with the screw that locks it in place.

jumper

Ditto to what leonz says. Pumps facilitate air infiltration & water exfiltration. The radiant floors may suffice without a circulator. A proper fitting on return may induce enough flow. If it's practical to locate expansion tank at top of system you can eliminate air there.