Dear All,
Novice here re: radiant heating.
Building a new house (3000-3400 sq ft), and want to put in radiant heating, and have some general questions regarding device: boiler vs hot water heater vs combo with pos (point of service) heater.
Looked at Al Rossetto’s FHB #161 article re: “Vermont’s most energy efficent home”, where he has a continuous circ system (even in the summer) run off only a hot water heater.
My area: chicago suburbs. My goal: above radiant heat, but since I’ll end up putting ductwork up anyways (ac in the summertime), I’d like to use that hot water again by putting hotwater coils on top of an air handler. I think I’m getting as much efficiency as I can by using that heated water twice.
Step up or step down? As I understand it, radiant heating needs water at 110-120, but “hot water” out of the tap is generally around 120-140. So, should I have a whole lot (say, 100gal) of water at 110-120 (via boiler), some hot water (say, 60gal) at 120-140 (via hot water heater) OR
some hot water (say, 60gal again) at 120-140 (via the hot water heater) but becoming a whole lot of water (say, 100gal again) at 110-120 by mixing with cold water (step down approach) OR
a whole lot of water at 110-120 (100gal via boiler again) but step up only at the pos (those little devices under the sink) (step up approach)?
I know it seems like I’m overthinking the prob, but my architect proposed 2 hot water heaters (dedicated to potable hot water) and a boiler (dedicated to radiant heating). It just seems like an incredible amt of hot water for what I originally thought was a fairly highly efficient system.
Any help would be appreciated.
watty
PS: idea for heating coils came from richard groff’s article, “mixing forced air and boiler heat” in taunton’s energy efficient building, a compliation of articles.
Replies
Watty, you need to find a qualified, innovative and reputable heating contractor that knows his #### with radiant heat. He will need to know you floor coverings, whether slab, gypcrete, hose under wood subfloor, etc.
Here's what I thought was the ticket 18 years ago for our 2700 sf home. All ceramic on concrete first floor (hose in slab), hose under subfloor / ceramic floors in upstairs bathrooms, 2-5' pcs of salvaged HW baseboard in the two extra BR's upstairs. Open floor plan, Tulikivi masonry heater in great room for fall/spring heat and backup and extra heat on very cold days. Passive solar. Domestic HW run off boiler (tank heated by boiler). Boiler temp 160. Return line tempers the floor heat going out.
My thinking on the boiler mate domestic HW tank was that the boiler would operate year round, keeping it in good condition. Recovery on the domestic HW is very good.
Oh yeah, one 12,000 btu window air cond in up MBR which cools the whole house, so the construction and insulation of the house is very important too!
I have all zones on their own pump, you can use zone valves and less pumps. There are controls that "think" of the outdoor conditions and prewarn the controls on the inside. Smart technology when you have a slow response system.
Best of luck, sorry for the rambling thoughts, watching a bowl game..........
Remodeling Contractor just outside the Glass City.
Quittin' Time
Briefly, I think FA plus RFH is nice combination, especially as in your case when the ductwork is going in anyway.
FA by itself is fast response, breezy, and creates stratified temps in the house.
RFH, by itself, can be slow repsonse, creates no windchill and gives very uiniform temperatures.
The combination can give you quick response without much stratification. And, since the RFH carries the base load, you don't need FA as often nor for the FA to as breezy.
If you build a VERY tight and well insulated house, expecct about 0.1 BTU/F/hr/sq ft heat load. 0.1 x (70F- -40F) x 3400 sq ft = 37,400 BTU/hour. Or maybe you want to use -40F as a worst case heat load. But 37,400 BTU/hour is what as much or more than most HWH capture into the water (their gross rating is 40,000 or 50,000, but 15-20% of that goes up the flue). So one standard HWH is not enough to keep up with -40F outside, much less the transient need to get the slab to a higher temp.
It works in my house at 1700 sq ft. Leaving about 15,000 BTU for the transient load and to make potable hot water.
I make more, hotter water for a mini, site-built air handler. And put it through a tempering valve for potable and another, easily adjustable one for the RFH.
"My area: chicago suburbs. My goal: above radiant heat, but since I'll end up putting ductwork up anyways (ac in the summertime), I'd like to use that hot water again by putting hotwater coils on top of an air handler. I think I'm getting as much efficiency as I can by using that heated water twice."
You can do this, but I'm not seeing the advantage. If you already have radiant floors, why would you want to supplement with air heat? Sounds like extra complexity and expense for no value added. If you do not extract all of the heat from the water on one pass, the remainder is not wasted. It recirculates.
"I know it seems like I'm overthinking the prob, but my architect proposed 2 hot water heaters (dedicated to potable hot water) and a boiler (dedicated to radiant heating). It just seems like an incredible amt of hot water for what I originally thought was a fairly highly efficient system."
Pardon my bluntness, but that design seems silly, complex, and expensive all at once. A single ultra-efficient boiler or HWH should be able to support all of your needs. There are several very efficient HWH lines that are good for 200KBTU, which should be more than enough for a well-sealed home of your size. In the article you reference, they used a Polaris HWH which can be had in up to 200KBTU. Keep in mind that some of the techniques used in that article are very controversial, and not even allowed by code in many places. For one thing, mixing potable and heating water is very controversial. I do not recommend it. A single boiler with an indirect DHW tank is another excellent (and more conventional) option.
"Step up or step down? As I understand it, radiant heating needs water at 110-120, but "hot water" out of the tap is generally around 120-140. So, should I have a whole lot (say, 100gal) of water at 110-120 (via boiler), some hot water (say, 60gal) at 120-140 (via hot water heater) OR
some hot water (say, 60gal again) at 120-140 (via the hot water heater) but becoming a whole lot of water (say, 100gal again) at 110-120 by mixing with cold water (step down approach) OR
a whole lot of water at 110-120 (100gal via boiler again) but step up only at the pos (those little devices under the sink) (step up approach)?"
The conventional way is to "step down". Have the boiler or HWH produce the highest temp water needed at all times, then everything else mixes down as needed. This works great with any single HWH, or with conventional boilers that need to operate above condensing temperatures. Having 'point of use' step up would add complexity with little (if any) benefit. A newer alternative is to get a modulating boiler that can have its output temperature set dynamically, depending upon what is making the call for heat. This is really the best of all worlds. A modulating condensing boiler can operate at very low input/output temperatures with full outdoor reset for space heating, producing water at the exact temperature needed to match the heatloss of the structure based upon outdoor temps. When a high temp zone (such as an indirect DHW tank on priority) calls for heat, the boiler produces hotter water. This may eliminate the need for downstream mixing devices, resulting in a simpler system with fewer moving parts. Condensing boilers are more efficient at low temperatures, so dynamic setpoints save a lot of fuel.
2 suppliers are:
http://www.htproducts.com/products/munchkin/residential/index.html
http://www.viessmann-us.com/web/canada/ca_publish.nsf/Content/Vitodens200_ca_english