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radiant heat

by Creede Fitch last modified Dec 10, 2012 11:50 PM
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radiant heat

Posted by Creede Fitch at November 20. 2004

My wife and I are thinking about [url href=]building a home[/url], and we would like to have radiant heated floors. The home will probably be a two story, slab on grade, sub 1200 square foot home. I love the idea of the tankless water heaters, and wondered if you could use one for both the heating, and hot water. Anyone have any experience?

Re: radiant heat

Posted by Gregory La Vardera at November 21. 2004

Radiant heat works just like a circulating hot water loop for domestic hot water - a loop keeps your hot water always at the tap so you don't have to wait for it to reach you from the water heater. You can use high efficiency water heaters to supply the hot water for a radiant loop, but it can't be a tankless type. You rely on that reservoir of hot water to draw your circulating water from and an on demand water heater does not work this way. A tank water heater heats a lot of water slowly and keeps it at that hot temperature, a tankless heats a little bit of water very quickly and does not store any.

Did that make any sense?

Re: radiant heat

Posted by DG at November 29. 2004

I hate to disagree with Lav since he is such a great resource on this and many other boards, but I have done an awful lot of research regarding radiant heating for the house we will be building this spring, and we will be going with a tankless heater to provide the radiant heat and domestic hot water. But there are a bunch of things to consider how you put it together, heat loss calculations that take into account the volume of the room, floor materials, windows, etc. We are having ours engineered to make sure we get all of this stuff taken into account and the zones and loop layouts done properly.

Just some more quick points to address from Lav's post:
- It's not always the case that you have to continually circulate water through the loops. You can run the system so that circulates as the heat is needed to keep the slab within the desired temp range. If you choose to design the system to run in the latter way, a tankless heater will work fine as long as the floor has fairly substantial thermal mass.

- A storage tank might helpfull to have as a capacitor or backup if you go with a less powerfull tankless or less thermal mass in your floors. But in this case you should probably be thinking about a high efficiency tank heater as Lav suggests.

Takagi makes several units that can be used in a radiant heat domestic hot water application. We are most likely going to use their 'Mobius' model. Their site has couple of pdf's showing a basic plumbing diagram for such a system.

Re: radiant heat

Posted by Gregory La Vardera at November 29. 2004

I stand humbly corrected and I appreciate hearing about your system. There are more possibilities than I was aware of and I'm glad it inspired you to speak up!

Re: radiant heat

Posted by Splatgirl at November 29. 2004

Check out this link:

I'm using radiant for whole-house heating in our new home. I suggest you do all the research you can on your own first. My experience has been that local contractors aren't as knowledgable as one would hope about the various options and products out there and will definitely try and sell you on whatever is easiest and most profitable for them, even if it's not what works best. Also check your local building code. What you've described would likely be an open system (where heating and domestic water mix versus a closed system where they are kept completely separate) and my understanding is that open systems are not allowed in all areas. I believe, however, that there are boilers on the market that would supply both domestic and heating water in a closed system.

Re: radiant heat

Posted by Joe Barthlow at November 29. 2004

I am not sure if combo units are the best choice. I don't think I would want to heat my home with a tankless water heater. I know the Takagi product mentioned, and it's a good one, but the radiant people who I know of aren't sold on it yet. I would suggest a tankless for domestic water and a medium-high efficient gas/propane boiler for the radiant panel. Check out Raypak boilers (low cost) and Laars-Teledyne boilers. I got a Raypak for $1600 and had a plumber install it for $1000. Works great!

Re: radiant heat

Posted by Gregory La Vardera at November 30. 2004

It seems like it would be possible to make a tankless unit part of a closed system, no? I am a little bit squeamish about the thought of mixing heating water and domestic water as well.

Re: radiant heat

Posted by Creede Fitch at November 30. 2004

One aspect that I really like about the open system, is that in the summer you can use it to passively cool your house. Every time you run the water it will pull cold water through the floor, which keeps it from stagnating, and cools your concrete slab.
I love the idea of tankless, I guess I need to read a little bit more on them, I'm just not sure it is best for a heating/domestic system.

Re: radiant heat

Posted by Joe Barthlow at November 30. 2004

as long as your slab is exposed, or you use a solid surface flooring, like VCT, ceramic tile, or a stone, your slab will be comfortably cool in the Summer. Be sure you get good cross ventilation too. Make sure you home is sited well and takes advantage of roof overhangs and proper landscaping. It's amazing what nature can do to heat and cool your home if it's designed well!

In the Northwest during coldest parts of winter, I run my boiler for two hours in the morning and three hours in the evening, while taking advantage of southern exposure during the day. Yes, believe it or not, we actually get some sunshine in western Oregon during the winter ( :

Re: radiant heat

Posted by Rich Carlson at December 03. 2004

I'm new to this group. Actually stumbled upon it while googling for takagi.

Wife and I are building a passive solar home in the spring, using radiant floor heating for backup. Pretty sure we will be using an MZ Boiler -- -- but we're still exploring other options. The MZ Boiler will handle both heating and domestic hot water using separate loops.

Re: radiant heat

Posted by Creede Fitch at December 05. 2004

rcwild, welcome to the forum. I would love to hear about your experiences, and how everything works out. I was thinking about using a tankless for radiant floor and domestic water, but I wasn't sure if it would be sufficient. What are the price differences between tankless, and a boiler?

Re: radiant heat

Posted by Kevin Dickson at December 05. 2004

There is enough info at The Wall to keep you interested for days.

Re: radiant heat

Posted by cdrmemphis at December 17. 2004

creede- I'm so excited for you and your wife building your cool house. My husband and I have discussed something similiar for awhile. Our trouble is finding a lot within the city of Memphis (that's where we live). We are searching and hope to build small, modern, and effiecient in the near future. Keep us posted on your house. Is it architecturally designed? Pre-Fab?

Re: radiant heat

Posted by Creede Fitch at December 21. 2004

Architectually designed. I have a friend who is an architect, we are looking at splitting a lot. We are in the process of trying to find a lot still. Easier said than done. Good luck to you.

Re: radiant heat

Posted by Lou Reynolds at December 23. 2004

Has anyone in this forum tried an outdoor woodfired boiler to supply hot water for radiant heat? A couple friends of mine in Wisconsin had systems and they worked great. I have heated my various homes for 20 years with woodburners, and I would now prefer to keep the wood burning outside (if I want the ambiance of a wood fire, I'll build a campfire).
There are some issues with smoke and wood boilers (bad design, bad wood, bad operation). Does anyone have favorable experience with any particular wood boiler?


Re: radiant heat

Posted by Eric Smith at March 13. 2007

Retrofitting a House with Radiant Heat

1. What do I need?

To properly size most components related to your project we highly recommend a heat loss calculation for your project if this is your primary heat source. Why? Heat loss is critical step, as we can estimate the average output of a radiant floor at 18-25 BTU’s per square foot but windows, doors, insulation, and degree days all make a major impact on getting you just what you need.

Taking the time to do a heat loss calculation will pay for it self many times over. The most common sizing mistake is in oversizing. This not only makes the new system cost more to install, but also forces it to operate inefficiently, break down more often, and cost more to operate. Oversized heating equipment also often creates uncomfortable and large temperature swings in the house plus it will short cycle the boiler and run outside the design parameters costing you more money.

We are not in the business of selling equipment that you don’t need and a little work up front can save you thousands of dollars in costs over the life of your system.

2. How do I calculate my heat loss?

  1. Have your architect or builder supply it to you as in many states like NH or CA it is required.
  2. Calculate it yourself using software like HVAC Calc which sells for $49.00.
  3. Or use one of the three different rough guides below.
Insulation Type and Climate Zone
(NOTE: We highly recommend that you do a heat loss calculation and provide the information below as a starting place)

  1. No insulation in walls, ceilings, or floors; no storm windows; windows and doors fit loosely .... 60 to 100 BTU’s per Sq. Ft.
  2. R-11 insulation in walls and ceilings; no insulation in floors over crawl spaces; no storm windows; doors and windows fit fairly tight .... 50 to 60 BTU’s per Sq. Ft.
  3. R-19 insulation in walls, R-30 in ceilings, and R-11 in floors; tight-fitting storm windows or double pane windows .... 29 to 35 BTU’s per Sq. Ft.
  4. "Energy Star Rated" house with R-24+ wall insulation, R-40 in ceilings, and R-19 in floor; tight-fitting storm windows or double pane windows; vapor barrier sealed carefully during construction .... 20 to 25 BTU’s per Sq. Ft.
  5. SIP or Earth-sheltered house with little exposure; argon filled windows, and R40+ insulated .... 10 to 15 BTU’s per Sq. Ft.
Climate Zone
Heating Sq. Footage by Climate Zone for a pre-1970's House
Houston, TX
Los Angles, CA
St. Louis, MO
New York, NY
Minneapolis, MN
15 - 25 Btu's
per sq. foot
25 - 30 Btu's
per sq. foot
30 - 40 Btu's
per sq. foot
40 - 50 Btu's
per sq. foot
50 - 60 Btu's
per sq. foot

Out Door Design Temperature

The Outdoor Design Temperature (ODT), also referred to as the 2.5% design day temperature, is not the coldest day ever, but rather a temperature that is achieved 97.5% of the time.

ODT Chicago = - 8°F
Denver = 1°F
Minnesota = -12°F
Washington = 17°F

Simply multiply the appropriate factor above by your home's total heated square footage to arrive at your approximate required heating capacity. For example, if you live in Zone 3, your home is well insulated, and you have 2000 heated square feet, the equation will look like this:

2000 square feet of "Energy Star" grade new construction but with lots of windows =

35 BTU's per sq ft.

70,000 Btu Load

Then, to calculate the output on a gas boiler, multiply its efficiency rating by its listed input rating for the actual Btu output of heat. For example, if a boiler has a listed input rating of 100,000 Btu's and an efficiency rating of 70%, it will produce:

87,000 Btu input of Embassy BMS 10/20 Boiler

X .86 efficiency =

73,000 Btu actual output

3. Existing Heating System

  1. All boilers sold in the U.S. must have a rating plate. Check the rating plate and get the:

    92,000 Btu Input of your Boiler
    X .80 efficiency of your Boiler
    73,000 Btu actual output

  2. Count the total linear footage of baseboard in the home. Multiply this number by 600BTU’s. This will give you the BTU output at 180°F. This number should be close to the Boilers actual output.
There are a few ways to calculate heat loss. Use the above information to get a rough idea. We strongly recommend that you download a heat loss calculator. Why? Because windows and doors make a huge difference to the heat load of your home. Once you have an idea of your requirements, we will be able to get you a quote.

4. Ways to install Radiant PEX with an existing floor

  1. Under floor with staple up with heat transfer plates
  2. Under floor with PEX with slide brackets
  3. Over floor with Warm Board, over the sub floor with 3/8 inch tubing
  4. Over floor with sleepers made of 2X4 with tubing and mortar over the sub floor
  5. New pour radiant Slab
  6. Thin pour over existing concrete

5. Radiant Slab on Grade

For residential slabs we recommend 1/2" PEX tubing to be 12" on center. Along walls with lots of glass or high heat loss the PEX should be 6" to 9" on center on the outside walls for the first 2 feet, and 12" on center everywhere else.

When figuring the over all length of tubing you will need you divide any 6" spacing area by .5, divide any 9" spacing area by .75 and any 12" spacing area by 1. This will give you the over all length of the PEX needed in the slab. You will need to add the length of tubing needed to get up to the manifold.

Typically manifolds are mounted 18" to 24" off the slab.

6. Installing PEX

Following good piping practices the maximum length of each 1/2" PEX tubing run should be no longer than 300 feet (300-foot maximum is code in many places). When the pipe loops exceed 300 feet you need to use larger circulators (pumps) to maintain this temperature drop. With larger circulators initial cost is higher and they usually require twice as much electricity to run. Most good radiant installers try to limit piping loops to below 300 feet.

There are many correct ways of installing PEX within a slab. The best way is tying the PEX to the reinforcing mesh or rebar. When attaching the PEX to wire reinforcing mesh or rebar it is recommend that a zip tie be used every 2 feet of PEX.

Another way to install PEX in a slab is attaching the PEX to ridged insulation. The use of insulation screw clips or large plastic staples is common.

We recommend an insulation screw clip or staple every 2 feet if installing the tubing over insulation only (no wire mesh). If you use 2" polystyrene insulation it is recommended that you use a 6 mil. polyethylene moisture barrier.

Installing the manifolds and keeping the lines under pressure (air or water pressure) for the concrete pour is highly recommended and required by code in many locations.

click here to review: Slab Packages

click here to review: Slab-Shield Insulation

7. Insulation

Insulation is always needed with any radiant application and especially needed under slabs. Why, if the soil has any moisture in it the moisture will wick away the heat at a tremendous rate making your system inefficient.

Today many radiant slabs being installed with insulation only around the perimeter. Their belief is that you should store the heat in the ground for use later. One problem with this notion is that a large portion of the heat is absorbed into the ground and never warms your home. Why do you want to pay to heat the ground?

We recommend Slab-Shield insulation which was designed specifically for under slab applications. Manufactured using two separate layers of 1/4" polyethylene foam with a pure aluminum center. This product is available in 4' x 63' rolls for easy application. It is simply unrolled and taped together (this is necessary for a complete vapor barrier to be achieved). With Slab-Shield there's no time wasted installing 4' x 8' foam boards. With a puncture resistance of 92.9psi you can work and walk on top of it without it crumbling apart.

8. Here's a rough idea of what it will cost

Below are some pricing guidelines. These numbers are higher than most proposals, but can act as a "stand-in" as you're creating your construction budget.
  • High Efficiency (87%+) boiler: $2500 to $4500
  • Tankless Water Heater as a heat source: $1200 to $1700
  • Per zone controls: $250.00 ea. zone
  • Slab on grade Radiant: $1.20 per square
  • Wood floor Radiant: $1.70 Per square
  • Myson radiators: $260 per 5000 BTU
People consider radiant heating for its superior economic and comfort advantages. But with energy prices rising 35%+ this year, whichever efficient system you choose, you’ll appreciate the cost savings!

Re: radiant heat

Posted by rose at December 10. 2012

Actually I am here to get knowledge about How can I Buy my best home heating system?
You mention very nice link for find out best place for this purpose.
So thanks for sharing this link with us...

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