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Radiant Heat

Glenn Sossin_2
Glenn Sossin_2 Member Posts: 592
There are lots of ways to approach heating a home with radiant, especially when the person asking for the quote is not knowledgeable enough to compare bids properly.

1.) Ask for the detailed heat loss calculations. If they can't provide it, they haven't done their homework. Run away hard and fast. I'll bet the low bidder can not provide it.

The detailed heat loss should list each room, the size and loss through each type of surface. For any given room, you should see a loss value for exterior walls, floors, glass windows, doors, skylights, ceilings and an infiltration value. Basically, you should see a value for any surface that will be exposed to cold air.

2. Ask how many zones - in theory, each zone would have a thermostat to control the room temperature. Think about how the home will be occupied relative to its size. If there are rooms that will be rarely used, it might be advisable to zone them so you can lower the temperature accordingly.

3. Is the contract/proposal providing an indoor/outdoor reset function. If you are heating the entire house with radiant, this would be a well advised option.

4. If the contractor is including the boiler, is it atmospheric or condensing. Again, for an all radiant house - a condensing boiler would be a well advised option.

5. Floor coverings - if the contractors didn't ask you to specify the floor coverings in each room, start running away from them. There is no way for a properly designed radiant heat system to work if specific floor coverings are not taken into account.

6.Is the radiant heating system using thermostatic tempering valves, or an injection system? You can think of it as a static system or one that uses feedback. The thermostatic mixing valve is a static device - another words it's fixed/set for a certain condition - the coldest expected outdoor temperature (outdoor design temp). So whether its 40F outside or -10F, that heating zone is provided the same hot water temp all the time when using a thermostatic mixing valve.

Is it a feedback system such as injection, or does it use a motorized modulating control? This means there is some form of temperature sensing occurring in the system to modulate either the boiler or the loop temperatures. Conceptually, the system is trying to create heat energy equal to what is being lost to the outside - the best scenario - prevents unnecessary cycling. The better systems will usually measure outdoor temperature as well as piping temperatures.

If you have multiple types of floor coverings, tile/wood/carpet, and your contractor says you can heat it all with one temperature water - I'd be very cautious.

Think of a room that's being heated with baseboard or a cast iron radiator. Now throw a sheet over it. It cuts the heat output right? Maybe you will have to run the system longer, or raise the temperature of the system water to a higher/different temperature to compensate for the sheet.

Now throw a lightweight blanket on it. Again, through common sense, you can recognize it won't heat the room as well, the system will have to run longer and/or run at higher temperatures.

Now throw your bedspread over it. Now we really have reduced the heat output of the radiator. This is what happens with the radiant floor. This is why we need to design different water temps, different tube spacing, different pex sizes, different pumps & different gpm rates in the loops. The effective type/r-value of your floor coverings will effect how much heat comes through the floor.

7 Thermostats - ask for spec/make/model information. Are their any with floor sensors? I usually find them desirable in a kitchen floor and master bath. In both of these areas, there can be external sources of heat that could prevent the thermostat from coming on - for example when your cooking, your creating excessive extra heat so the thermostat will be satisfied but the floor could be cool. In the master bath, you could be taking a shower, using the hair dryer, but the thermostat could be satisfied while standing barefoot on a cool floor.

8. Most important, check references. Get a list of the 3 most recent radiant jobs they did. Due your due diligence. Contact the prior customers and find out if they were satisfied. Check with your local better business bureau and consumer affairs departments.

Unfortunately, there are plumbing contractors who think since they can solder copper, and install boilers, this makes them capable of installing radiant systems. The actual product installation is different from designing a system. Much like a carpenter can install kitchen cabinets, he may not be able to design/layout a kitchen efficiently. While not necessary, if possible get an RPA certified designer/installer. This will be someone who has taken multiple hours of class room training and has passed stringent tests.

This is the link to RPA. They can also answer questions for you. http://www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=1

9. Installation methods. Know and understand the method of installation. I suspect the low bidder was going just suspend pex in the joist bays. While this is cheap and easy to do, it may not work where you have carpet or wood floors. This is where the heatloss design calculations come in. There are several common ways to install radiant heat - panel systems such as from Roth, Rehau, Viega, Wirsbo, gypsum pours, joist bay heating with different types of emitter plates, mud-jobs, etc. Make sure you know what your contractor is proposing for each area so you can accurately compare your quotes. There are advantages/disadvantages to each type.

10.) Know who is providing the insulation and the type. Radiant heat requires adequate floor insulation. Make sure someone has figured it into their quote - either the radiant contractor or you general contractor.

Don't feel like you can't ask questions of your potential contractors - it's your money your spending. Get educated - the more you know, the more comfortable you'll be with your decision.

I'm sure other people can add some suggestions to this list. The above should be a good start. Good luck on your project. The more you know, the better the decision you will make.

My $.02 + /Advice


  • Rick Regan
    Rick Regan Member Posts: 1
    radiant heat

    A friend, who is handicapped and working with a limited budget is installing radiant heat in a new construction Geodesic Dome, (his dream house) in upstate New York. A local radiant contractor with a 25 year track record was undercut by 40+% by a company called Radiant Tech who his GC is potentially brining in. Any input on this scenario?
  • Brad White
    Brad White Member Posts: 2,398

    There is a DIY type company by a similar name in a state between NH and NY, north of MA and south of PQ. I believe they do not do installation so is this apples to apples?
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • I agree with Brad,

    tell your friend to avoid all guesswork , it`s rampant in my area,,he`ll be sorry!

  • singh
    singh Member Posts: 866
    Tell him

    not to cheap out on his dream. It can turn into a nightmare.
    Don't let the GC do the install, go with a heating contractor.

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  • Rob Blair
    Rob Blair Member Posts: 227

    I agree. They supply materials only, and the tubing is odd sized, 7/8". I have bid jobs, materials only, against them with Rehau products and still been able to match or beat their number. Plus, I live nearby the customer.

  • singh
    singh Member Posts: 866

    you must type fast. Your post are always so well written, and detailed. It would take me a half a day to type that much. I do look forward to reading your responses.

    Anyway, I do have a question for you. You wrote:
    6.Is the radiant heating system using thermostatic tempering valves, or an injection system? You can think of it as a static system or one that uses feedback. The thermostatic mixing valve is a static device - another words it's fixed/set for a certain condition - the coldest expected outdoor temperature (outdoor design temp). So whether its 40F outside or -10F, that heating zone is provided the same hot water temp all the time when using a thermostatic mixing valve.

    Explain, my understanding is fixed type thermostatic mixing valves actually cannot provide a fixed supply temperature, as the heat load increases, the delta t return should also comeback cooler, therefore the fixed valve cannot adjust, and in theory should the supply temperature gets cooler, as we approach design day temps. I'd only use those in non - critical areas like a garage or small zone.
    Also, I'm no longer a big fan of injection, although great control of supply temps, I think needing a primary, secondary, and injection pump to accomplish that is a waste of energy.

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  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
    static versus feedback

    Singh, your correct to point out the mixing valve may not reach the water temp of design conditions. The point should have been that it tries to.

    If you are modulating the boiler temperature (indoor/outdoor reset), the tempering valve will rarely reach it's set point because design conditions are rarely reached. This could happen when the tempering valve is set to the same value as the high limit of the heating curve or boiler.

    Suppose the reset curve is set to 170F, and your tempering valve for a given set of radiant loops is set @ 120F (design temp for those loops). Now the tempering valve will always be able to supply water hotter than the loops actually need - potentially wasting energy, and increasing cycling.

    An easy way to demonstrate the concept of static and feedback systems is when you take a shower.

    Assume we start with a standard mixing valve. When you first start the shower, you are adjusting the position of the dial to provide the water temperature you want (set point).

    As the hot water starts to reach the valve, you start to turn the dial to left to get less hot water – you eventually find the right setting after things stabilize and you jump in. Now you’re taking your shower - everything is fine. A few minutes go by. All this time, suppose you're drawing water out of the hot water tank faster than it can recover.

    As the hot water is drawn out of the tank, the cold water goes in. The output temperature of the water tank starts to drop. In response, you turn the dial a little to the right to increase the portion of hot water, relative to cold water, thereby keeping your target temperature.

    The point here is, you operated the mixing valve – actually it's a proportional valve, - you changed the proportions by moving the dial back and forth. This simulates a system using feedback. You reacted to the change in water temperature (provided the feed back) and mechanically changed the position (proportions) of the valve.

    Now lets consider the same situation, this time with a thermostatic mixing valve (not pressure balanced). This is similar in function to the radiant mixing valve. It will open and close the hot and cold ports in an attempt to provide the target mix temperature. It requires no interaction from you. It is self adjusting – it reacts to the changing temperatures, usually mechanically through means of a bi-metallic element or some capillary fluid similar to a TRV. This would be similar in concept to an injection station, or a motorized 3-way,4-way valve. They adjust based on some feedback from the system.

    Keep in mind, the heat energy required to heat any structure is a function of it's heatloss. Under normal conditions (excluding heat gains), the heatloss is a variable based on outdoor temperatures, the radiant mixing valve output is fixed and therefore can not adjust to the varied needs of the system. That’s why it is typically set for the worst outdoor design conditions.

    I hope this answered your question and explained the difference between a static system, and a system using feedback.

    If you don't like injection, I will assume you think it's too complicated. You might want to look at Viega. They have an injection system which can operate off one circulator and does not require primary-secondary piping. They have a state of the art training facility and do an excellent job of training in this area. You might want to look into it.

    My $.02 +
  • Bob_28
    Bob_28 Member Posts: 7
    New Boiler/Water Heater

    Glenn, I think I learned more about radiant systems from your postings than I did from having a system installed in my new home and then living with it for five years. Really great info and I wish I had it when I was building.

    While my system has worked OK, I'm now faced with replacing the water heater that powers it. That's right...a plain old water heater. I've got a 100 gallon, 199K Btu water heater that provides hot water for my radiant heating (7,100 sq. ft.) and DHW for half the house. It's leaking and I'm replacing it with an HTP Phoenix condensing boiler/water heater (80 gal, 130K Btu). (see http://www.htproducts.com/products/phoenix/index.html.)

    I live in No. California with winter lows in the high 20's. My annual gas bill is about $2,700 and looking at seasonal variations, it appears that about $1,700 of that is radiant heating. Am I going to save enough through improved efficiency to offset the higher cost of installation on this type of unit ($12,900)? It's on order but I'm having buyer's remorse.


  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
    Sounds somewhat Steep

    Without knowing the particulars, based on the info provided, I may have suggested a modcon around 175-250K (depending on heat loss), a 65-80 gallon indirect and a 120gal storage tank.

    Basically, by having your entire system focus on this one appliance you are putting yourself at a higher financial risk. If/when this unit fails, you are looking at a costly replacement number.

    Assuming space isn't an issue, I like using an indirect and 1 or more storage tanks. They will likely outlive the boiler because they don't see high differential thermal shocks. If there is a product failure, the boiler can typically be replaced by a single mechanic in a matter of hours at a much lower replacement cost. No draining down and removing a 100 gallon tank. If you find you need more hot water, you can add a 2nd tank. In some of the jobs I design, we put in 2 - 120 gallon storage tanks in but valve one off. During a high usage event, like a party or family get together for Thanksgiving, you can open the valves on the 2nd tank. Personally, this is what I did in my home but I used 2 - 50 gallon tanks.

    I am not directly familiar with this unit but from what I surmise from the specs, it tries to reach a set point (target temp)you specify. Like the thermostatic tempering valve, this is a static fixed value that you will rarely need for heating, which is what this unit will be doing most of each day during the heating season. The difference is, this burner will modulate it's firing rates in an attempt to meet that target temperature (energy efficient). The problem is, the target temperature is fixed but the climate around your home isn't. This heater will try to make the same temperature water when its 50F outside as when its 20F –( not system efficient).

    Most modcons, will be equipped with an indoor/outdoor reset control. The control calculates a variable target temperature based on actual outdoor temperatures, as well as modulating the burner. They likely will have 2 target temps- one variable (based on feedback) for heating using the indoor/outdoor reset, and one (fixed) for DWH (domestic hot water).

    Given the size of your home, a separate modcon would have been my recommendation. As far as fuel savings, I don’t think anyone could answer that without a lot more information. It would depend on the time the heater spends servicing DHW demands vs. space heating.

    Don’t get too upset about this. Things could be worse. You probably have a beautiful home. Health, family & happiness are more important than your water heater. Enjoy them while you can.
  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592

    Brad Not that it will matter too much but where/what is PQ? Province of Quebec??
  • Bob Parrish
    Bob Parrish Member Posts: 3
    Thanks, Glenn...

    I appreciate you sharing your take on my situation. And while it's true that it won't be the end of the world if my purchase isn't economically optimal, I'd still like to think that it would be better than replacing the existing, low efficiency water heater with this unit.

    You described a modcon's use of an indoor/outdoor reset control. Isn't the unit that I've ordered a modcon? And, as such, does that mean that my installer should be installing a reset control?

    One of the other factors affecting my situation is that, while the house is large, we typically use only a fraction of it and the radiant system has 14 thermostats, allowing us to close off rooms that aren't being used. I guess all that argues for buying the cheapest possible system, doesn't it?

    Again, thanks for your comments.

  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
    Perhaps someone else knows

    I'm not familiar with this product. I downloaded the spec sheet but didn't see provisions for the typical TT type connection which could allow for a reset control like the Tekmar 256 to be wired in. On its face, I would say you can't add a reset control. I would suggest you have your contractor call Heat Transfer Products to inquire.

    I wouldn't be overly concerned. Sometimes we all go to far in search of the ideal system. You certainly will realize substantive savings over the prior year because its a modulating burner, condensing abilities, and if you control/pipe all those heat zones efficiently.

    Good luck on your project.

  • your unit is a modulating/condensing water heater, not a boiler. You don't want to put reset on a big tank of water. You use reset mixing out of the tank. Hopefully your radiant system can be heated with less than domestic temperature water?

    With this many zones, the buffer capacity of that tank will probably serve you better than worrying about whether you can drop the tank from 120 to 100. However, reset mixing to the rest of the system will typically improve comfort noticeably.

    If you have a 14 zone system, even if you turn down rooms, I doubt you want the "cheapest possible system".. you probably have a real heat load here and efficiency should be worthwhile.

    The phoenix will probably be just fine. though it is a fairly new unit, that's my only concern.
  • Bob Parrish
    Bob Parrish Member Posts: 3
    More good info...

    Rob, as I understand it, my radiant system is intended to use 85 degree water, suggesting that it employs a mixing valve as you stated. Thanks for clarifying things. BTW, you can see pictures of the existing system here: www.geocities.com/pics2send

    I'll put a note on my calendar to post an update on this forum in December, reporting on how my new water heater works once we start using the radiant heat in conjunction with a house full of holiday guests.

    Again, many thanks to those who shared their knowledge with me.


  • If you only need 85 degrees, skip the reset entirely. Just use the 3 way tempering valve set to 85. The water heater will most likely be set to domestic temp and would need to be in any case. use a heat exchanger though or some other form of separation if possible (maybe you are, I just mention it to be sure).
  • Bob Parrish
    Bob Parrish Member Posts: 3
    Thanks, Rob.

    Yes, the radiant system already has a heat exchanger. Thanks again for your help.


  • See RadiantTec


    As you surmise they seem to sell product packages on the internet.
This discussion has been closed.