1 Boiler for Both Radiant Heat and Hot Water Possible?

Hi everyone, we are in the process of rehabbing a house. We plan on installing radiant heat. Does it make sense and/or is it even possible to have 1 boiler preform both the heating for the radiant and for the bath/kitchen hot water? House is 1800 sq ft in total.

I have built 5 houses using one Polaris condensing water heater for the radiant and hot water. The oldest must be 20 years and never with a hitch except to be sure ONE check valve continues to work. I never use electric valves and the idiotic and non-maintainable complex valving and sensing controls. Each zone has a small ballvalve for flow control, and I use one properly chosen circulator pump. If you have a monster house you could use several of the smallest pumps, one to a zone or two. By simply placing the check valve in the correct location, all incoming cold water destined for the water heater is routed through all of the floor zones. In the summer this automatically provides some free "heat" in the slab to the incoming cold water. Unless you are a techno-nut and like 500$ service calls, this is the hands down only way to plumb up a house. One of the radiant parts site used to have a schematic of the system on line. I have passed out my own plan to many builders and still get thank you calls.

A few valves, 2 manifolds, 1 top quality checkvalve, and a pump - thats it! The system also tells you elegantly when the check valve fails: You will have hot water at the hot water heater, and hot water at all fixtures when the circulator pump is running OR when the check valve is good. The moment hot water fails to reach any and all fixtures when the pump is NOT running, the check valve is bad.

After 11 trouble free years, my wife suddenly decided our hot water heater was bad. She Was not getting hot water in a distant bath. By the time I got to check it, I had hot water. The hot water heater was at normal temp. We almost duked it out when I told her she did'nt know left from right anymore. Seems each time the pump kicked on before I went to inspect her screams. Finally I got stiffed for hot water, and found a tiny piece of a hot water connector washer in the check valve. So many years and jobs without callbacks that I forgot my own design spec. {thank god she was'nt home and I did'nt have to grovel at her feet in apology} So outside of marital harmony once each decade or so, its so simple and perfect that you will laugh at the guys with the nuclear controlroom boards in their basements. Use a SS checkvalve one or 2 sizes larger than the piping with a very low force spring closed valve disc or ball. I use double unions and wire a spare check valve to the pipes with the schematic. Many plumbers have never heard of this system and it could drive them insane trying to figure out hot water at the heater and NOT at the fixtures.

ballvalve, thanks for the detailed reply! Thats a great tidbit to know about the check valve. The guy I am going to have design my radiant system also recommends a Polaris consequently.

Just out of curiosity, typically what size of gas piping have you run to these units in the past? I am having the gas pipe run before the system gets designed do to time constraint from the gas company. Two plumbers I have had over recommend a 3/4 line as being oversized.

Better read the Polaris manual. A few years ago they required at least 20' of 3/4 pipe before the heater and after their regulator. Use the 3/4. It wont work with restricted gas flow. [speaking of propane in my instance] This unit has a negative flow gas valve. Dont let your designer add a ton of electronic junque - keep it simple with one big shared loop of zones. I use 300' runs of pex per loop, sometimes 500. A simple 20$ electric heat thermostat, line voltage can switch your pump. One thing I noticed is problems when the exhaust run is too short - received lots of bad ignition warning lights. I extended the exhaust by sleeving the inlet air inside the outlet 20' [preheats the incoming air] and now I never get error faults. Keep a hot surface igniter spare in stock. By the way the early Polaris INCLUDED this preheat rig ,in a short version, but it seems no longer. Its easy to fabricate.

cloves said:

Hi everyone, we are in the process of rehabbing a house. We plan on installing radiant heat. Does it make sense and/or is it even possible to have 1 boiler preform both the heating for the radiant and for the bath/kitchen hot water? House is 1800 sq ft in total.

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This is VERY commonly done, and there are many ways to skin this cat.

The most typical high-efficiency way it's done is with a low-mass condensing boiler running the radiant, but having an indirect-fired tank controlled as a "priority zone".

If the radiant is low mass (not a concrete/gypcrete slab) and is micro-zoned with each room controllled separately it's usually more efficient to set it up with a buffer tank (or if the water temps are typicaly 130F or higher a "reverse indirect" hot water tank) feeding the zones, with the boiler serving the buffer rather than short-cycling on micro-zone calls. But there are many other approaches that can work as well.

Unless you want to to a LOT of reading up and are good at the math, it's probably best to let a pro design it (and I mean somebody who is local, visits & measures the house, and is on the hook for it when it's done, not some "internet pro" hawking combi or radiant systems.)

NJ's climate isn't very severe, and even the smallest Polaris may be 3x oversized for your peak heat load, (or even 10x bigger than your average.) While the thermal mass of it's tank spares it from excessive cycling, it's not necessarily the most efficient or best bang-for-buck. If you're heating a slab you'll do measurably better with a right-sized low-mass modulating condensing boiler and an indirect-fired tank.

If the house currently has a functional cast-iron/steel/copper/whatever boiler in good repair, using reverse-indirect plumbed as a buffer/hydraulic separator will be far cheaper than a new boiler, and set up properly, will deliver better efficiency than when it was running baseboards, etc.

Using one boiler for both hot water and heating is the normal configuration in most cases. There's many ways to do this depending on your heating requirements. Options include oversized water heaters with space heating capabilities (e.g. Polaris), on-demand water heaters rated for space heating (e.g. Takagi JR), space heating boilers that can act as on-demand water heaters (e.g. Trinity Ti series combi), and, finally, piping a space heating boiler to heat potable water through a heat exchanger.

Ideally, a good plumber will figure out what's best for your application, but good plumbers are extremely rare. Spend as long as it takes to research the options so you can compare notes with potential contractors. You really don't want to hire anyone who knows less than you do.

Lightwave said:

Using one boiler for both hot water and heating is the normal configuration in most cases. There's many ways to do this depending on your heating requirements. Options include oversized water heaters with space heating capabilities (e.g. Polaris), on-demand water heaters rated for space heating (e.g. Takagi JR), space heating boilers that can act as on-demand water heaters (e.g. Trinity Ti series combi), and, finally, piping a space heating boiler to heat potable water through a heat exchanger.

Ideally, a good plumber will figure out what's best for your application, but good plumbers are extremely rare. Spend as long as it takes to research the options so you can compare notes with potential contractors. You really don't want to hire anyone who knows less than you do.

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After encountering the wealth of ignorance out there, I no longer rely on "heating & plumbing" on the side of the truck counting for anything. Lately I've been leaning more toward recommending paying an expert to design the system, then put it out for bid between competing heating & plumbing installers rather than letting the installer design the system. Of course, that means you also have to figure out the competence of the designer...

At my own home I ended up designing it myself- the installer's designer gave dire warnings that it wasn't going to work, even putting up a cash bet, which he promptly lost. YMMV

On a rehab project, unless the boiler is in bad shape or running very inefficiently it's hard to make an economic rationale for buying whole new system to run radiant & hot water. There are many many retrofit radiant systems built around reverse-indirects that look roughly like this:



With the boiler slaved to the tank as it's only zone and the heating zones running off their own pumps this is a classic primary/secondary configuration with a couple of twists- the hydraulic separator has significant (and insulated) thermal mass, and an internal heat exchanger for potable hot water. The only significant difference between this and many buffered primary/secondary systems is the internal heat exchanger. The mass of the tank allows the designer to micro-zone the system with abandon rather than trying to balance the radiation across a large single zone (which can be difficult or impossible to do well in many homes), since it now can't short-cycle even under the tiniest zone's load. Also the domestic hot water load shares gracefully with the heating load: As the temp in the tank falls, the radiation emits less, so a larger share is available for the domestic hot water. In most homes this will have only slightly lower hot water capacity than a priority-zoned indirect that turns off heating zones during hot water heating calls. Note the architectural similarty of the reverse-indirect buffered system to this standard-buffer as hydraulic separator:



There's at least one contractor in NJ who has installed hundreds of systems using a Takagi hot water heaters as the boiler, with reverse indirects as the point of hydraulic separation as sort of his "signature system". If you need higher radiation temps you need to pick your tankless water heater carefully or use a different configuration, since some have safety limits that turn the flame off when incoming water temp exceeds ~130F. (eg. Takagi TK1 & TK2).

If retrofitted to a gas-fired cast iron boiler setting up a reverse indirect for 140F hot water + radiation is dead-simple. You probably don't want to drop it much below 125F ever for hot-water reasons, but if you do you'd need to add a small "boiler bypass" loop to protect the boiler, but it's not a big deal. Most under-floor staple ups/plated radiant systems will likely need water temps in that range anyway (a bit higher, if unplated). If your using an above-the-subfloor system (eg WarmBoard tm) the temps can be much lower and thermostatic mixing valves can mix down the 125-140F water to whatever the radiation temp requirements are, and you'd still have pretty much endless hot water while maxing out the efficiency of the boiler.

You can spent a whole lot of money for a higher efficiency condensing system and save at most $100-150/year in ~4800 heating degree-day climates like NJ. If there's a functional non-decrepit boiler already there that has a 78% AFUE or higher test rating, use it, and configure the system with an ErgoMax/TurboMax/Everhot-EA or other as a hydraulic separator. When the boiler finally gives up 10-15-25 years down the road, the system is relatively insensitive to the heat source, and can easily accomodate a higher efficiency (or different fuel) boiler. There's nothing special about lower temp radiation like radiant floors that requires a different boiler, but if the temps are low enough it can take advantage of condensing boiler for higher operating efficiency, but in relatively mild heating climates like NJ the payback on fuel savings will take a very long time, even for new construction.

Go to "Radiant floor company" and click on "Open systems" . You will see a great schematic and photos. If you are getting rid of an old boiler and do not want all of those problematic connections in the above drawings, this is your way to go. If working with pex, most of the copper work goes away, and the installation is very quick and inexpensive. I have done these with just a regular water heater [or two] also, in a mild climate with wood stove backup. Then you are really on the cheap for equipment. I admit the Polaris is a bit high priced.... perhaps not so on propane though.

And yes, "good plumber" is almost an oxymoron, so best if you decide what you want and then get a "mechanic" to plumb to your spec's. If you buy your manifolds and pumps from these internet radiant guys, they will help you get things right - without any interest in making money on labor.

The photo on the site shows several circ. pumps, but 1 high head rig or a few on 2 or 3 zones works fine. Control the rooms temp. with the flow valves.

Of course many people in this day and age think turning a ballvalve while thinking about what they are doing is too difficult. Then you will need a thermostat in each room and your mehanic to stay over for a week on the install.

Open systems are always bad idea, and not even legal in many states. (In MA, they're legal only with control provisions to circulate the water 5 minutes out of every hour 365 days/year to limit stagnation issues.) I'm not sure what the legal status is in NJ, but it's a recipe for water-borne health issues- if not this year, sometime down the line.

And doing system design via internet-experts is a LOUSY way to go- they're not there to deal with the installation, and aren't on the hook for anything other than the hardware they ship.

A single-zoned system with a buffer or reverse-indirect can be done with 2 pumps, and they don't need to be monster-flow if you've done your radiation design reasonably. It's not any better/worse or more complicated to plumb than the internet open-systems based on hot water heaters IMHO. How is this any less complex than the Ergomax sketch?


http://www.radiantcompany.com/images/muni.gif

Tank HW heaters can be impressed into service as space heaters in areas with relatively light peak loads (under 25KBTU/hr on design day, or you'll be out of hot water when it's really cold out) but don't expect more than ~70-75% net efficiency out of any non-condensing unit, and if you get 20 years of service out of it you're doing well. With heating loads higher than that you'd need a bigger burner/higher cost tank, and you'd get better net efficiency out of a small cast-iron or copper tube boiler for similar cash investment. But if you go the tank route, use a heat exchanger for the heating loops, keeping the potable water separate- open systems are an open invitation to health problems, and you're at the mercy of whomever is controlling the quality of the incoming water.


http://www.radiantcompany.com/images/newstdclsd.jpg

In NJ the peak loads can still be high despite fairly modest averages. Unlike CA, NJ is sometimes subject to continental air flows from arctic regions. I'd only go with a standard tank HW heater combi as a radiant solution there to support a single zone( or a VERY small house) and then only if the main heating system couldn't be used.

Dana said:

You can spent a whole lot of money for a higher efficiency condensing system and save at most $100-150/year in ~4800 heating degree-day climates like NJ.

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Is there any comfort advantage to a mod/con boiler or is the modest cost savings the only benefit?

Closed heat exchange systems are always the LAST bad choice

Dana said:

Open systems are always bad idea, and not even legal in many states. (In MA, they're legal only with control provisions to circulate the water 5 minutes out of every hour 365 days/year to limit stagnation issues.) I'm not sure what the legal status is in NJ, but it's a recipe for water-borne health issues- if not this year, sometime down the line.

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Dana said:

Go to RADIANTEC website and read about their preferred open loop system. In my not SHO, and based on many systems myself and thousands have done, this Voodoo about water borne illness is absurd. Absolutely off base and without merit. Water is always, automatically circulating through the loops,winter and summer. Does the mother of all world states, the oh so great MA. know more than the rest of the world? And tell me, in great MA. when gramma on chemotherapy goes to the upstairs guest bath in the far corner of the house that was visited a year ago, and takes a big swig of that fine closed loop "clean" water that sat in the pipe for all that time, what does she get? A big dose of regulated MA crud water that might kill her as she stands. Do they regulate the standing time of water in normal dead end plumbing systems?

You want to get sick and adore the fine Toto throne for a week or two? Just wait for the day when your inefficient heat exchanger leaks [a definite, given event] its closed loop slop silently into the regular system. Your intestines will try and wrap themselves around your knees. You'll need a 7 hp pressure washer in the bathroom - forget Terry's wash-o-let!

Got a house thats gonna freeze solid? Do a REAL closed radiant system all on its own with no exchangers and no connection to the potable and fill it with pipe compatible anti freeze.

I also see ast least 6 valves out of the 9 in the radiant company schematic that are not needed.

I think the guideline must remain KISS, or you end up with a SNAFU

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Lightwave said:

Is there any comfort advantage to a mod/con boiler or is the modest cost savings the only benefit?

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It's all just in fuel savings- the comfort from radiant the same regardless of the heat source or it's relative efficiency.

Ballvalve: "Representative system" schematics are never to be taken literally, all systems need to be designed, not hacked. I've never been impressed with either Radiantec or Radiant Company (or any other internet vendor of canned DIY radiant systems). Combis running off hot water heaters isolated with heat exchangers are neither complicated nor expensive. Radiantec's love affair with Polaris is cute & all but the notion that a 100K burner & condensing heat exchanger is the best or most efficient for the money is laughable, as is their stress on it's "95% efficiency", which is can only be true when the return temp from radiation is under 90F, which would require a pretty big delta-T on any staple-up. They're also obfuscating the difference between steady state thermal efficiency, EF, and AFUE. As-used in a staple up heating system with a design day heat load of 35K that requires 140F water, without doing the math I'd hazard a Polaris would pull 86-88% AFUE, best-case. In a radiant slab it could beat 90%, but it'll never hit 95%. Between lower modulation and outdoor reset control for a higher condensing fraction you'd do better with a cheaper, smaller condensing boiler like the smallest Munchkin Contender + indirect for the hot water, and that's with either slab or staple-up. (It's not rocket science to design or build with mod-cons, and they meet code everywhere, as do much cheaper hot water heater combis with isolating heat exchangers when the design-day heat loads call for a much smaller burner.)

But for a single zoned slab system for somebody who either doesn't know how to or is unwilling to design a system, I s'pose there are worse options than a one-pump one valve Polaris powered system, eh? ;-)

BTW: Leaking heat exchangers are far from "...a definite, given event..."- where are you getting that? (That's even more voodoo than overblown Legionella fears, methinks. )

Plumbing with previously treated water staganating at less than 75F temps has very little bio hazard. Plumbing that runs between 80-115F (typical shoulder season or summer heat radiant tubing temps) has some. It doesn't cost a lot to render the point moot, and it's a point that can come up at resale time, whether an actual hazard exists or not. The advantages of open systems are so slight as to be irrelevant, even where legal.

I may have mis-remembered the spec on what constitutes a legal "open system" in MA- don't count on the details as accurate. (I've never had an impulse to go the open-system route, never done it, probably never will. There's no point to it, really.)

It sure reads like you drank the Radiantec Kool-Aid though (and I'm glad it's working out for you), but it's a pretty silly thing to recommend right out of the gate for a rehab job on a house in NJ where they might not even be legal.

I never drink anyones Kool-aid. Using Jim Jones as a mentor, I always mix my own after reading a dozen recipes. I present those "links" as a means to see the schematic. I have never purchased a part from the noted parties; always built my own manifolds, and specified and installed my own collection of mechanical parts. I am no benighted lover of the Polaris, except that when I installed what was likely the first radiant slab system in this county many decades ago, with polybutylene pipe, that was the only thing on the market trying to make a dent in energy waste. [and still no problems with the PB pipe]

I did a few with the "Nautilus" a little cheap brother to the Polaris, that still work well. With natural gas prices falling,a standard water heater or two would calc out better. And you are probably correct to say the Polaris is over rated and cheating on its specs. They also made some units that blew up several houses, but even Toyota can do that.

All heat exchangers fail, as do polaris internals eventually. I simply advocate absolute simplicity in the mechanicals. The new mini nuclear self regulating and inherently non melt-downable plant designs I would compare to the open system VS. the closed, electric valved heat exchange designs that looks like a Chernobyl to me.

Simple can be good, and if the room-to-room radiation balance is near-perfection, running it as a single zone primary/secondary using an ErgoMax or TurboMax as the hydraulic separator the controls are dead-simple (a 2 pump, no valves system.) And a medium to low mass boiler behind it it'll downright kill any non-condensing non-modulating hot-water heater based system on efficiency. Iif the radiation balance ISN'T good, micro zoning with either additional pumps & 1 checkvalve per pump or a zone-valving scheme carries no efficiency penalty, where it otherwise would if un-buffered.

Polaris isn't cheating on it's steady state specs- it's just an irrelevant spec. When used strictly as a hot water heater it only scores an EF of 0.84 (84% average efficiency). Steady state thermal efficiency with continuous flow of 40-50F water entering is a useless spec for a heating system, and barely relevant to a storage-based hot water heater, since in real use it only sees the cold-flow at the heat exchanger while the water is running. During recovery as the temp rises the combustion efficiency falls. The raw combustion efficiency is highly dependent on the temperature of the water entering the heat exchanger. With natural gas exhaust the cold end of the HX has to be ~90F to hit 95% efficiency, and at 120F (the minimum you'd ever keep a hot water tank) it's already fallen to ~88%. And that's just raw combustion efficiency, no disounting for jacket losses, etc. If your staple-up needs 140F water and returns 120F water, you simply can't hit 90% with a Polaris if it's oversized for the load (which it almost certainly is). If your heating system needs 150F water and returns 130F water, there's very little benefit at all to a condensing appliance at all unless you can lower the temp when the heat load is lower, but with a hot water heater based system you'll never really want to run it colder than 125F, with a ~100-105F return, which yields (best case) ~92% in raw combustion efficiency (which is about what you'd see with a mod-con and an ErgoMax at normal hot water temps.)

But with a modulating condensing boiler and a buffer tank as the hydraulic seperator (using an indirect tank as a seperate zone) you can spend most of the season at very low temps and hit 95%, even in a staple-up. A mod-con and a slab (no buffer) + indirect can hit the high-90s.

But a key component to getting those efficiencies is to size the burner appropriately to the load, and unless this is the leakiest least-well insulated 1800 foot house in NJ a Polaris is oversized, and would only squeak mid-80s efficiencies by virtue of it's thermal mass (it's self-buffered). If there's already a functional 80%+ boiler in place, for the price of a reverse indirect and another pump it'll perform nearly as well as the Polaris (even if it's a 4-5x oversized behemoth) spending may be $100 more/year, and within $200/year of a pretty-good mod-con in a 4800HDD climate. The thermal mass of the water in ErgoMax providing the same cycle lengthening function as the mass of the water in the Polaris reducing the hit from oversizing. Oversizing still hurts some, but it is no longer an efficiency-disaster. Similarly, with slab-radiant the thermal mass of the slab itself can mitigate a world of ills, but staple-up retrofits are inherently low mass, and may need a bit extra to keep cycling losses low with on/off burners (even when "right sized"), which is why reverse-indirects are can be a good solution for hot water + heating with a single boiler. If the heat load is low enough a standard hot water heater (~78-80% raw combustion efficiency) can handle the load, but the standby losses will still limit it to mid-70s efficiency- a buffered somewhat oversized 80% AFUE boiler will meet or beat it on efficiency and provide far more hot water capacity.

But every case is different, and "design by web-forum" is just plain lousy, fraught with unforeseen hazards. But knowing the options when sitting down with a real designer (who is on the hook for the results) can help.

Sounds like Polaris would be a good unit if one could regulate its BTU output to the needs of the system. Does anyone know if a Polaris can be downsized with different gas jets?