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Thread: hydronic heat changes in bathroom reno

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    DIY Junior Member rainy island's Avatar
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    Default hydronic heat changes in bathroom reno

    I was hoping for some advice/insight to help me plan heating changes as part of my bathroom reno.

    The salient details are these....I apologize if I'm adding too much information:

    1976 bungalow in Victoria BC....moderate climate with relatively few days below freezing. House has one-year old Lochnivar WHN 110 boiler heating 160 linear feet of baseboards in three zones (designed with a primary loop and secondary loops) The boiler uses an outdoor reset and typically heats the house comfortably running at 20% and heating the system water to 115-118 degrees. The zone in which bathroom to be renovated operates has 76 feet of baseboards and the bathroom is question is 12X6 and is currently heated by 2 three-foot baseboards. I have good access to the bathroom from the crawlspace underneath, so piping changes are not a problem.

    I want to put in underfloor heating and replace one or both of the baseboards with a hydronic towel rack/heater. The question is given my system's operating perimeters, what combination makes the most sense?

    1) electric underfloor heat and replace one or both of the baseboards with a towel bar?

    2) replace the baseboards with pex/hydronic underfloor heating and a hydronic towel bar?

    I ask because I'm not sure how to calculate out heating requirements. My suspicion is that option 1 makes the most sense, but I would appreciate any guidance the boiler experts here could offer. I should add that Victoria is not exactly a hotbed of hydronic heating use....underfloor heating appears to becoming more popular in newer homes, by large it appears to be a town of electric baseboards or heat pumps.

    Thanks for any and all advice

    Eric

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    In the trades Dana's Avatar
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    How many square feet of floor do you have available for the radiant floor- I'm assuming it's not the full 12' x 6' (or is it?) Give yourself at least a foot of clearance around the toilet, since you shouldn't be heating it, and don't count anything that is under a floor-mounted cabinet, etc., just the raw open square footage.

    And, what is the floor stackup starting with the underside of the sub-floor. Materials & thicknesses please.

    Is the existing baseboard is all ~600BTU/foot @ 180F type fin-tube? If yes, the existing 6' of fin-tube puts out about 160-170BTU/ft, or ~1000 BTU at your water temps. You can then work backward from there using the output of the radiant floor & towel rack specs.

    With the material stackup you can estimate the R value of the floor, and from there the BTU/hr per square foot you can get out of the using extruded aluminum heat spreaders at your ~115F water temp using the nomograph on p.4.

    Say your actual free floor area works out to about 50 square feet, and it's a subfloor of 3/4" plywood with thinset & ceramic tile above it. The plywood has an R value just shy of R1, and the thinset & tile add up to another ~R 0.6, call it R1.5 for the stackup. If you follow the R1.5 line on the nomograph in that other document to the 110-115F range you'll see it will deliver about 15-17 BTU/hr per square foot, so the floor will be good for 50' x 16 BTU/hr 800 BTU/hr.

    If the room is reasonably balanced with the rest of the zone at 1000 BTU/hr that means that if the floor is delivering 800BTU/hr, you can find a towel rack that puts out ~200BTU/hr @ 110F and be rid of the baseboard. The specified BTU output of radiators (including hydronic towel bars) is at a nominal 180F average water temp- the output at your water temps will be about 30% of that number. So a towel bar that's rated at about 600 BTU/hr @ 180F AWT (give or take a hundred or so) would be about right. If you preferred a bigger towel warmer, you could cut back on the amount of radiant floor, but if it's currently balancing temperature wise with the zone you want to keep it the total floor + rack to run between 900-1100 BTU/hr @ 110F AWT.

    Other heat spreader types will have different output, and the responsiveness & consistency will vary. Extruded aluminum heat spreaders are pretty good- more expensive than the stamped sheet metal, but for the amount of floor area you're talking it won't break the bank. You're looking at probably 3 joist bays 12' long with a pair of runs per bay for a total of 3 x 12' x 2= 72' of extrusion, at about $2/foot.

    Running it as a single run of 75-80' of half-inch PEX adds quite a bit of length to the zone loop, and it's advisable to set it up as a pair of equal-length loops with 2 x manifolds. The 6' of 3/4" baseboard has miniscule pumping head compared to 80' of half-inch PEX.
    Last edited by Terry; 03-18-2014 at 10:45 AM.

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    DIY Junior Member rainy island's Avatar
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    Dana,

    Thanks very much for your detailed reply....its exactly what I need and I'll take some time to chew through the materials you provided and make the calculations! Incidentally, I have about 33 square feet of "heatable" floor (netting out the obstructions you mentioned), and the final stack up will be 5/8 ply, 1/8 Hardiboard, thinset and then 1/4 ceramic tile.

    Thanks again for the informed response!

    Eric

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    In the trades Dana's Avatar
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    At that stackup and 33' of space you're looking at about 500-550 BTU/hr out of the floor, so you'll need another 500-600 or so out of the towel rack, which would be something that puts about 1600-2000 BTU/hr @ 180F. (Runtal TW9-16 or TW9-20, or Buderus NB2432 are possiblilties.) If you err slightly to the high-side you'll have warmer towels and the bathroom might run slightly warmer if you normally keep the door closed when unoccupied, neither of which is a bad thing, and with towels hanging on it the output to the room will be reduced from it's free-air ratings. What you DON'T want do is put a rack rated at 3500BTU/hr @ 180F or higher or it could get downright sweaty in there.

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    DIY Junior Member rainy island's Avatar
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    Thanks Dana. I used your suggested parameters and came to the same conclusion....I would need equal btu/h output from both the floor and the towel rack if I want to replicate the current heating. After measuring the bays in the crawl, I believe I have room for about 54 linear feet of pex and transfer plates. From your perspective, would this make the most sense if installed as a pair of 27-foot loops rather than in a single run? Also, I'd be interested in your opinion as to whether you think the addition of this much pex (and its additional pumping head), combined with a towel rack (that creates more of a pumping head than the slant fin baseboards) would throw the zone significantly out of kilter or whether its impact is immaterial?

    Thanks again for your insight!

    Eric

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    In the trades Dana's Avatar
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    As long as you can get 2gpm or so it won't change much. The pumping head of a towel rack is very low, the pumping head of 55' of half-inch pex is something, but might not be a deal breaker. Splitting it into two 27' parallel loops is the rough equivalent of 27' of 3/4".

    Do the math on lengths of all of the existing plumbing, including the "equivalent length" of all the ells tees, etc (you can google up tables of such), and see where it comes out relative to your pump's output curve if you add 55-60' of PEX. A primer on how to do that lives here.

    Typical residential systems are designed for delta-Ts of 10-20F @ 2-4gpm. With outdoor reset the delta is smaller when the output temp is low, but higher when you're at peak temp, peak load. It doesn't hurt to measure the delta on the zone you're modifying first. (Hockey tape on copper pipe and an infrared thermometer is good enough measuring method), but if it's over 50F outside don't be surprised if it's under 10F. If you slow the flow significantly the delta will creep up, which can affect the room-to-room balance if it goes from a 10F delta to a 30F delta. Estimate the flow from your head calculations, as long as it doesn't drop by more than 30% with the single-loop pex it won't change the balance a whole lot, if it cuts the flow by more than half, it might.

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    DIY Junior Member rainy island's Avatar
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    Dana,

    Thanks for the advice. you've given me a lot to think on and I'll do some more reading before I decide next steps.

    As always, I really appreciate the insight.

    Eric

  8. #8
    DIY Junior Member rainy island's Avatar
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    Quote Originally Posted by Dana View Post
    As long as you can get 2gpm or so it won't change much. The pumping head of a towel rack is very low, the pumping head of 55' of half-inch pex is something, but might not be a deal breaker. Splitting it into two 27' parallel loops is the rough equivalent of 27' of 3/4".

    Do the math on lengths of all of the existing plumbing, including the "equivalent length" of all the ells tees, etc (you can google up tables of such), and see where it comes out relative to your pump's output curve if you add 55-60' of PEX. A primer on how to do that lives here.

    Typical residential systems are designed for delta-Ts of 10-20F @ 2-4gpm. With outdoor reset the delta is smaller when the output temp is low, but higher when you're at peak temp, peak load. It doesn't hurt to measure the delta on the zone you're modifying first. (Hockey tape on copper pipe and an infrared thermometer is good enough measuring method), but if it's over 50F outside don't be surprised if it's under 10F. If you slow the flow significantly the delta will creep up, which can affect the room-to-room balance if it goes from a 10F delta to a 30F delta. Estimate the flow from your head calculations, as long as it doesn't drop by more than 30% with the single-loop pex it won't change the balance a whole lot, if it cuts the flow by more than half, it might.
    Dana,

    Following your excellent suggestions, I did my best to measure the total piping, els tees ect. and have arrived at the following:

    I have ~150 feet of 3/4 inch pipe in the zone, along with ~80 feet of baseboard and ~54 els. based on the primer you provided (and assuming the baseboards are the same as 3/4 inch pipe) my totals should be (150' + 80' + (54X2.00=108')=338' EFP. Multiplied by 0.04=13.5 feet of head. I also discovered the zone in question is tied to the front of my house which means 1/3 of the piping is covered by finished ceiling…hence the estimates as opposed to exact measurements.

    My circulation pump is a Grundfos UPS 26-99-FC set on low which according to the pump curve chart pushes ~7 GPM (or can go as high as 15 GPM or 22 GPM on speed 2 or 3 respectively). Thus adding two 27’ loop of ˝ pex adds ~27’ of 3/4 inch pipe or 8% to the loop and increases the head from 13.5 feet to 14.6 feet and drops the flow to 6GPM.

    I haven't measured the delta T of the zone yet because we've had unseasonably warm weather, but based on the information above, does adding two 27-foot loops for the underfloor sound reasonable?

    Thanks again

    Eric

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    In the Trades Tom Sawyer's Avatar
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    Boy, talk about overcomplicating a pretty simple problem. LOL Look, running pex or any underfloor heating is going to be expensive and a pain in the backside as well considering the
    small space you are dealing with. Several companies make flat panel steel radiators with the heated towle bar option. Another option is a toe space heater under the vanity cabinet.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    In the trades Dana's Avatar
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    But Tom, people who want radiant floors want radiant floors- doesn't really matter if it's overly complicated for dealing with a tiny load!

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    DIY Junior Member rainy island's Avatar
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    I appreciate that I'm likely overthinking this project and I agree that the easiest solution would be to install a hydronic towel bar and leave a baseboard in place. That said, my wife wants warm tiles, so I have to choose between underfloor hydronics or an electric radiant system. Judging by the latest comments, it appears that the addition of the the small underfloor loop(s) is a viable option so I'll have to choose between that and the electric option (and the addition of a new dedicated circuit and thermostat).

    Thanks again for the feedback and insight The primers on calculating flow and load etc were really informative and have given me a much greater appreciation for the mechanics of my heating system.

    Eric

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    In the Trades Tom Sawyer's Avatar
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    Ok so staple up as much 1/2" pex as you can get in the bays and put a tempering valve on that loop. Just adjust the tempering valve until you get the floor temperature you want.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    In the trades Dana's Avatar
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    A tempering valve is just adding a plumbing complication, and probably isn't necessary. His problem isn't keeping the floor temp down, but rather bringing the temp UP, given his reported low boiler output temps. This is a very temperate climate, where the mid-winter weekly highs are comparable to the mid-summer weekly lows.

    He claims he's able to heat with 115-118F boiler output almost all of the time. The average mid-winter heat load in winter in Victoria (outdoor temp= +37F) isn't that different from what it is at the 99% outside design temp (+26F). He's in no danger of overheating the floor with a staple-up, and even with extruded plates it won't be super-warm to bare feet even at design condition. Unplated it probably wouldn't feel very warm at the average outdoor temps. On that rare one day out of 100,000 that it hits +4F in Victoria (the all time record low, back in 1968, before his house was built) it might feel uncomfortable, but it wont' be dangerous. The coldest recorded temp of the past 25 years is +14F. The typical low temp over a Victoria winter is about +20F (which is how we can tell we're not in Maine anymore, eh, Toto? ) Over the life of the system he might need 140F boiler output during below design temp cold snaps but it'll probably stay under 130F otherwise.

    At 50F outdoor temps with <115F boiler output, probably a <105 AWT, a plated sub-floor installation won't be not going to be a hot-foot- the floor won't be nearly as warm as the shower he just stepped out of. When it's 30F outside it'll feel pretty toasty but still comfortable.

    If my WAG on water temps base on his "...comfortably running at 20% and heating the system water to 115-118 degrees..." turns out to be wrong, a tempering valve can always be added if the floor gets uncomfortably warm during the colder weather.

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    DIY Junior Member rainy island's Avatar
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    Interesting observations on Victoria weather....though I had to do a quick refresher on my C to F conversions.....I haven't had to think about those since we went metric up here in 1977 (though I defy you to find a Canadian who can truly understand fuel economy in something other than miles-per-gallon, or who can tell you how big a hockey player is in centimeters and kilos). For the record, I do own a snow shovel, but its gets 99% of its use moving leaves in the fall.

    That said, Dana's observations on the challenge posed by my low water temperatures are, I believe, correct. My house was originally built with an electric boiler that typically ran at 160F (and consumed a breathtaking amount of electricity even at British Columbia's very low hydro-electric costs). The baseboards would get really hot for 20 minutes then shut down cold for 20 and so on, irrespective of the outside temperature. I switched to natural gas Lochnivar and the installer (from a a firm with an excellent reputation in this town) set the boiler up so that it ran 115-118 degrees (according to the digital panel....the analogue temp/pressure gauge on the primary loop usually reads 110. As a result, my baseboards are warm for long periods, but never hot and the house temperature "feels" more stable instead of the hot/cold cycling we experienced through the first 6 months in the house with the electric only. In fact, I kept my electric boiler plumbed into the system on the assumption that, with apologies to Mr. Murphy, the gas boiler will conk out on the coldest Friday evening of a long weekend. The installer suggested that If I ever needed to heat my house rapidly, I should switch on the electric boiler because it would heat the system much faster than the gas boiler was programmed to. Indeed, other than for a couple of minutes following ignition, the system invariably turns down to a 20% rate of fire.

    I asked all these questions because my neighbour (yes, up here we spell it with a "u") installed three underfloor loops into his system and is pleased with the results. He runs an electric boiler at a higher water temperature, and didn't really do any research other than to simply trust that adding a whole bunch more pex would not impact the heating efficiency and balance of his system. As it turns out, it worked, but I suspect more due to intuition and luck than design and planning. In my case, I knew that with lower average water temperatures I had other factors to consider, and wanted to get some advice from folks like you who have given a lot of thought to the intricacies of mod/con boilers combined with the wrinkle of moderate outdoor temperatures.

    Again, I greatly appreciate your feedback and insight. It has been a tremendous help in my planning process.

    Thanks again

    Eric

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    In the trades Dana's Avatar
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    With an electric boiler it's the same efficiency whether it's 180F water or 90F water, so running higher temp with unplated PEX has nearly zero effect on operating cost. With much higher temps you can push a lot more heat through the floor, and can lose the plates.

    But with a condensing gas boiler it has to stay under 120F at the return (where the water enters the boiler) to stay in the mid-90s for efficiency. At 180F output, 160F return even a condensing boiler is only delivering ~86% efficiency. At your operating temps you're probably north of 95%.

    Being a science nerd in my deep dark past, and an engineer in my current profession the F/C conversion thing is something I'm stuck with often, and in an heating design context it's gotten to the point I've memorized the fives: -5C is +23F, -10C is +14F, -15C is +5F, -20C is -4F, -25C is -13F, and -30C is too damned cold- colder than I ever want to think about. ;-) Don't ask me to convert cubic meters of natural gas to BTUs or therms & back without looking it up though...

    In Victoria your 99% design temp is quite a bit warmer than Tom's (and my) mean January temp, and above his daily high for many days of the year. I heat part of my home with about R2.5 floors with ~125F water, but it takes extruded plates to get there. A staple-up wouldn't come close to making it without bumping the water temps way up. Rather than making yours a dual-temp system, plating your patch of radiant bathroom floor seems comparatively cheap & reliable.

    I'm pretty familiar with the climate in your area having spent most of my childhood in the Puget Sound region. I have relatives across the Salish Sea from you on Whidbey Island as well as further down on the Kitsap Peninsula who I visit annually. It's been over 30 years since I've been to Victoria, but it strikes me as a very nice place to call home, one of the nicest small cities ever!

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