Low voltage electric radiant floor heat
Ine
| Posted in Mechanicals on
There was a question about electric radiant floor heat mats several days ago. I’ve seen a product called Step Warm Floor that uses a graphite based strip and runs on low voltage electric that the manufacturer claims is much more efficient – they suggest is one of the most efficient heating methods around. It also costs about 4 times as much as the normal voltage electric mats. I’m no expert on electric theory so can’t tell from the numbers if this would really work as advertised. Does this product have a track record of verified use? Any experience among participants here at GBA? Thanks.
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Replies
Donald,
Electric resistance heat, whether 24 VDC or 120 VAC, is always 100% efficient, and always expensive.
The only possible advantage to 24 VDC is the lower chance of electrocution when your kids start practicing with their hatchets on the bathroom floor.
Assuming your house isn't an off-grid home operating on 24 VDC, this system will be less efficient than a line-voltage system, because of the inevitable inefficiency of the transformer. Of course, the heat given up by the transformer still warms your house (unless you are foolish enough to locate the transformer in the garage) -- it just doesn't warm your floor.
Martin,
Thanks for the explanation. Your explanation makes sense to me and I assumed there was no "free lunch" It took me a few days to remember where I saw this but here http://www.warmfloor.com/literature/HANDBOOK/07%20-%20EFFICIENCY/7.3.%20Operating%20Cost%20Comparison.pdf is the chart from the manufacturer that compares operating costs of various heating systems. I'd be interested in you take on their numbers.
Donald,
You'll notice that all of the different types of electric resistance heat shown in the chart — the Step WarmFloor, the electric resistance baseboard, and the electric radiant ceiling — all have exactly the same price per BTU ($15.39 in the chart). That confirms what I wrote (electric resistance heat is always 100% efficient and always expensive), although their price per BTU is obviously ridiculous. The real price of electric resistance heat is about $0.00003 per BTU.
The most obvious trick the the chart -- the deliberately deceptive part -- is that the cost for every other type of heat is based on 68 million BTU, whereas the cost of the Step WarmFloor is based on 26 million BTU. The only reason it is cheaper is because it delivers less heat! You save money by shivering.
Thanks for the reading lesson Martin. The 26 vs. 68 million BTU detail is the what I didn't catch.
Martin & Donald,
The $15.39 electric cost is per million Btus, but that's based on a 5 cents/kWh cost (compared to fuel oil at 86 cents a gallon and propane at $1 per gallon. Those must be 1980 costs.
The 26 million Btu trick is because they claim a 1300 sf test house in Norway operated with a 61% energy savings compared to a a "typical home heating system". The Norwegian family slept with the windows open at night and aired the house out every morning (I assume they turned the heat off, then, at night), and the house apparently received ample sunshine every afternoon.
So, their math-a-magic is based on a very atypical family and house compared to whatever they consider a "typical" house and family. And their comparison to other heating types assumed that all non-electric systems lose 10% to unconditioned crawlspaces and required a 340 watt blower.
Even the Reflectix and Icynene magicians can't top this sleight of hand.
I am a consumer who purchase a whole-house heating system from STEP Warmfloors in 2003. It was supposed to reduce my heating bills, but instead my bills more than doubled. The system had been mis-designed by STEP Warmfloors. I specifcally told them my house was made of cinderblocks, but they miscalculated everything and my PG&E bills went up to over $400 a month. The transformers in the system were very noisy. I could not live with the system, which had cost me over $7,000.! I had to pull the whole thing out, throw it away, and then spend $6000 on a new central heating system. I wrote and called the company repeatedly and just got the runaround. I would never, ever recommend them!
Natasha: Thanks for sharing your pain, and preventing others from getting into this (apparently) snake oil business. Your experience is sick, and I hope you have some legal recourse to recoup some of your money. Good luck.
John & Natasha, I installed Step warmfloor in my own house about twelve years ago during our bathroom remodel project with great success and no issues. I did quite a bit of research and knew I didn't want a cable system as they consumed way to much power plus I wanted to install it in the shower floor and on the seat. We have sold floor covering products for twenty eight years, six years ago we began selling the product in our store because of it's versatility, It could be installed in so many ways under so many flooring products it was a natural fit. I can say that we've had very few complaints. Not to say there haven't been any issues because as with any product to say never any issues is a totally bogus thing to claim. No product is perfect and and the very few issues we have had were taken care of and resolved for the customer.
Two years ago we remodeled our kitchen when doing so we took out the hot water baseboard heat in the kitchen so we could install more cabinets and installed warmfloor under about 500 sq ft of ceramic tile. also installed it under the granite counter tops. I installed the thermostat in the kitchen, we keep that set at 68 degrees and the thermostat for the rest of the first floor at 63. The heat from the radiant heat in the kitchen flowing into the front of the house kept it warm enough so the circulator pump for the first floor never turned on. My electric bill did increase but it's a little hard to tell exactly how much we spent for heating the first floor because we also have a 36 foot sidewalk outside heated with the product as well. Yes it is a total waste of money but it rarely gets any sun in the winter and the kids never shovel it. It's nice coming home and not having to shovel the walk. Trust me I don't have any regrets.
Please understand I'm no building or heating scientist but in the last six years I've learned how this system works and the importance of the heat loss calculations in designing a heating system for a client. I've done quite a few total heating jobs with this product and haven't had an unhappy customer that I'm aware of yet. I have entire houses heated with the product with no complaints. I'm truly sorry for Natasha's problems and I have no idea what the circumstances were with her particular job. Someone could have made a mistake with the heat loss calculations or possibly it was with the installation. Although it is a simple system to install it can be installed improperly and not have the desired affect. This in turn would make for a very unhappy customer and depending on who sold it to her and who installed it could be a key to the problem here, have no idea at this point. I just wanted to say that this product is a legitimate heating system and not just for floor warming. Keep in mind if you don't provide the correct information on R- values, room sizes, ceiling heights, window & door sizes along with U or R values then the heat loss calculations will be incorrect and again you could end up with a system that doesn't work as expected. I just read your blog and felt I had to say something to defend this product. Also with regard to Martins comment on the high voltage electric cable system being more efficient I would beg to differ, but that is another discussion. Sorry for the lengthy dissertation.
Just have to jump in to say: Martin is 100% correct - any electric *resistance heating* (as opposed to a heat pump) is inherently 100% efficient, unless you're throwing heat away into unheated spaces or running a blower. The only efficiency difference between a low voltage system and a line voltage system is the losses in the transformer... which would of course also become heat(except for the small amount that turns into annoying noise, see Natasha's post!) - that heat just might not be where you want or can use it.
I do agree. My argument is how the heat is delivered to the living space. Electric baseboard heaters and electric heating cables do not efficiently deliver the heat to the room. Consider the size of a typical baseboard heater. Your essentially just pouring the power to it till it gets so hot you can barley touch it without burning yourself. It's drawing maximum power till the heat builds up in the room (from the ceiling down to the thermostat level) and doesn't shut off till the warmth comes down to the level of the thermostat. Now take heating cables they cover less than 2% of the floor surface and because they are essentially a controlled overload of wire to generate the heat for the room they operate on 12 to 15 watts per ft when installed. Now lets look at the warmfloor product operating on 24 volts, there are no wires that have to heat up, the mat has a flat bus braid wire embedded into it on each side that's energized with the stepped down voltage from the transformer. The plastic mat is what heats up, the mat is is twelve inches wide and when installed covers approximately 60% of the floors surface creating a much larger radiator so to speak and has the ability to deliver a larger volume of heat into the room. Now here is the bonus that make it even more efficient. The plastic polymer element is conductive and has self regulating properties, meaning that when the ambient and floor temperature is cooler the conductive particles in the heating element are closer together allowing more electricity to pass through it which in turn heats up the element and whatever it's touching, as it heats up the floor and the ambient temperature the conductive particles in the plastic get farther apart and the result is less electricity passes through it and your amp draw actually drops and so does the cost of maintaining the heat in the living space. This system likes to be running more than it's off because it requires less current when it's warm and running. When all is said and done the warmfloor is running at anywhere from 4 to 6 watts per foot. All depends on what the heat loss calculations determine as to how many watts per ft needs to be installed. Your also right when it comes to the transformers, installing in the basement or a closet in the living space will make more efficient use of the heat generated from reducing the high voltage down to the 24 volts. Essentially the end result after independent testing proved that it requires 2 1/2 times the power to do the same job with heating cables and 2 times the energy for 1/2" water tubing. Here is another thought, this product is made of 20% recycled content, it's biodegradable, and any scrap and waste from the manufacture of the product is reclaimed, reground and reused in the manufacturing process. Depending on your utility supplier you can purchase green power for as little as .05 additional per kw/hr and have a totally green heating system. My understanding is pex tubing is a polyethylene cross link product, so even if you were to heat water with solar panels and use photo voltaic panels to run your circulator pumps you still cant technically have a green heating system as the pex tubing if thrown in a landfill will still be there a million years from now. I could be wrong but at least that is my understanding.