Adding radiant floor loops to hydronic boiler system

Mike T., Swampeast MO

Sorry, but I can't give a preference. Much will depend on the existing system as well as the heat loss in the new spaces. Do though be aware that firing a boiler that's large enough to heat the entire house in the coldest weather just to satisfy a single bathroom is a prescription for poor efficiency and possible damage to a simple cast iron boiler unless measures are taken to protect it from condensation damage.

Please forgive me for repeating, but all of these issues should have been addressed <I>before</I> the project began.

Were heat loss calculations made for the sun room and baths? How was the tube size and spacing determined for the sun room? If you did the first and used it to determine the second, then you know the required supply temperature, flow rate and design floor surface temperature... Then you have to compare to the rest of the system to see if the required temperature higher, lower, or about the same as what's generally available.

If these things haven't been considered than anyone trying to "hook it up" is at a loss unless they reverse engineer. Anything less is nothing but a stab in the dark and your chances of a cost-effective, efficient and comfortable system decline.

Tim_39

Adding radiant floor loops to hydronic boiler system

I am adding three zones of radiant underfloor heat to an existing cast iron boiler hydronic system and I need some basic help understanding exactly what items I need to add to my existing system. My new loops are all 1/2" pex to relatively small rooms (2 bathrooms and a sunroom). The pex is in a thin slab in the sun room. I'll probably use a sleeper-type application in the bathrooms. I hope to zone each of these rooms separately. I have already run thermostat wire back to the boiler with my pex. These loops will be my only heat in each zone.

I've done a heck of a lot of reading on-line and I can't seem to find a consistent approach to the control aspects of my project. Do I want a single mixing valve? 4 way? Do I have a separate pump for each loop, or one pump with zone controls? How do I size the pump? Am I better off with a mixing valve that can adjust itself to climate conditions? What kind of relays or control box is best here?

Or do I even want to fire my boiler for this? Should I just let the system run as a slave to my existing thermostat?

Finally, another, more specific question is this: I "killed" an old radiator loop during this project, leaving me with two capped 1 1/2" black iron nipples; these would be a VERY convenient tie-in point for my new manifold loop. But this tie-in point is about 15' from my boiler. Am I okay setting up my manifold loop that far from the boiler, or should I install Tees right next to the boiler?

I don't want to spend a fortune, but I want to do it right. I realize this is a pretty elementary and stupid question, but advice would help, even if it's just advice about where I ought to look. Thanks,

Tim

Wayco Wayne_2

Actually Tim

This is the most complex part of a radiant floor job. When I give a price for a radiant floor job the boiler repiping and controls are often the more expensive part of the job. Since radiant floors work on boiler water that is not as hot as your radiators you must step down the temperature from that of you radiator loop. That's easy enough to do, however.... when the cooler water comes back into your boiler system the temperature must be raised back up so it doesnt harm your cast iron boiler with condensation or even thermal shock. I've been called into jobs where brand new boilers have been ruined in a couple of years time because they have had no protection from the cool water return. Basically you want return water no cooler than 140 degrees F. Yo might want to get professional help for this part of the job. WW

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Tim_39

I hear you, but contractors are in strong disagreement

Wayne,

Thanks for the advice. Believe me, I'm consulting professionals, but part of the problem is the wide variety of answers I get. I can only conclude that there's not a strong industry consensus about how to tackle this kind of application.

For example, one contractor claims that I need a separate pump and mixing valve for each area where the water temperature needs to be adjusted. This contractor is specifying two Watts Isotherm Pump/mixing valves plus a stainless manifold for each zone. He is also strongly recommending a floor sensing thermostat--which runs almost 300 bucks, plus labor.

Another contractor has insisted in the strongest terms that there is no need for a pump and mixing valve on every zone. Yes, he says, it can be useful in systems where the installations of different zones are different (i.e.- a zone under the subfloor and a zone in concrete), but it’s not necessary in systems like mine. I don't think this contractor is specifying anything other than a mixing valve, pump, manifold, supply/return temperature sensors, and a relay for the thermostat...

Can you or someone out there tell me what I ought to be looking for as a bare minimum in this kind of application? What is the gold standard? What would you do?

Thanks in advance,

Tim

Mike T., Swampeast MO

Tim,

I fear you've put the cart before the horse.

Before a single inch of tube was laid--much less embedded--you should have know how you're going to control. That's why you're getting such divergent opinions.

"Sun rooms" are probably the most difficult to control of all radiantly heated spaces. The finest and most experienced people in the business spend LOTS of time pondering how to control them well and often wind up with "you need automatic shades".

From your description of your desires for the bathrooms you could well have a WILDLY different supply temperature requirement compared to the sun room. Then you ask if everything can just be tapped into connections that have for some reason been recently abandoned. No mention of the nature of the rest of the system or how it is controlled.

It sounds to me as if you're a well-intentioned DIYer who has built a clusterfu#k and expects a pro to produce a working system. This said from a hard-core DIYer who made sure he understood the horse (and learned its limitations) before adjusting the cart.

Couderay

and there ya have it well said

Tim_39

Note to self: horse first, cart second

Mike,

Yes, I'm a DIY-er and yeah I've got good intentions. For what it's worth, I've pulled and closed mechanical, electrical, and plumbing permits for many projects in the past with no problems. I've also learned a lot from contractors by shutting my mouth and not pretending to know what I don't know. So I'll be the first to admit I'm in the early stages of learning about radiant heat.

But I highly doubt I've screwed up my job. I have two excellent HVAC contractors--one put in my A/C; the other has maintained my boiler and repaired a couple of radiators. Both bid my job and both specified 1/2" pex installed in exactly the same way in my little sunroom. (The pex hasn't been pulled to the bathrooms.) The only problem is, they disagree on the controls.

I hear what you and Wayne are saying to me: let someone with a lot of experience make my boiler and control connections. Point taken.

But which proposal seems more authoritative in describing the proper controls for the job? If I read between the lines of your answer, Mike, I take it you'd be inclined to favor the contractor who's advocating a separate pump and mixing valve for each area, because the heating needs of the two areas are so different.

Unknown

-you need to know the water temps each zone will require. this will tell you if one temperature will do the job well, or not.

-Sunroom at least should have floor sensing to maintain a minimum temperature when the sun hits, to avoid a slow start up later, unless you are using indoor feedback sensors linked to a central control.

-you must protect the boiler from cold return temps.

-you should mix the radiant based on at least outdoor temperature

-sounds like small zones, I would run one manifold with loop actuators on it for the multiple zones. You could also use 3 small circulators. Doesn't really matter on a project this size.

Once you have done all these things, you may find it not too expensive to use a better controller that instead of doing just mixing and boiler protection to also add in boiler reset for the high temp system.

There are many ways to skin this cat. Many are good and a few are not. If the water temperatures work out, and you can vary with load, and you are protecting the boiler, then it's all good. Make sure your bids are doing these things and let the fellows rip.

rb_6

test question

...and the fluid flow velocity through the ½” pex for the bathroom is______?

You can always tell a competent designer if they have evaluated the velocities.

Paul Pollets

My choices

I'd be using an Oventrop UniBox for each of the small zones, using the old radiator stubouts for supply and return. Each UniBox can provide a max output of 10,000-11,000BTU's or appx. 1.2GPM The Unibox can be used with a distribution manifold for more than 1 loop, if required. This mixing device does not require any changes in "Near-Boiler" piping. The UniBox can be ordered with a limiter, to prevent scorching the floors when using a med. or high temp supply. Sorta like having 2 TRV's in the same box.

The sunroom, as mentioned requires carefull calcs as the heatloss may be more than the tubing can provide, even on 6" centers.

You will still need to address the boiler's return temps and protect from condensation.

You'll get varying answers to preferences for zoning. If you ask 5 hydronic techs, you'll probably get 5 different piping and control strategies to do the same project. The less componentry, the better, as long as the zone is balanced and controllable.


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Mike T., Swampeast MO

Yep! Velocity rules.

Velocity was PRIME in the minds of the dead men when they designed steam and gravity systems.

Now it's again PRIME with regards to condensing/modulating boilers. Such is the VERY reason why W/M had issues in a recent post regarding the rather small piping for one of their larger output units. (Even if the black iron piping was very nicely screwed.)

Tim_39

Thanks

to everyone who offered advice on this job. I appreciate it.

Brandon_4

Look at this website: www.hpscontrols.com
They might have something for you.

KAG

$300.00

I have slab sensors in all my floor, each room is it's own zone. I did this because I install radiant systems and wanted to be able to tell my customers how they worked. As far as the price of the units ($300.00). In the words of the great Homer Simpsom "mmmmmmmmmmm warm floors". It is worth the extra $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$.

[Deleted User]

Fluid velocities???

We don't need no stinkin' fluuid velocities. We've called proportional control in the form of non electric TRV's. Proportional over drive in CRUISE control.

Now THAT'S a sign of an EXPERIENCED succesful designer.

Now you know why the Europeans are doing it that way...

Homer Simpson rules!

That should be our national battle cry... "MMMMmmmm, warm floors!!"

If you don't mind, I could use that line in an article or two:-)

ME