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Hydronic radiant: how do I evaluate H-E tank vs tankless systems?

Bdgray | Posted in Mechanicals on

Hello GBA forum,

I’m a homeowner who has just lost a debate with my wife over the new heat source to replace our ancient furnace. She wants radiant. I wanted a high-heat mini-split. The ceiling drywall in our basement is currently down so a radiant installation won’t require a lot of demolition. Since she is my wife, she has won the day. I’m now trying to educate myself for a closed-system between-joist hydronic radiant installation. I’ll do this myself with a lot of help from youtube and several books I’ve just purchased on Amazon. I plan to do this over the course of the winter so that I can test the performance before I rip out our forced air old system.

With that said, I have several questions for which I’m hoping to receive guidance:

1) In a closed-system, what is the best way to evaluate and select the best heat source. The options I am considering are A) a *High-Efficiency* tankless (93%) or B) a H-E tank water heater (95%). Both claim to be H-E, does that make the comparison apples-apples?
2) Are there trade-offs in going with one vs the other (other than the 2% efficiency difference)? Reliability, performance, noise (I’m assuming a tankless will be on a lot), etc?
3) Tangentially related. Any thoughts/suggestions on heating a finished basement with a mixed-bag of insulation (walls are R-10 XPS but floor is only R-3). My thought is to have two layers of pex between the joists – one “pointing” up, one down. The flooring is just installed so floor-based heat is not an option. I need heat the basement either from the ceiling or perhaps via supplemental electric on walls.

We live in Chicago, zone 5. The house is a 2,500 sqft, pretty-well insulated/sealed ranch (R-20 walls, R-50 ceiling, lots of crawling around to air-seal). We have a finished basement that will also need to be heated. And we are a family of 5.

If you want more details, I’m happy to provide. I wanted to keep this post relatively short.

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  1. GBA Editor
    Martin Holladay | | #1

    The discussion could be long and detailed -- but here is the short version: you want the tank-type water heater (or a boiler), not a tankless water heater, to serve your hydronic system.

  2. Expert Member
    Dana Dorsett | | #2

    What he said. Under the floor radiant floor systems have very little water volume/thermal mass, and the modulation of the output of a boiler or tankless is finite, so it'll short-cycle, taking a toll on longevity and efficiency.

    A tank type hot water heater has a lot of thermal mass that can make up for the fact that the burner is non-modulating.

    All good heating system solutions begin with room-by-room heat load calculations. With a reasonable heat load estimate you can then calculate how many BTU per square foot you'll need out of the radiant floor, which will determine what will/won't work, and the water temperatures required to make it work. If you need 150 F water to stay warm there is no point to heating it with a condensing hot water heater of any type, since the temperatures will be too high, preventing the condensing efficiency, and you'd be stuck at 85-87% instead of 95%+. A modulating condensing boiler can typically deliver even180F water when it needs to, but under "outdoor reset" control the water temps can run in the condensing range most of the time, when the heat loads aren't as severe.

    To run a crude I=B=R heat load calculation for your first floor, assume the R20 batts in a 2x6 studwall with wood sheathing and half inch gypsum will have a U-factor of about 0.07 BTU per square foot per degree-F. Look up the U-factors for the windows in those rooms- if you can't, and it's clear double-panes, call it U0.5, if low-E, call it U0.35. Exterior doors call it U0.5. Ignore the R50 attic- it's "in the noise", of the error on the rest of it. In Chicago the outside design temp is about 0F, and assume an indoor design temp of 70F, so you have delta-T of 7F. The it's

    U-factor x square foot of exterior type x 70F= BTU per hour.

    eg: Say you have 15' x 12' room with a 9 x 15' exterior wall with 40 square feet of U0.35 window. That's 135' of gross wall area, less 40' of window leaves 95' of wall. The approximate load of that room is:

    Wall losses: U0.07 x 95' x 70F= 465 BTU/hr.

    Window losses: U0.35 x 40' x 70F= 980 BTU/hr.

    Add them up and you have 1445 BTU/hr, plus ventilation & infiltration losses what we'll guesstimate at 300 BTU/hr (about a 20% adder to the raw heat load number) minus internal heat gains from the Tivo at 350 BTU/hr and the large sleeping dog at 150BTU/hr for a net about 1245 BTU/hr

    Say you have clear access to the subfloor to all 180 square feet of floor, for a ratio of about 7 BTU per square foot of radiant floor.

    Do this on a room-by-room basis, add up the exterior wall & window & door losses for each room separately, and look realistically at how many square feet of subfloor under those rooms can be readily accessed for the retrofit. Then calculate the BTU per hour per square foot for each room and report back, or start digging around on the internet for what it takes to deliver that kind of BTU/ft with different radiant approaches through whatever your subfloor + finish floor R-values are.

    If you can't deliver design-day heat at condensing temperatures, you won't get condensing efficiency out of a water heater, but you probably still can with a mod-con boiler.

    Basement loads in insulated basements are a fraction of above-grade loads, and radiant ceilings put out more BTU/ft^2 at any given water temp than under-the-subfloor radiant floors do. You'll want to put at least some crummy R11-R19 batts between the floor solution and ceiling solution for zone isolation.

    Easier & cheaper than radiant ceiling for the basement would be to use low-temp panel radiators. Figure out what water temps you need for the radiant floor first, and with a heat load calculation for the basement you can figure out how much panel radiator it would actually take to heat the basement. (It's surprisingly small, in an insulated basement, unless it's a walk-out with big glass sliders.)

  3. Bdgray | | #3

    Dana, Martin,

    Thank you for the response. I will dump the tankless idea. I actually have a Manual J that was run a few months ago when I nearly pulled the trigger on a mini-split system. I think their estimates were somewhat generic but it is all I have. They used a program from RightSoft to map the house and generate the peak load. I have the full output and they did map out all rooms, windows, doors, & basement. So while generic, I have to believe it is reasonably accurate.

    This firm set the outside temp to -5 and the inside design temp to 75 (both aggressive). The house has 2328 sqft, nearly all of it is accessible. The peak heating loads across all of the rooms add up to 36,500 BTU/hr or ~16BTU/hr/ft^2. At this level, I would be curious to know your thoughts on a hot water heater vs boiler.

  4. user-2890856 | | #4

    Brian ,

    this is a evry ambitious undertaking that can bite you in the butt . Be very cautious when getting information or even books from the internet . Zoning radiant to optimize it's efficiency and insure your comfort is a detailed venture . You don't want a Goldilocks house . My advice since your wife has won the argument is to seek out the assistance of a radiant heat design professional , the investment will be quite worthwhile and make your project something that will be fulfilling as opposed to frustrating . Both Martin and Dana are quite correct about mass and the choice you would be best served by . There are not many good units that fall into this category and do not let price alone determine what you will use . Water heaters are water heaters , here is a true boiler that has everything you require for a proper install with mass , I install these and have yet to be anything but amazed that more don't use it . Check out the videos and anything you like , get the opinions of professioanls and don't let them tell you a wall hung unit is better . Although this is a 100K unit you should not be concerned with the high end modulation as much as the low end and with this mass you can almost insure that if fproperly commissioned this unit will fire for a nice efficient 15 minute cycle every hour near design and possibly every 2 to 3 hours during shoulder seasons .
    This is a low maintenance unit since the heat exchanger is of such a generous size , we all owe the gentlemen at HTP , especially Brian French for coming up with this heat exchanger .
    Dana is also thinking well about Panel radiators (low temp) with TRVs in basement .
    dana and Martin , please look at this boiler also and give this man your opinion . By the way , it is less expensive than many low mass , tight channeled hx units .

    Here is another version of the same unit that does DHW also . Although I don't think this unit would suffice for a family of your size , you could add a small preheat tank as a heating zone to decrease the temp rise through the DHW Hx thus increasing it's output .

  5. Bdgray | | #5

    Richard, Dana, Martin,

    Thank you for the guidance. Last night as I sat down in front of MNF I cracked open a radiant heating textbook that I bought online "Modern Hydronic Heating For Residential and Light Commercial Buildings". It's an impressive piece of academic text and it made me realize how naive it was to think that all this would take is a decent PEX layout and a high-efficiency hot water heater (which BTW many companies are insisting work just fine). I figured I'd drill some holes in some joists, run the pex, tack in some diffuser plates, sweat a few pipes, connect the WH and a few peripherals. How hard could it be?

    Despite the realization that this is far more complicated and involved than I expected, I'm an ambitious guy who likes challenges as well as the sense of accomplishment that comes along with it. First things first, I know I need a more complete & accurate Manual J. That's fine, I know there is software that can assist me there. Once I have that I need to design the PEX layout to be efficient. That also can be assisted with software (hopefully the same that performed the Manual J). Even if the software costs me $1,000 I'll still be saving vs a professional install. Plus I'll understand the system and will know it has been designed properly (or at least I'll have nobody to blame but myself if its not). The area where I'm certain to be over my head is selecting the optimal heat source. I had no clue there was such a huge variety of systems, nor did I realize that performance could vary so dramatically based on the demand set or design variables.

    I will definitely need several weeks to better educate myself and plan for this. Once I'm ready I look forward to asking slightly less naive questions on this forum.


  6. Expert Member
    Dana Dorsett | | #6

    To re-calibrate the Manual-J to realistic indoor/oudoor design conditions is pretty easy. If they set it up for -5F / +75F, an 80F delta-T and 0F/ +70F, a 70F delta-T is more realistic it's dead easy. Heat load is pretty linear with delta-T, so just multiply their room & whole house heat load numbers by (70F/80F=) 7/8. That 16BTU/ft now comes in at 14 BTU/ft, and the 36.5K whole-house heat load is now about 32K, which feels about right (or at least not insanely high) for a house that size.

    But you don't really have full access to all floor area, so that 14 BTU/ft number is optimistic- reality is probably 20-25BTU/ft of ACTIVE floor area in those rooms.

    That's do-able with under the floor systems using extruded heat exchanger plates and half-inch PEX, assuming you don't have hardwood or tile floors and not rugs with padded underlayment. A typical stackup of 3/4" subfloor + 3/4" hardwood finish floor will come in somewhere between R1.5-R2. If you look at the nomograph on the last page of this document, you'll see that you can deliver 20 BTU/ft with ~125F +/- 5F average water temp, which could put you in the low end of the condensing range for return water temps:

    The extruded heat transfer plates aren't cheap. If you are doing most of the house with radiant floor it adds up. But to do it with sheet metal plates requires much higher water temps which will cut into the system efficiency. Panel radiators are MUCH cheaper than radiant floor, so think about whether you really need a radiant floor in every room or zone, or how much you're willing to pay for that marginal uptick in comfort.

    The HTP Pioneer and Versa series are nice units and inherently self-buffering, but may be more expensive than a mod-con boiler + indirect tank solution, provided the output of the smallest output zone at condensing temps is a large-fraction of the minimum modulation of the boiler. (If you micro-zone the hell out of the place the mod-con + indirect solution usually doesn't work, unless you start adding buffer tanks etc.) The very smallest Versa or Pioneer would be more than enough to manage this, since the min-fire modulation of the burner is roughly the whole house heat load, with another 65, 000 - 95,000 BTU/hr to cover the domestic hot water load.

    The burner on a typical 50 gallon standalone HW heater delivers less than 40,000BTU/hr, and is usually good enough for a family of 5. With a 55 gallon buffer and 1.5-2x the BTU-rate of "spare" burner, it should be good enough. Even 65K of excess burner capacity will support a 1.75 gpm shower pretty much forever, even without the buffering capacity of the tank. Recovery rates would be pretty fast, even after a tub fill at 5AM on the coldest night of the year.

    From a Jr. hydronic designer perspective the Versa is pretty idiot-proof- you'd have to literally break it to actually get it into a short-cycling mode, and it has burner to spare for even fairly high domestic hot water loads. It's not cheap, but it's a well engineered piece of kit. With one of those at the heart of it you can then spend your design time on getting the radiation sizing & flow rates right, making sure that you can run the zones at sufficiently low water temps to max out the efficiency of the system.

  7. user-2890856 | | #7

    Dana & Brian ,

    As a comparison for a well designed equipment package that is protected from short cycling and all kinds of nasty stuff .

    Average cost of 50,000 mod con boiler of a quality - 2,800.00
    Indirect hot water tank ,Stainless steel - 900.00
    Buffer tank , smallest that is practical 750.00
    Second pump for P&S piping , high head loss boiler loop 250.00
    Labor to build it all 1,000.00

    TOTAL 5,700.00

    Pioneer 100K boiler 2,600.00

    Versa Flame 130K 55 gallon mass 4,514.00

    Versa Hydro is another unit to check out . A water heater with the flat plate for space heating , again no buffer tank needed . 130 K / 100K CH capable , 55 gallon storage . 5,100.00

    These units are really a good tool for a designer installer to insure a quality system that has few if any shortcomings while staying competitive while still needing a smaller foot print for equipment . While being kinda idiot proof , smart designers and hydronicians sue these to theirs and their customers advantage . I don't need to build a wall of pumps and equipment to stroke my ego . By the way if you need any modular stuff built to really tight tolerances you can contact me .

  8. Expert Member
    Dana Dorsett | | #8

    Richard: A competent scroungy DIYer will discount the labor, hunt around for a $200 electric hot water heater on sale to use for as a massive hydraulic separator / buffer etc. (which is fine at the likely flow rate & buffering requirements for this type of system.)

    And of course there is ZERO labor cost to installing the Versa, right? :-)

    It's OK to play the sales & marketing game here I suppose, but there are many ways to skin this cat with a mod-con without adding another $1000 buffer & boiler loop hardware, and another $1000 in labor, especially for those who can do the math.

    This isn't a knock on HTP solutions- they are well engineered units easy to design around, removing a good chunk of the design risk. It's a heluva lot cheaper to take the HTP route than to debug FIX a badly designed system, to be sure!

  9. user-2890856 | | #9

    Why , yes Dana , there certainly is labor cost to installing the Versa . However , I only added the labor for tasks not included nor necessary with the Versa . I did use the assumption that this gentleman wanted to use quality in his house and that he had some sort of pride , so let's look at it from that view point . And to be perfectly clear , I do not play sales and marketing games . I , after 30+ years of looking at and fixing the very worst jobs done by people whom claimed they were the very best , I am always skeptical and have taken pride in the fact that I am not one of those .
    Maybe you could enlighten me on how to skin this cat using a mod con w/o a buffer . You see , I am not someone who does not know how to do the math , in fact I probably know too much math . Sales and marketing is not what I di , it is what I despise . I am more interested in designing / installing systems that are as close to Exergy as possible . You can have the small little channels in a mod con and the required maintenance that the owner will never do . I'll keep being the guy who installs really good systems , thart provide comfort through a wide range of conditions , that don't short cycle , and cost the end user the least to use .
    I did not get the impression that this was a scroungy DIYer nor do I ever get that impression of anyone . So hey if someone around me wants to know how to do something I can tell em , then if they decide to buy a 200.00 electric water heater and spend 1.40 for app. 10 Kw as opposed to .32 for a third of a therm of NG , so be it . I would be interested in discussing what you do as opposed to what I do privately though . Let me know if you're interested .

  10. Expert Member
    Dana Dorsett | | #10

    Mayhaps "scroungy" sounds a bit too much like disparagement, which is not my intent at all!

    "Resourceful" would be more appropriate, given that he has expressed the desire to learn the math, and to do as much of the installation himself.

    Using an electric HW heater as a buffer tank specifically does NOT include hooking up power, only plumbing it as a buffer tank. It doesn't take a purpose made $750 tank to buffer small residential systems with tiny mod-cons, and the longevity of the hot water tank in this sort of application is much longer than when used as a hot water heater. With bigger design flows purpose designed buffer tank may be called for, but I don't see this system evolving into one of those. If he does the math right and doesn't micro-zone it he may not need a buffer at all. Electric HW tanks are cheap, well insulated, and have multiple ways of using them as a thermal buffer in hydronic systems if you're willing to go there.

    I have a 48 gallon Ergomax on my home system, but that was as much for the domestic hot water as the system buffer. It could have been done more cheaply using other means but would have taken more space. As it is the system takes about as much space as the Versa, ( but it took more math to get there than it would have with a Versa. :-) )

    We've all seen nightmarish installations from people who should have known better, as well as DIY systems from those who hadn't a clue.

    For me any hardware up-charge in going with a Versa solution would be paid back in reduced installation time, a cleaner installation, and a simpler system design. I really DON'T knock it at all (indeed, I prefer it! ) But it's not always the cheapest solution for a 32K heat load, especially if the sweat-equity labor is deemed a cost of education for the DIYer rather than a dollar cost to the system. (I really DO think it's closer to the right solution for Brian, even if he does want to play Jr. Hydronic Designer.)

    I apologize for rubbing you the wrong way (I tried to put a smiley on it, eh?). I respect what you do, I just didn't think the all labor and hardware costs needed to be there when looking at a mostly-DIY solution.

    When a particular solution is pressed repeatedly, with perhaps up-sized cost estimates for other solutions it begins to feel like you're selling something. I'm happy to hear that's not where you're coming from.

  11. user-2890856 | | #11

    All good Dana . Was just trying to steer the man since he asked about an HE water heater for source ( about 2,000.00) to a heating unit with a quality heat exchanger and some real efficiency . Then of course that there would be a solution to making DHW within the same footprint .

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