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radiant heat w/ HW heater

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ALH_4
ALH_4 Member Posts: 1,790
It depends on the heat loss. For very small systems, a combination water heater with an internal coil heat exchanger for a heating load (ie. Bradford White Combi2) might be the best bet.

Anything over about 40,000 btu/h design heat load, and it is probably best to go with a modulating condensing boiler with an indirect water heater.

I assume this will be gas-fired.
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  • Patrick O'Connor
    Patrick O'Connor Member Posts: 1
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    radiant heat w/ HW heater

    Hello,

    I'm a homeowner considering radiant heat for my new house. What is the consensus on using a HW heater instead of a boiler for the heat source?
  • Brad White_9
    Brad White_9 Member Posts: 2,440
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    I agree with Andrew and will add

    first and foremost that the DHWH you are using is really dedicated to heating only. Do NOT use a regular DHWH which also serves your domestic needs. The two must be entirely different systems.

    As Andrew said, heat loss is key. I wish ModCons came in a 30-40 MBH package but until then, consider these points:

    1) A DHWH has a rating of 150 PSIG.

    2) A DHWH is designed to last a reasonable amount of time taking in raw, oxygenated, high-mineral content water and sending it out again, over and over. Imagine how long the same heater would last if using recirculated water kept at a much lower pressure? I have no data, but given the duty compared to the ratings, I think you have an idea.

    The downside is that controls are not perfect. Pilot light and manual adjustment unless there is something new on the market that I am not aware of. You also have to run it at higher temperatures and mix-down or use a heat exchanger to avoid condensation. (See Hot Rod's note below- EDIT)

    I would set it up like a boiler (expansion tank, maybe a WAGS valve, 30# relief valve, spill switch, etc. Run it by your local authorities first.
  • hot_rod
    hot_rod Member Posts: 22,147
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    One important concern

    If you connect a typical tank type gas fired WH to a low temperature radiant you need to be concerned with extended condensing mode operation.

    I have seen WH burners rust away in a few years when the WH's run below dewpoint consistently. CO issues come to mind...

    I'd suggest a 140 operating temperature with a 3 way thermostatic mix valve for the radiant.

    Code compliance is another issue to address. Not all AHJ's buy into DHW tanks used a boilers.

    Bradford White has some cool products for this application. The new Combi 2 is a dual purpose tank. With a 1-1/4 steel coil inside the tank.

    BW also has a new 25 gallon high BTU tank. Talk about a nice small package for properly applied radiant applications :)

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
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    No real problem with the following caveats:

    1) Must be allowed by your jurisdiction. Some do, some don't.

    2) Please do not use the same heated tank water for both space heating and domestic hot water. Bradford-White (and perhaps others) make a "dual use" model "water heater" but the domestic hot water is kept SEPARATE from the space heating water.

    3) Appropriate safety devices must be used. For instance the temperature and pressure safety installed on a "water heater" is designed for domestic water pressure, MANY TIMES higher than the pressure (12 psi typical) in space heating systems. Nor do typical "water heaters" have a high-limit safety much less a low-water cutoff.

    4) Your design heat loss should be quite low--say no more than 30,000 - 40,000 btu. Much more and you truly should use a condensing/modulating boiler to drive your radiant system.
  • Unknown
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    It's important to note that in a real heating climate, 30 to 40 kBTUs/hr I would consider average, not low. In running some numbers I generally find that if you're over, say 5000 heating degree days a year I would personally use 25kBTU design load, or even lower as the bar for when it makes sense to go to a water heater... ASSUMING both are gas, and the other option is a mod/con hitting 95% on a low temp radiant system (as this must be, or a water heater isn't a very good choice either...)

    The less degree days, the higher the design load that a water heater can handle adequately.
  • Unknown
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    HR, what's that high BTU tank called?
  • hot_rod
    hot_rod Member Posts: 22,147
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    Bradford White high recovery #

    GX-2 2596BN is a 25 gallon 78,000 input 18" diameter, 60" tall.

    Also a 55 gallon 80,000 GX-1 5598BN

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Unknown
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    thanks HR!
  • Unknown
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    won't use Bradford White

    Seemed Brandford-White heaters in this part failing prematurely and my supply houses stopped carry them as they are horribly don't honor their own warranty...
    I use regular water heater , using the flat plate for heating zone... Needs to use extra long heat sink pipe with spring loaded check to prevent ghost flow, overheating... otherwise, no plm with heating 900 sq ft outbuilding. Of course using the 2" board on outside walls and underslab with constant circ.
  • Darrell
    Darrell Member Posts: 303
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    I did a down and dirty system one time for a friend of mine where I used water heaters. His house was slab on grade single story. We split his house in half right down the hallway and used a water heater for each side. The operational load almost perfectly matched the realized output of the waterheaters. We ran the waterheaters at design temp...say 125 or so and fired a circ pump with a line volt t-stat. I added the appropriate temp controls and changed the relief valves. So he wound up with three 50 gallon water heaters...one for each zone and one for DHW. We ran a diluted mix of cryotec in the heating units because of the exposed slab rim and potential standing heating water between cycles. The end result is a system that requires extremely low maintenance and he swears his gas bill is half of mine...and his house is twice as big as mine. The only problem he is having is that the bank will not finance his home to a buyer because the tanks are not specifically rated and stamped for space heating...we should have read the fine print and got tanks that were. When he gets ready to sell he wants to replace the existing tanks with a pair that are rated for space heating...doesn't even want to talk about a "fancy boiler that will cost a fortune to install and run." He has a point...sometimes we engineer a project beyond its capability for any real payback. Don't shoot me...I understand the save the world arguement...just one mans experience with heating via water heaters.

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  • Unknown
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    Darren, I see you are in AK.. that's low load territory, isn't it? I'm only seeing 3000 to 3500 heating DD's in that region.

    If you're running a regular boiler especially, I would not be at all surprised to see a lower load system running more efficiently on water heaters. More surprised if you're running a mod/con, but then.. your room temperatures can affect that quite a bit too, as well as house construction.

    What were the load calcs on this building and yours?
  • kal_2
    kal_2 Member Posts: 60
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    How come no one mentioned legionella bacillus…

    I know some one who did it for a while and took a big chance…
    it is one of the biggest problems of piping domestic water with pex instead of copper, because, missing is, the copper-sulfate (that green stuff”) on the inside of the pipe that’s a natural enemy of legionaries’ disease – not only are you going to have large standing bodies of water in your radiant floor, but quite possible at the perfect 115f incubation temp, and then, you are going to atomize it in your shower and breathe it in, talk about a bad case of legionella based respiratory disease

    I once suggested heating the bath wall radiant with water coming to the shower – and go the idea shot down for this very reason – and that would only have been 1-2 gallons of water max

  • Unknown
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    mike mentioned separation. You can use a HWH no problem if you're separated with a coil or HE.
  • ALH_4
    ALH_4 Member Posts: 1,790
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    Yep

    Which is why I mentioned the Combi2. That is the only way I would do this. External heat exchangers are an unnecessary complication, and loads higher than what the Combi2 can provide are too large to consider a water heater as a heat source.

    There are applications where this makes sense. How about a very-low-heat-load, off-grid house? It seems the Combi2 is well suited because of the low electricity consumption and small space required.
  • Unknown
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    Unnecessary complication?

    external HE's can be sized for the load, potentially saving quite a bit of money.

    Plus, it leaves you all set up for VSI with the addition of an inexpensive controller.

    Finally, if either the heater or the HE fails, you replace what fails, instead of both.

    I do like that with the coil in the combi though you can have a 1 pump mixing valve system. 1 pump systems make me feel funny... in a good way.
  • ALH_4
    ALH_4 Member Posts: 1,790
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    hx's

    I do like that with the coil in the combi though you can have a 1 pump mixing valve system. 1 pump systems make me feel funny... in a good way.

    I agree with that statement. I am just not a fan of situations where stainless or bronze pumps are required. It could easily be less expensive up front to use the external HX. One issue with that is that the domestic side of the heat exchanger could sit stagnant all summer, and there is potential for bacteria to grow in that piping. When the heating system is turned on in the fall, that stagnant water is introduced into the water heater. However, there is a good chance it would thermosiphon a little during the summer eliminating the stagnant water...

    As usual, you make excellent points. I prefer the coil-in-tank from a design standpoint.
  • Jerry_15
    Jerry_15 Member Posts: 379
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    Let's get real here. The only reason to use a hot water heater is to save a buck. Standing pilot, 40 gal kept hot all the time, AFUE? What's that? They use the don't ask, don't tell approach. Pretty much defeats the advantages of a closed loop system, doesn't it? A flash heater does a much better job if you must forgo a real boiler. Triangle Tube has a combi-unit that is the real deal. Check it out.
  • Unknown
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    Bull.

    With an on demand, on low loads, you're stuck with cycling issues.

    If you add a buffer tank, as you should... well congrats. You have a tank water heater with a remote burner.

    Plus the pumps you need to use "flash heaters" are much more powerful than you would need on a tank setup. That's more cost up front, and more energy forever.

    There are many where I would *prefer* a water heater to a "real boiler". I suppose a mod/con and buffer would be better.

    But on those systems, a mod/con and buffer would be a ridiculous amount of expense for the heat load you're servicing.

    You can use electric ignition and put the tank in conditioned space.
  • Jerry_15
    Jerry_15 Member Posts: 379
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    S'cuse me Bull. They don't short cycle (at least the ones I use); they modulate and do a nice long burn. Not my first choice but work quite well on a small system. A three speed Grundfos can help dial in the delta-t, without over pumping and cavitation. They often qualify for IRS tax credits and sometimes local utility rebates. Yeah, they cost a bit more.
  • Noel
    Noel Member Posts: 177
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    They also have all of the safeties required on heating appliance

    > S'cuse me Bull. They don't short cycle (at least

    > the ones I use); they modulate and do a nice long

    > burn. Not my first choice but work quite well on

    > a small system. A three speed Grundfos can help

    > dial in the delta-t, without over pumping and

    > cavitation. They often qualify for IRS tax

    > credits and sometimes local utility rebates.

    > Yeah, they cost a bit more.



  • Noel
    Noel Member Posts: 177
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    They also have all of the safeties required for heating

    The saftey switches that aren't required on domestic water heaters (because they don't run the length of time in a day that space heating appliances run) make water heaters ideal for people that want to keep the costs low while ignoring risks of carbon monoxide poisoning, and fire due to roll out. If there is no inspection of code compliance, the entire risk is on the installer (Duty of Care).

    I'd not do it, myself, nor recommend it to anyone else. To much risk and legal liability. Not worth ending my carrer over.

    Some water heater installations intermittantly vent poorly. I'd not dare to risk that happening for 10 to 20 hours of every day in the winter. Houses are too tight, these days.

    Noel
  • Unknown
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    if they aren't short cycling, then your loads aren't low enough to be using a water heater of any kind, on demand or not.
  • Jerry_15
    Jerry_15 Member Posts: 379
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    Modulating, modulating, not short-cycling. Works dandy.
  • Unknown
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    They do have minimum modulation rates.

    If you aren't below that, most of the time, then you have a load that is probably high enough to use a small mod/con cost effectively. That is, if you're generally over a 16k load.. use a mod/con.

    If you're not, then you should be short cycling. I suppose if you do bang-bang concrete systems, the concrete would be your buffer. Are you doing mostly 'crete?
  • Dave Yates (GrandPAH)_1
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    koolaide!

    Drink up & be merry gents. Personally, I'd much prefer being given cold hard facts.

    How low do she go? Modulation is great, but let's see where the boundaries lie.

    Give me the resistance to flow at the required flow rates for the hydronic side of the mix. Just cause it's 3-speed doesn't make it the right match. Head & flow & let's go look at the chart.
  • Jerry_15
    Jerry_15 Member Posts: 379
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    Of course you have to match the pump to the system, of course a good mod/con is better than a flasher, of course you can do a primary and secondary. It's just a good compromise that works nicely, simply and cheaply on small simple systems without resorting to a water heater. I'm done. Good luck.
  • Jerry_15
    Jerry_15 Member Posts: 379
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    One caution. You cannot use a flasher on a steel/iron system. They run off a magnetic impeller now instead of a diaphragm that will collect filings in the system and shut it down pretty quick. You can clean it out, even put on a y-strainer with a magnet stuck on, but it's not worth it.
  • Dave Yates (GrandPAH)_2
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    so

    "Of course you have to match the pump to the system"

    Ok then - teach me or not. If you don't really know the details, that's ok too. But I think you owe it to those who read these posts and might be lead astray by posts that have the potential for problematic situations.

    Give us the required info to use a "flasher" correctly.

    No innuendo or guess-work mate, just the cold hard facts. You owe us nothing less.

    Also as noted in a previous post: a storage vessel (AKA: water heater) still requires a T&P valve (150 if'n ye wants to be within specs) be installed within the top 6" of the tank, so ye'd best be installin' a 30-LB vlv too due to the dual duty. What's in yer wallet?
  • Plumdog_2
    Plumdog_2 Member Posts: 873
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    He is in ALASKA.
  • Unknown
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    what is a \"flasher\"?

    What's a "flasher" ?
  • Jerry_15
    Jerry_15 Member Posts: 379
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    OK here we go heating 101. Flasher is a flash heater or instantaneous water heater, without clothes. They don't need a tpr valve since there is no tank. Prv 30# psi is just dandy and suits all codes. They work dandy on small simple systems with new piping and basic control. Single zone line voltage set-back t-stat to the pump, or a Taco with a built in relay and you're done. From there you can design a lunar lander, make it an injection system with multiple manifolds, pumps, zone valves, t-stats, 4 way mixing valve, and outdoor sensor tied in by satellite to the national weather service to anticipate local, and global issues. I would never use it on a large system, or one with existing tired piping. I always remove the fine mesh strainer and put in a regular y-strainer. Flow rate through the unit is limited, but if you can get a good delta-t at 5 gpm,it shines a lot of shoes. By all means work out the details. None were ever provided here.
  • Dave Yates (GrandPAH)_2
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    not what was asked for

    1 - I know what a "flasher" is.

    2 - The T&P reference was for an earlier post where someone installed a tank-style water heater and replaced the T&P with a P-only relief vlv. Both are required if a tank-style water heater is used in a dedicated hydronic application. As for tankless, I know they require a P-only relief vlv, but I've seen more than a few installed without any relief vlvs.

    3 - You have not answered the questions. If I were someone not already familiar with hydronics 101, I would get the impression from your posts that using a tankless on-demand water heater would be a snap and fit virtually any application. I am not interested in what's beyond the tankless - be it a Yugo or a Hummer - not yet. If you can't answer the questions I asked, then simply admit so & move on. They're basic hydronic design 101 questions.

    * The GPM rate for your hydronic design calculations.

    * The head loss at that GPM flow rate through the tankless.

    Head loss & flow rates & which pump - a Grundfos SB in your earlier post.

    *Which of the three SB 3-speed models are you using? With or without integral flow check? Which speed?

    Once we get beyond the tankless issues, then we'll need the system info to complete the picture.

    Paint us a mental picture, or better yet, take some pics of installs & add them to your post.

    4 - Let's add one more question. How do you handle recirculation with your tankless "flasher" water heater installations?
  • Andrew Hagen_2
    Andrew Hagen_2 Member Posts: 236
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    Saving a buck

    I disagree that using a water heater is just about saving a buck.

    When it comes to producing domestic hot water, there are applications where instantaneous water heaters are appropriate. But for the vast majority of residential applications, instantaneous water heaters are not ideal. That is not to say they do not work.

    The reason I recommend a water heater like the BW Combi2 is because one device will serve both purposes and will do so safely. The Combi2 is not an inexpensive unit when compared to standard gas water heaters. When the load is very low, there simply is not a unit that I would feel comfortable installing in a client's home that fits the "below 40,000 design load" home. Additionally, a few points of efficiency buys you much less with a 20,000 btu/h load home than with a 200,000 btu/h load home.

    Instantaneous water heaters are designed for a specific purpose. Wide delta-T, relatively low flow, and high delta-P. Why use an appliance designed for a specific purpose for a purpose with very different design requirements?

    Noritz has a 380MBH unit. Why buy two Vitodens when one instantaneous water heater will do?
  • Unknown
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    hahahaha... thanks, plumdog. I always think that AK is Arkansas.

    Darren, southern handle or mainland?
  • Jerry_15
    Jerry_15 Member Posts: 379
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    I keep saying that I don't recommend, and will not install this system with many controls and variables. Dirt dawg simple only, or go to the real deal and run the numbers. For example: small house (900 sq '), two floors, no space for equipment room. Hung a Rinnai commercial (180 deg keypad, 180K) outside (no flue), popped into the wall behind the beauty skirt(walls open inside) dropped down 3' to crawl space (access door right there). Ran 4 fan convectors and 2 towel warmers to a simple manifold with 3/8" pex, (each on it's own run). Had balencing valves which I never needed to use. Used a Taco 0014 then with a relay. One t-stat. They control the heat with t-stat, and leave some of the fans off, (It heats up like a rocket), but the flow is always the same when the system is on. Only problem was the super fine mesh strainer on the unit which I now remove on the install. 28 deg delta t. Why Rinnai? They will flame out and re-start on the differential rather than continue at lowest firing rate and over-limit (manual re-set). Be sure to check for this feature on other brands or your phone will be ringing in the middle of the night. I think the whole job took a day, maybe two with a long lunch. Six years ago, still works just fine.
    That included sticking a residential unit (120 max on keypad) for domestic next to it. Like it or not, there is an argument for separate systems. Much as I like a good combi, losing both heat and hot water at the same time is a real pisser.
    Sample two: 500 sq' slab addition with existing forced-air heat on the main house. Towel warmer and underfloor 4 runs, line voltage t-stat (no relay needed), 1/2" pex, balancing manifold, but it all runs wide open off t-stat. 120 temp worked fine. Used the little grundfos 3-speed on high, lot cheaper then the Taco; yes I took out the back-flow.
    I must confess that I have a habit of assuming that the equipment I used on the last 10 jobs which has worked perfectly will work on #11. Mea culpa. I do check the delta t and adjust if needed. Since I use iso flanges a quick change out takes me less time than, well, writing this note.
  • Weezbo
    Weezbo Member Posts: 6,232
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    Darren lives south of us .....

    He lives down by Noahs Arch :)

    where there are two of everything.....somewhere....
  • Weezbo
    Weezbo Member Posts: 6,232
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    a sick depraved individual who thinks the world

    should be prepared for the unsuspected...

    go the other way if you see someone in a Wellington rain cloak and goulashes on a sunny day ...:)
  • Jerry_15
    Jerry_15 Member Posts: 379
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    Speak for yourself weez - I recall a warm summer afternoon 30 years ago when a bunch of ladies and gents doffed their duds, ran right through the boat-yard and the shop, and dove off the docks. We turned off the equipment, locked the doors, and dove in. None of the customers complained. They were too busy swimming. Keep your raincoat on, but remember that even body armour has it's limitations.
  • Chris_82
    Chris_82 Member Posts: 321
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    I mentioned this a while back and have yet to get any reasonable response about pex and the slime that is now forming in just about every application in addition to the funny brown/black spots that you can see. Is this the next huge recall that we are all going to face? And by the way running pure clorox thru the tubing didn't help with the spots. We left in in the tubeing for a day and then ran it, purged it for about six hours before the clorox taste went away? I ask'd if any of the manufacturers would step up to the plate with an answer and as of yet none have!!!
  • Dave Yates (GrandPAH)_2
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    combined?

    Are these dual use potable/hydronic "open" systems?
This discussion has been closed.