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For various reasons we've been considering adding hydronic radiant floor heat to some parts of our circa 1910 foursquare. Access to those areas is excellent as we have a full unfinished basement, so installation is not a worry for me, but instead efficiency. In the few places I've had to tear up a floor it seems to me that my old house has significantly more wood than most new homes, and I was wondering if the increased r-value would make radiant floor ineffective. I didn't measure my flooring when I had it torn apart, but from memory it seemed like my subfloor (fir or pine) was at least 3/4" or more, with my douglas fir flooring above it about as thick. Obviously if I install it I will heavily insulate below the tubing to increase efficiency.

Does anyone have experience retrofitting hydronic radiant floor heating into antique floors? If so, are you happy with it? Are there any dangers that I'm unaware of with this; such as the heat drying out/damaging the wood? Any knowledge would be greatly appreciated!

Jon

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You already know the BIG downside, over time the heat will shrink, check and generally screw the floor up. 

I was just guessing, I have no knowledge of this. Is there anything that could be done to prevent this? I assume the problem would be caused by the moisture level changes between a humid Iowa summer and then baking the wood dry in winter? What if the water temperature of the hydronic were slightly lower, such as 85-110 degrees instead of 120-140?

Dropping the water temperature would greatly increase the cost of heating the room and isn't a way to go given the current energy cost situation.

I've seen many attempts at hydronic floors with wood floors, including plywood glued to the joist, and have yet to see a really good system.  About the best you can do involves constructing a thermal slab on top of the wood floor.  This involves insulating with foil faced foam and then installing the PEX.  There is a gypsum product that works well for the slab, and ceramic tile can be directly adhered to it.  

The reason we are considering adding radiant floor heating is because we have an ideal location to install a solar collector on our property (that wouldn't even be too ugly!). If we are building a pre-heater for our domestic hot water, it is only marginally more expensive to size the collector to also act as a supplement to our gas forced air furnace, hence the radiant floor heat. We aren't really expecting it to heat our house with solar during the dead of winter, hopefully just keep the furnace off for a little while longer in the fall and spring, and maybe take the edge off in winter. Picked up a really neat vintage thermostat at an auction in a box of junk, and we would have the radiant floor set up completely independent of our forced air system.

With that in mind, and since the "heat" would be free (solar) I wouldn't mind losing some efficiency by running lower temperatures if it would save my floors.

Would lower temperatures save my floor, or will they still be damaged by the large moisture content swings?

Frankly I don't see your proposed plan as a winner in any respect since it amounts to basically converting your 1½"± floor into a wood drying kiln.  Wood by its nature is not a thermal conductor, and fastening PEX to wood in a manner that turns the assembly into a radiating surface is a task that has escaped everyone I ever saw try it.  Without 100% contact PEX will give off little heat.

Now that you add in the existing hot air heating system and solar availability, My suggestion would be to create a solar heated storage battery in the cellar or even in an insulated bunker next to the house.

Solar can give you 200° stored eutectic fluid 3 seasons of the year, with the only energy cost being circulation,  A modified A coil can be placed in the plenum of the existing HA furnace to deliver heat to the house from the storage.

Systems of this design were installed in Pennsylvania's ABE area in the late 80s.  They employed off peak electric at very low KWH rates to heat the fluid.  Solar would be an easy substitution for electric resistance heating in a storage tank.

Interesting idea, I will look into it!

Perhaps I'll limit my radiant floor dreams to the kitchen and bathroom, which will eventually be tiled...

Greetings Johnathan,

I would like to counter some of the other comments I observe in these posts regarding your (worthy) interest in underfloor hot water radiant heat. As you know it is the most comfortable heat ever developed and was used in villas in ancient Rome. Hydronic radiant heat is used extensively throughout europe with excellent results. Now, regarding your desire to employ it underneath existing wooden floors you should know that it is indeed quite possible to carry out. In order to ensure no drying or shrinkage occurs to your floors, the circulating water temperature should be limited to about 80 degrees F. Counter to the comments seen elsewhere here, there are numerous examples of this type of system in use in historic homes with good results. For you to go forward with your idea it is recommend that you consult with a professional engineer experienced in such work. He will measure the thickness of your floors and compare it to the heat requirements of each room  to determine their ability to radiate heat based upon your location and the heating requirements for your area. I would like you to know that I am myself planning to install underfloor radiant heat throughout a 6000+ SF 1838 estate that I am restoring and I have complete confidence in the way it will operate.

I hope that you do not give up so easily on your dream of warm floors and the freedom from radiators or baseboards around your home. Please let me know if you would like any suggestions about with whom you could consult.

 

Best regards!

 

Chris

Thank you for the vote of confidence in the system! I have decided to try to have it both ways and see what happens. I'm going to downsize the plans for my solar collectors (I can always add more later) and heat storage tank and experiment on my kitchen and dining room. The floors in both of those rooms are in pretty rough shape and we have talked about replacing them anyway (one has SEVERE pet damage and the other had another layer of floor glued and nailed every 4 inches). If the radiant heat ruins those floors I figure I'm not out much, and then I won't install it in the rest of the house! If it doesn't bother it, I'll increase the size of my collectors and install it in the rest of the house.

I am really glad to have been of some help to you in this respect. Do you have access to Natural Gas in your area? The combination of the latest technology fully-modulating gas boilers together with a properly designed underfloor radiant heating system will result in the most comfortable  as well as competetively priced heating available. One last comment: the capital cost of these tends to be higher but that is mainly due to the high labor cost of installing all of the PEX tubing and aluminum frames, etc, under each floor. If you like to do reduce costs, you may like to partner with a licensed contractor and/or engineer to perform some of the labor intensive work yourself. This can be inspected and tested by the contractor after installation and before it is concealed - with this acting to reduce costs on your side. Once installed, underfoor radiant heat syatems are less expensive to operate than conventional systems.

Do you have any heating already, like radiators? We had the same thought process as you. Full basement, access to all the 1st floor floors, but once you consider the age of the floors and the fact you are going to subject them to an unusual environment, you may be opening a can worms. So we elected to stay with the radiators. I could not find any solid information that radiant floors were a good idea in our case.

However, I would certainly look into using solar to heat any water for your heating system.

Hi Bill,

 

I am going to dig into several books at home and pull out references if helpful for you and Johnathan - will revert later on. Am not sure if you meant for me to respond or Johnathan since he began this discussion, but my house has old and obsolete radiators which I will be removing. My house was heated with 12 fireplaces in its original service and the radiators were added in the 1930s. For underfloor radiant heat,  the floors in old houses are frequently and for many years exposed to 80 degree temps in the summer time - with sunny spots even getting direct exposure. Mild warmth from below will not affect old dry wood. Heart pine - with fewer pores that trap air will transmit heat better than a lesser quality wooood.Will send some additional information on this later.  

Yes, we have a ten year old LP gas furnace. In our case, the hydronic floor heat would only be supplemental, heated with the solar collectors we are planning on installing anyway to preheat domestic hot water. I'm going to oversize the system a bit and try it out on the floors I consider expendable.

I agree, I am concerned with the effects of underfloor heat. I am hoping that if I keep the water temperature low enough it will be ok. Yes, I'm quite sure that in the summer time my floor, when exposed to direct sunlight, is exposed to temperatures quite a bit higher than I am planning for my radiant floor heating. On the other hand, the amount of moisture in the air that is available to that floor is also significantly higher in summer.

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