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Radiant Heating System Design

I looked into it a little bit back when we were first designing the house but not very seriously. How much would a geothermal system cost and how much would it save me?


  • Dave_4
    Dave_4 Member Posts: 1,405

    I need some advice. I am currently building a home (I am the owner) that is three levels (basement, main floor, second floor) and will be heated with radiant heat. The home will have 15 zones with a total of about 10,000 feet of piping (the main floor and the second floor use warmboard and the basement has tubes in the concrete). The heat load for the whole house is 185,000 BTU/hr on design day. I also intend to heat my domestic hot water with my boiler (I'm thinking of using a 90 or 120 gal tank).

    My question is what brand and size of boiler would be best suited to comfortably and efficiently heat a home like this? I am thinking that I should use two boilers that put out a little over half the required BTUs for my house for redundancy purposes (I still want some heat if one goes out). I am currently considering the Knight boiler and the Vitodens. I would be interested in hearing any suggestions (including that you think I am going in completely the wrong direction, if you think that) that will help me to get the most comfortable and efficient design.

    By the way, I am not doing this myself. I just want to be as educated as possible so that I can make a good decision in choosing the equipment and contractor to install it.
  • A/C?

    Will you be cooling the place as well? Where (what state) are you located?
  • Brad White
    Brad White Member Posts: 2,398
    If natural gas is available

    it would be hard to beat an array of Vitodens 200's.

    For 185,000 BTUH, the 11/44 model has an input of 172 MBH and an output at 92% of 158.4 MBH. The turn-down takes you to 55 MBH input or about 50.6 MBH output, probably way to high on a mild day and too much boiler if paired.

    My optimum would be two 8-32's each with inputs of 124 MBH and outputs of 114 MBH. That is a comfortable "2/3" sizing that will allow modulation down to 37 MBH input on a warm day.

    A pair of the next size down (6-24's the smallest) might cut it too close. I would go with a pair of 8-32's to allow good lead-lag control and reliable standby and plenty of DHW generation capacity.

    Now, if you have reasonable electrical costs, you could supplement your hot water and allow summer cooling with a ground-source water to water or water to air heat pump. If water to water, you can make chilled water in summer and run that to fan coil units, possibly use the waste heat to pre-heat domestic hot water. In winter,make low temperature hot water, perfect for radiant. That can also cover your heating needs in the spring and fall when your lowest turn-down on the boiler is still to much. Save cycle wear on the boilers you see.

    You have options, plenty of them.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • dblagent007
    dblagent007 Member Posts: 31

    The house is in Utah and I will be cooling it in the summer. The cooling system I am going with consists of three Trane variable speed air handlers and variable compressors (total is 9 tons of cooling).

  • Providing you can find someone willing to work with these. There are only two intalled in the entire USA. They're not cheap but offer a lot of advantages. In your case two would be right and they offer a Direct Digital Control (DDC) that operates them like a plant as opposed to separate units. You would eliminate the direct expansion cooling and use chilled water instead. No heavy amperage necessary for cooling, to start with. The US branch is located in Evansville, Indiana. They may be able to find you a local distributor.


  • Have you considered Geothermal? This is a huge load; savings could be quite significant, and you're using all low temp heating.
  • dblagent007_2
    dblagent007_2 Member Posts: 6
    Okay, what are those things

    I have looked at the site you linked to, but I can't figure out how those things work. I think I understand that on the heating side they operate much like a regular boiler. However, how do they cool my house? Does air pass over a coil of chilled water thereby cooling the air, which is then distributed through the house? Also, do these systems have a built in air handler or do you have to buy that separate?
  • They...

    heat and cool water, you then ship the water via pumps and tubes to air handlers or floors or radiant panels or anywhere else you'd like. I wouldn't put chilled water through a floor though. Not yet anyways. They are nothing like a boiler. What they have done is taken conventional ammonia based cooling and developed a way to reverse it and make it a heat pump. You can get straight heat, straight cool or reversible heat AND cool models. They are also either air source or water / geo source. The air boasts an average efficiency of 126% and the water source about 140% in heat mode. Yes, that means they put out more heat than the amount of fuel introduced can produce on it's own. They do that by extracting heat from the air or water source and adding it to the heat produced by the nat gas burner. Very slick stuff. The cool side runs at about 14 SEER per Robur, you are burning nat gas to cool as well and, theoretically, with gas costs down during the summer becasue of low demand you should see savings there as well. They only require a 200V/15 amp circuit and draw ~4.5 amps when running. One drawback is if you put in the reversible HP you will not be able to use it to make domestic hot water in the summer, because it is cooling and making cold water not hot. We just put in a high efficieny gas water heater on our job. There IS one model that will run heat and cool simultaneously but the distribution system needed to make it all work is probably a bit cost restrictive. That system is more for factories where they can pull heat from one area and transfer it to another. Which is also pretty slick.

    Air handlers are separate but only need one coil and one set of tubes. The warm and cold water travels through the same tubes. You just size the A/H to satisfy the cool load and they are then more than adequate to heat. I'm running the whole system on 110* water and the air out of my A/Hs feels hot because they are basically oversized for heat.
  • A little more...

    Let me tell you what I did with the house, sounds very similar to what you are describing. I heated the slab in the basement, no insulation between the basement and first floor. Then I only tubed traffic areas which would preferably be ceramic tile, or stone. I don't like doing under hardwood or carpet. So I did kitchen, bathrooms and foyers/entranceways. I did it strap up with no transfer plates. Foil-foam-foil underneath forming a 1.5" deep void. No problem holding them at 77* all winter with 110* water. The A/Hs pick up the rest. In our case we put 6 A/H: 1-3 ton and 5-1.5 ton. 3 floors and a bonus room. 9,000 sq ft. This house has the sprayed in insulation and superwindows so I can't claim yours will perform the same but the numbers we've seen so far are very impressive. The gas company guy told me he did a double take when he saw their gas usage for last winter because he couldn't believe it. He thought he read it wrong. Another drawback, the air source only works down to -20* ODT then it shuts down! It's efficiency also varies with outdoor temp, it ranges anywhere from 140+% on a 45 or warmer day down to 100% at 0* ODT. The water source claims a consistent 140%, I've read it can go even higher but I don't want to make any claims to that effect, I've got no proof.
  • dblagent007
    dblagent007 Member Posts: 31

    Rob, since you posted this I have done a lot of research on the issue and here is what I have found.

    First a little background. I am insulating my home with closed cell spray foam so that it will be very tight. I also used a third party company to do the heat load calcs (EnergyWise out of Texas) because almost every HVAC guy I could find uses rules of thumb. EnergyWise guaranteed me that my utilities would average less than $486/month (I gave them my electricity and gas rates).

    I have talked to the geothermal guys and they are saying that I could probably cut my utility costs in half, but the cost to install all of the geothermal tubes would be about $30,000. If I am saving $243/month that means it will take about 10 years to recover the cost. However, the value of my $30,000 in ten years will be much more. Assuming that my $30,000 returns 5% (this would be a typical CD, but I can easily do better than that), then in 10 years it will be worth $49,000. Using this analysis, I don't think the geothermal will ever pay off. I am glad I looked into this more, but I don't think it will work out.

  • $500/month on that heat load? Something wouldn't seem to add up on a gut reaction there.

    what kind of degree days are you seeing?

    If you had a zero degree design temp, that's about 32000 BTUs/dd, or about 0.50/dd for $1.50/CCF of natural gas or gallon of propane.
  • dblagent007
    dblagent007 Member Posts: 31

    The winter design temp is 5 F and the summer design temp is 95 F. The price of gas is $1.10/ccf.

  • I guess it is in the ballpark. Not figuring any inflation in the cost of fuel?

    Not trying to push you into it, just exploring further.
  • dblagent007
    dblagent007 Member Posts: 31

    I have attached the heat load calcs for you to look at if you want. The assumptions that went into determining the utilities cost is also included in the calcs
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