Hello and thank you for a great site and forum.  I am doing a small bathroom (25 sq. ft.) remodel and would like to remove the baseboard on one wall to accommodate a sink and vanity that are flush to the wall.  I am gutting the entire floor and replacing with marble tile.  I understand that usually radiant floor heat is run at a much lower temperature but would there be any harm in running a small radiant loop under the marble tile at current baseboard temperatures ( I am on a tight budget [I got the marble for $1 a sqft.!] and don't want to get into adding zones and mixing valves, etc. )?  If pex's temperature limitations were the only issue I could use copper being that it is such a small area?  Also with pex or copper the loop material would be a smaller diameter than the current baseboard pipe, would that be an issue?  Thank you for any input and assistance, I was finding nothing on the internet regarding such a renovation.
-Thomas

PEX can handle 180F without a problem, but that's not to say you can run 180F water in a slab or plated to a wooden subfloor without issues. If the system needs to run 200F water, you're a bit beyond the ratings for the tubing (but it'll still take it- Wirsbo has a test sample that has been running more than 20 years at temps slightly over 200F.) The PEX iisn't likely to be your limitation.

If the marble is just being placed on a standard wooden subfloor with thinset, running the radiant as suspended tube from the floor joists and inch or so below the subfloor will likely get you there. You may be able to run the entire heating system at a much lower temp and still deliver the heat too. If the system delivers (or can deliver) design day heat at temps under 140F you can get the BTUs into the floor with aluminum plates under the subfloor and get quicker response.

But balancing the bathroom with others on the same zone will require a bit more analysis than is reasonably done on a web forum. If you know what temp and size the previous baseboard was running, you could then work backwards to determine which radiant approach might deliver the same order of magnitude of heat at the temp & duty cycle that the zone normally runs. The temperature of the floor would fluctuate more than would be ideal, and may be uncomfortably warm at times, but you can probably get it to deliver the heat. The more thermal mass you can incorporate the slower the response time will be, but the more constant the floor temp. Longer cycles at lower water temps also helps.

With any variation on radiant, be sure to put at least R13 of fiber or foam insulation between the tubing and the room below (R19 min, if it's over an unconditioned basement or crawlspace that ever drops below 60F.) If it's higher temp & suspended tube, radiant barrier 1" below the tubing (in conjunction with, not instead of, the fiber insulation below that) is also worthwhile.

My baseboard runs off 1" copper, will there be a problem with flow or pressure reducing the diameter to 1/2" for the pex loop? Thanks for the prompt reply earlier, I am trying to get this done before mid-April (chiild on the way)!

The head presented to the pump is a function of pipe cross section, length & flow. If it's set up as a single loop of 1/2" PEX in series with the 1", with 8" spacing under the floor, for 25 square feet it'll add ~40-50' of 1/2 PEX into the chain, which may be a significant hit to the flow (don't know for sure, without all of the other details.) If you made that a pair of 20-25' loops of PEX (exactly matched lengths for balance flow), the additional head is cut by more than half, and unless it's a fairly high-flow system will probably be fine.

But this is all standard hydronic design stuff you should be doing on paper ahead of time rather than hacking. When you do all the math first stuff tends to work. Fixing it later would likely be a real PITA.

My wife's childhood home had a similar setup. Wifey says that bathroom floor was so hot they couldn't stand on the tile in bare feet. The room was warm but the feet were not happy.

If the radiation on that entire zone is capable of delivering the heat at low temps and you do the math right, the peak floor temps might be a bit uncomfortable on design-day, and it would cycle a lot on other days, but won't stay in foot-burner mode.  Back in the day baseboard was all 180F, and radiant was embedded in concrete, in which case a full-flow of 180F into a thin slab could be quite uncomfortable indeed.  Unless the radiating area was a small fraction of the room it was probably overheating the room as well, not just frying feet.


He really needs to design it, not just hack it to get it right.  25 square feet of tile radiator can put out quite a bit of heat, even at modest floor temps. Could be he only needs a single loop of 1" PEX per bay, set up as suspended tube to get the requisite BTUs into the floor.  But without the particulars on the rest of the radiation and it's temp requirements it's all kinda fuzzy...

Thanks for the reply, but... I was asking about problems with materials and the validity of the concept, considering I felt it odd that the topic had not been covered before in this forum or on the internet (that I could find anyway). If, through this venue and others, I determined it was the best solution for the given situation I would apply heating load and other design calculations. Obviously in a 25 square foot bathroom not all of the floor will be heated (none under the bathtub, toilet, vanity, linen closet). Before you go calling someone a hack perhaps you should inquire a little deeper. Comments like that really undermine the usefulness of a great site like this and my desire to post what I believe are useful and relevant questions.
-Thomas

I think you asked a good question and got some good answers. I wouldn't worry about putting heat under everything - just limiting the output (possibly to about 25%) is the issue.

Good grief.

I think there is no human trait more distasteful than ungratefulness.

Dana is right and always gracious-or is it gracious and always right. Either way the problem with DIY design is not knowing the questions and being insulted by intelligent answers.

Proper HVAC design-and more especially hydronic radiant floor design-starts with a heat load analysis (no DIY free programs will due). Once you have the load you can consider your options. A well-designed program will limit areas with high floor traffic to 85°F for comfort and about 30 BTUH/sq.ft. output. If your load is higher you have to supplement the floor with another emitter. Areas without foot traffic can be driven to higher temperatures but floor coverings have to be more carefully considered as response time and the integrity of the floor come into play.

As for water temperature, tube size and length; if you have to ask, a professional should be involved. Water temperature, flow, tube size and controls are all part of a system, which is, in every case, custom to the application. Questions have to be asked and answers scrutinized by an experienced designer if success in the goal. If you can't afford a professional there are many books on the market that address radiant floor heating.

DIY design...always the first mistake.

I think 180 would be a bit warm for the floor,
how about this,
2 T's 1x1x1/2 T#1 feed a 1/2# TACO 3 way mix valve next 005 TACO pump next pex pipe return to T#2
Reduced temperature, Switch pump with line volt stat or parallel with heat system pump.
Dan

I meant no disrespect towards Dana and have learned much from his many posts as well as Badger and Dan's. I am extremely grateful for all of your help and the numerous posts that have guided others and myself, as I have followed many of your threads. I just wanted to make sure I was on the right track before wasting anyone's time with unnecessary specifics. In hind sight I guess you can never be over-specific when asking for advice. What programs would you recommend for heat load analysis?

To be clear, I wasn't calling you a hack, only pointing out that design-by web-forum without the fundamental data to work with is in the realm of guesstimating/hacking/hoping that it's "about right", and that this project is more complicated than separately controlled micro-zone.

From the nature of the questions (and the fact that you've specified neither the water temp &  flow, nor the heat load of the room) gives me the sense that the necessary math on the hydronic design isn't being done.  By referring to designing rather than hacking, I'm only strongly encouraging you (or anyone) to learn all you can about hydronic & radiant design before installing it.  Were it a micro-zone (which it isn't) with it's own controls, the design would be a lot more forgiving (you can often over-design the radiation capacity by several fold and let the controls dial it back without discomfort.) But if I understand this correctly, you need to balance the radiation of the bathroom with the radiation on the rest of the zone, which makes it a different problem (to be solved with a sharper pencil.)

Yes, it can be done, yes PEX can take the heat, but that's not even the beginning.

Hell, I do hacks all the time (and usually get away with it), but this is one I'd want to be very careful about.  Does that make me a hack? (Yeah, well, kinda- just ask Morgan or Rob! )

Now, in the spirit of HACKING this together at indeterminate water temp without mixing valves...

(and mind you, this isn't a recommendation- it's a hack...)

...I'd plumb it with a section of 1" copper in parallel with a 2-loop manifold/balanced-length pair of 1/2" PEX  under the floor, with separate ball valves on both the parallel stub and the radiant side to be able to control the flow to to the radiant.  As a design methodology it sucks, but you'd at least have a chance at tweaking it after the fact, and wouldn't likely (TBD) be adding so much head to the zone pump that the system stalls/can't deliver enough flow to deliver design-day heat to the zone.

You'd still need to ball park how many BTUs/square foot it needs to deliver on design day, determine whether that ends up being foot-frying misery for floor temps at that load (in which case you'd need to provide for supplemental radiation, like a heated towel rack or section of cast-iron baseboard or something), and have a plan for getting at LEAST that much heat out of the tubing & into the floor at whatever temperature you need to run the water to the baseboards on the rest of the heating zone (if it can't do it suspended-tube, figure out how to plate it- possibly above the subloor WarmBoard style- but that could be a hazard in high-temp operation.)  That's about as crude a hack as  might actually work...

If you're running 180F water the flow though the radiant will be pretty low, with hot-spots on the floor and a big delta-T.   If the entire heating system can be run at 140F or lower it'll be higher flow & a more even temp across the radiant.  (Not to mention the system as a whole saves ~3% in fuel for every 10F you can drop the temp, which is why outdoor reset control can often save high-single-digit or even low double-digit percentages in fuel, even systems with high-temp baseboards & cast-iron boilers. But I digress...)  

You don't mention what boiler you have, or whether it's either inherently (or plumbed to be) low-temp tolerant (w/boiler bypass or something), but most real-world systems are oversized/overdesigned, and can be run cooler & more efficiently than the day they went in. (Pros are often in the habit of hacking it too, eh?)

Awesome, thank you. Remodeling the bathroom has called into play a much larger question which I will present in a new post in 'general-residential' here:

http://www.greenbuildingtalk.com/Forums/tabid/53/forumid/14/postid/74722/view/topic/Default.aspx

I will follow up on the bathroom heat later in this post, for those following this thread.