Xafshirt
Registered Users
Posts:2
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| 07/15/2008 9:42 PM |
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I'm new to this avenue of information. I am also in the middle of designing an ICF home with Geothermal and Radiant systems.
While considering the options I began to think about coupling an On-Demand heater input with a collection tank on the outflow of the radiant system that feeds the One-Demand to reduce the delta on heating the water throughout the system.
My thinking is that except for the initial heating of the slab this would be an efficient and cost savings method of applying the radiant heat process.
Your thoughts would be much appreciated.
w/r
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NRT.Rob
Registered Users
Posts:327
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| 07/16/2008 8:54 AM |
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if you have Geo, what is the "on demand" doing?
Geo wants a high temperature drop.. cooler return water means more efficient Geo exchange. |
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-=Northeast Radiant Technology=-
NRTradiant.com |
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BadgerBoilerMN
Registered Users
Posts:38
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| 07/16/2008 10:46 AM |
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Rob;
I think he wants to run potable water through the floor to preheat his DHW going to the tankless? Is this the case 'X'?
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MA
www.badgerboilerservice.com |
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Xafshirt
Registered Users
Posts:2
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| 07/17/2008 7:20 AM |
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My idea was a closed loop using the tankless WH as the system boiler and the storage tank as a reservoir to collect the return water to use it as the input for the tankless.
I got here after looking at prices for the water-to-water and water-to-air Geo systems. Considering the cost of the tankless system I thought that using it in this application would be a cost saver instead of using the water-to-water side of the geothermal.
I looked at a modulated natural gas system (which does basically the same thing I am talking about) and wondered if anyone had used a tankless electric system instead.
DHW would be handled with a different system.
I lived in England for 9 years and used the tankless systems there, they worked great and were a real money saver.
I hope I've explained myself a little better this time.
Thanks |
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NRT.Rob
Registered Users
Posts:327
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| 07/17/2008 9:23 AM |
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initial cost saver, yes, of course: anything is cheaper than Geo. but you are then heating with electric or gas instead of geo... not exactly equal, and you'll spend a lot more on heat.
If you want to use an electric boiler, that works too, if electric heating makes any sense for you based on electric rates.
If you want to use gas, you have two scenarios:
1. You have low enough heating requirements that a water heater would do the trick; if that's the case, use a tank water heater instead of a tank with a tankless.
2. Your heating requirements are high enough that a better heat source makes sense, then using a modulating/condensing gas boiler. |
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-=Northeast Radiant Technology=-
NRTradiant.com |
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jmagill
Registered Users
Posts:77
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| 07/17/2008 9:49 AM |
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I have a tankless used as the heat source for a radiant system. It works very well. I don't think there is any need for a tank.
I think what you need to consider is how much will the tankless run. My home is SIPs with a stained concrete floor. The combination of good insulation and mass means my system runs for a couple of hours at a time once or twice a day., during winter. This means the tankless is working as it should( periodically not consistantly).
If your system will be required to run off and on because there is no mass to buffer the heating cycle it might not be the best heating source for you.
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BadgerBoilerMN
Registered Users
Posts:38
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| 07/17/2008 10:34 AM |
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Tankless water heaters are great compared to a conventional water heater. But, they are typically can only reach 82% thermal efficiency. This is a low efficiency gas fired appliance. Unless it has a PVC vent you waste 18% of every dollar spent on fuel. Tankless water heaters are usually wall hung; saving on floor space, sealed combustion; saving on infiltration, direct vent; eliminating the chimney and quiet reliable. But they are generally not designed for heating application and require technical design considerations to be applied properly.
For instance; the tankless lacks outdoor reset; standard on Mod/Con boilers, as is PVC low temperature venting (unless it is a Navian or similar), the Mod/Con modulates the burner to provide heat as it is being lost, contributing to comfort, fuel savings and equipment longevity. The Mod/Con may have a built-in domestic hot water heater or use an indirect at efficiencies a conventional tankless can not achieve.
I would start your query with the cost of electricity per therm and the cost of gas per therm. Figure equipment cost, expected life and fuel savings over the life of the equipment.
In most cases the Mod/Con with domestic hot water wins the math test.
The time that the unit runs is based on the heat load and the size (usually too big) heating appliances output. Heat sinks are best left to passive solar systems.
As Rob rightly points out, electric (even modulating) boilers are made for the purpose if you have a competitive eletric rate. |
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MA
www.badgerboilerservice.com |
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NRT.Rob
Registered Users
Posts:327
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| 07/17/2008 10:42 AM |
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I personally doubt that the vast majority of tankless water heaters acheive anywhere near their rated efficiency in a radiant application.
low zone count, high mass slab systems, maybe. but everyone else, I find myself highly skeptical. |
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-=Northeast Radiant Technology=-
NRTradiant.com |
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kurt
Registered Users
Posts:7
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| 07/21/2008 7:43 AM |
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Hey guys, At what heat loads does it make sence to pay the larger upfront cost of a modcon plus something for dhw versus using a tank water heater like the bradford white combi for both space heat and dhw. It seems to me that you need a lot of fuel savings to recoup the costs. Would using a combi with a 80gallon solar preheat tank make any sence for a load of under 40K BTU with 3 people for DHW. This way you would get a solar benefit for the DHW and possibly some solar benefit for the space heating side of things in the shoulder seasons. What do you think?
My second question is what about the Navien tankless for heat backup. They claim to be 98% efficient, designed for heating applications, use stainless steel for the HX and waterways use PVC venting and cost about $1200 with builtin recirc, small buffer tank and operate at any flow. They make a pumping station for about $1500 called a heating box to distribute the HW and space heat in a closed system. It really looks tempting as a cheaper alternative to a modcon with indirect for use in a low BTU demand heating situation. It would be nice to find something that does the job cheaply and efficiently Thanks Kurt
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thuley
Registered Users
Posts:2
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| 07/21/2008 9:03 AM |
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Kurt:
Monitor makes a very efficient combination unit that functions as both a boiler and on demand domestic hot water heater.
Also you might want to consider Solar thermal depending on your location. We have just done a hybrid system that combines Solar and geothermal for heating and cooling.
Also with an ICF house you will realize a big savings in energy cost if you use a more efficient radiant system, we are using a new LEED certified in floor system that is heating 4000sqft houses in up State NY for around $65.00 a month. Remember designing the entire house as a system will yield you the most efficient and comfortable house |
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BadgerBoilerMN
Registered Users
Posts:38
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| 07/21/2008 9:06 AM |
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The Navien is a horse of a different color; raising the standard for DHW heating. I spec. the Navien and the Combi on a regular basis. The The estimable Combi-2 is low-tech (read, no maintenance) with perhaps a 10+ life expectancy as it is just an 82% water heater with a coil in it for heating. The Navien is one of the only "tankless" high efficiency water heaters on the market today and by contrast a machine designed to last 20 years. But high efficiency equipment should only be installed by factory trained personnel and must be professionally maintained (read certified tech every year or so}.
As you correctly point out, the smaller the load the less one can justify the added expense of high efficiency equipment, this would of course include all forms of solar in all but the sunniest (and Southern) climes. But there is more than one reason to conserve energy and sound business practice rarely dictates human behavior. So if your are going to splurge, why not invest in products that hedge against inflation, high fuel prices and cold homes?
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MA
www.badgerboilerservice.com |
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kurt
Registered Users
Posts:7
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| 07/21/2008 12:31 PM |
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| My situation is a 1500sf reasonably tight house with passive solar gains from south facing windows and a small epa rated wood stove. I also have four 3 x 7 solar panels kicking around. Although they aren't going to solve my winter heating woes they still will reduce my dhw loads for half the year. With these things in mind wouldn't I still get high efficiency by using the Navien cc180 after a solar preheat tank and then using the heating box or a HX to split the DHW from the space heating? |
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NRT.Rob
Registered Users
Posts:327
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| 07/21/2008 12:42 PM |
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that's $3700 kurt. not much cheaper than a mod/con w/indirect in that case, and I would be pretty skeptical of claims of 98% but I can't really check it out since their website isn't very detailed on the commercial units you need to use for heating applications (according to what their lit was saying, at least)
the tank heater vs mod/con debates depends on your heat load AND your heating degree day count. a house with a 50k max load in san diego will not use as much energy yearly as a house with the same maximum load in maine. max load is one snapshot of one period of time. the yearly curve matters just as much.
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-=Northeast Radiant Technology=-
NRTradiant.com |
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BadgerBoilerMN
Registered Users
Posts:38
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| 07/21/2008 1:13 PM |
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The Navien is a condensing unit and claims 98% thermal efficiency (not to be confused with the nebulous AFUE). In moderate climates the domestic hot water load (especially for tankless WH) is greater than the design-heating load, so if your water heater modulates or stores heat with a buffer, you are on the right track.
However, you still don't have outdoor reset, a critical component in my designs where comfort and efficiency are the top two goals.
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MA
www.badgerboilerservice.com |
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kurt
Registered Users
Posts:7
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| 07/21/2008 1:30 PM |
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| Hi Rob I'm getting a clearer picture now. I'm in Connecticut I believe we are around 6000 degree days. I too have questions about the Navien. They've been introducing this product for a long time and still don't have any real info on it. Not much is availability in this area anyway. If not a BW Combi, is there a small modcon with DHW available? Just one bath Thanks for all the info Kurt |
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NRT.Rob
Registered Users
Posts:327
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| 07/21/2008 1:44 PM |
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ah, "thermal efficiency". always liked that one. just wish I knew what it meant, really. You could say the same thing for AFUE too, though, I suppose.
Kurt; Triangle Tube Prestige Excellence is about the best all in one heat source for heating and small DHW loads I know of. super simple piping, buffer capacity, "on demand" hot water with 14 gallons of storage built in, reset, etc.... pretty sweet unit. for one-temperature radiant systems, you can just about slam a zone valve header on it and call it a day. |
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-=Northeast Radiant Technology=-
NRTradiant.com |
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BadgerBoilerMN
Registered Users
Posts:38
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| 07/21/2008 2:08 PM |
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| Mod/Con's with built-in DHW include the Baxi Luna and a similar unit the Mascot by Laars, not to mention NTI's Trinity. The Prestige w. Mini=Smart by Triangle Tube, though an excellent unit, does not offer "built-in" domestic hot water with a plate heat exchanger for a clean, one-box integrated heating and domestic hot water heating wall-hung boiler. One has to consider water hardness with any DHW application and keep it below 5 grains. |
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MA
www.badgerboilerservice.com |
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NRT.Rob
Registered Users
Posts:327
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| 07/21/2008 2:23 PM |
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You are incorrect Morgan. well, partially; the Prestige Excellence does not have a plate exchanger, it's true. It does, however, have a built in 14 gallon storage tank for DHW which is completely separate from the heating system for a clean, "one-box" integrated system. for any 3 GPM to 4 GPM domestic system (depending on ground water temp) it is a fantastic setup.
The Prestige Solo does not have built in DHW and requires the use of an external storage tank. But the Excellence does have it all built in.
we would not recommend the Baxi to anyone these days though we have used it in the past, and the Trinity is a decent low cost unit but requires a high head pump and primary/secondary piping, so some of the savings there is eaten up with additional trim items and energy usage will be noticeably higher on the electrical end. Plus it does not have the built in buffer capacity for heating or for DHW, so it's not truly equivalent. Just cheaper. |
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-=Northeast Radiant Technology=-
NRTradiant.com |
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BadgerBoilerMN
Registered Users
Posts:38
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| 07/21/2008 2:40 PM |
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| What would I do without you? The Excellence is an excellent choice, but you know I love choices. I never met a Mod/Con I didn't like. |
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MA
www.badgerboilerservice.com |
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