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There are apparently 2 ways to go for electric resistance radiant heat w/ slab. One is mat or “electric blanket” buried in thinset under tile. The other–which some claim is more efficient–is resistance cable buried in slab or under slab in sand.The mat seems to be favored for spot applications like bathrooms.
I’ve found interesting references on the web, like at least 2 electric coops in upper midwest that give special (.035 KWH) rates to people who agree to use electric off peak for such applications; they install radio transmitters at participating homes to regulate the power going to what they call radiant electric storage.
But electric resistance radiant is definately in the minority, probably because most people have to pay too much for electric. But I’ll be paying a flat rate of .05 KWH on Oregon coast, so am intrigued with this option.
Who knows about this stuff?
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Jim: Most electric HWHs are 7000 watts or less. That is 24,000 BTU/hour. For one-story, 500 sq feet (ever going to expand?) and (rule of thumb) 0.3 BTU/hour/degree F and a -10F design temp, I get that you'd need 12,000 BTU/hour. So unless you live in Fairbanks, or have a very leaky house, that electric HWH would do it.
The radiant would take half its capacity away from its already slow recovery period. So be sure to take the first shower every mornhing. Or use separate water heaters - one for radiant and one for domestic hot water.
The mat may be used in bathrooms because it can fit into the thinset of tile. And is therefore way eaiser to retrofit than pouring a slab.
An electric slab will be more responsive if the cables are in the concete, not the sand. Jog them down into the sand for the expansion joints, but otherwise keep them in the middle of the slab.
I'm not too concerned about repairing a wire in a concrete floor. Any radiant slab has tubing or wires that conceivable might need to be repaired in the future. And tracing wires is really easy with a voltage sniffer ($5). But compact the sub-base, use appropriate reinforcement, etc. and a slab lasts forever. -David
*I was getting a little wordy, so I'm posting separately with thoughts about energy costs.Any electrical heating system will really make the meter spin. When nuclear plants were first going in, they predicted electricity would be too cheap to meter. Now people see spikes in fuel prices and think electric rates (which are regulated and can't change quickly) are not so bad. Well, most electricity is generated from fuel at 30% to 45% efficiency. So electricity will always cost more than fossil fuel. Unless you believe in Ponds & Flieschman. Which I don't. $0.035 is a really good price, only possibly in areas where the water or neutrons flow day and night, sometimes above base demand. $0.035 is 40% of my electric price and only 20% above my natural gas price. But electric is more efficient - no hot gases going up the stack. Can you use most of your electricity when it is cheap? The slab will have a fair thermal mass, so that is in your favor. It might push me towards a few electric HWH tanks. As a way to store BTU's (for radiant, showers, laundry, etc).Will the power company commit to offer these rates in the long term? If the program is unpopular or unprofitable and they dump it in 2 years, will you be left paying $0.12/kwh and wishing for a gasfired system? That would be an advantage of running tubing with water in it. You could always scrap the electric HWH's and put in a gas one.Putting wires in the floor has the advantage of being a simpler system. No pumps. No water. No tubing. If I didn't have an electric inspector to deal with, I could do the heating cable in a 500 sq ft electric slab for about $600, retail. There may well be similarly cheap, but UL-approved options. -David
*Thanks, Dave, and tell me: where would you get the cable? How is it specified? Spacing?I'm giving thought to resistance because there's no natural gas there, and propane is at $1.65 and will likely never drop to $1, which is what it would have to do to compete w/ electric. Electric price has been stable there for a good while.One thing that makes me leery: I don't know how many and what length power outages to expect up there, but I imagine I could run an electric WH off a moderate sized generator but would need a whopper to run resistance cable in slab. But I guess I could run a little space heater if need be. Kerosun or something.
*Dave, we cross posted.The .035 is the flat rate up there; as far as I know they don't offer any of the off-peak rates I've seen in other places.But you're right: I could also drop PEX in the slab to be on the safe side. Probably $300.
*I've used roof-heating cable for a variety of "off-label" (as the docs say) uses. Heating groundwater within a well. INSIDE a water hose to allow thawing after freezing solid, etc. They stuff lasts for years in wet, freezing weather, exposed to the sun, etc. About $30-50 for a 600 to 1000-watt roll. I'd put at least 20 kw of cable into your slab. The cable would go in on something like 6" on centers (which is really easy on 6" x 6" #10 WWF).20 kw seems to dictate a BIG generator, but you could run one zone at a time. Bring the multiple cables back to one stop with the option of run a few at a time. I'm really leary of kerosine heaters. I'd feel better about a propane stove, range hood and a carbon monoxide detector. -David
*Oops, wishful thinking up there: meant to say .05 KWH.Thanks for cable advice. So when you come out of slab, you go into a junction box and come out with regular romex? The amount of juice is starting to scare me. And for thermostats?
*Or just come out of the slab into a thermostat-switched 120-volt receptacle.The thermostats can be a problem. Most of the fancy termostats are for 24-volt system. By fancy, I mean ones that can react to the trend in the temperatures ("differential control"), not just the current temperature. And/or look at outside temperatures (when they fall, a smart thermostat can anticipate the need for more heat in the future). You could always use a smart 24-volt thermostat and use it to close a relay with a 24-volt coil and contacts rated for enough 120 or 240 to do the job.Line voltage thermostats would make the wiring really easy. But most of them assume a low-mass heating system, like electric baseboards. In a high mass system, they are neither quick enough nor smart enough to hold the temeperature with reasonable limites.Think about zones. Think about them a lot. Which areas are similar in terms of wall-area to floor space and solar input? Put those in the same zone. A north corner room with a small window will need more heat than a south room. And at different times. So put them in separate zones. With electric cables, adding a few more zones is cheap is easy, at least with line-voltage thermostats. With a hydronic system, is means more pumps or more control valves and a bigger manifold. With electric, it is just a few more wires. -David
*Dave,So the idea, is that the cable will come into a receptacle box, containing regular line voltage thermostat, which governs on/off. You just don't worry about the fact that a certain amount of resistance cable is inside wall? Because it doesn't get VERY hot, anyway?Guess I don't exactly know how this resistance cable acts. It gets.....how warm? Not too hot to touch? Or what? I'll call my local electric supply house next week, but here in central valley California, I don't know if they'll know what it is!Where I'm going to build, on southern Oregon coast, is pretty mild. Typical winter weather, day after day, is 45F low, 55F high. (Probably considered balmy up where you are, and perhaps not in need of heat.) Sure there are cold snaps, and I read somewhere that it once got down to 17F, but in general just mild. So perhaps fancy thermostats not needed anyway.
*JimAt 55F David is concered about getting heat stroke if the AC fails .DavidI am in the Kansas City, MO area and here are our ratesSummerFirst 600 kWh 0.0700Next 400 0.0720All ohter 0.0756WinterFirst 600 kWh 0.0700Next 400 0.0479All other 0.0479Winter-with heatingFirst 600 kWh 0.0700Next 400 0.0378All others 0.0313All rates also have a 6.71/month customer charge.Winter is Oct 1 to May 31. You are suppose to get the heating rates, if you basic main source of heat is electric (any type) and include all usage (no separate meter).But (quickly let me change my name in these posting) but I am getting the heating rate, although I use natural gas.The utility has had this type of rates for a long time and has push to get people to go to heat pumps.Slight off target, but I have a 2 story house on a slab with the slab being at ground level on the front and 1/2 of the two sides. When I built in 1979, all the HVAC people wanted to treat the 1st floor as a "basement" with just a couple vents in the ceiling. I knew that would not be comfortable for main living space so I finally found one that would put in dual FWH systems with ducts in the slab for the first floor. However, they grossly oversized it.Just before I was getting ready to start the I got the though of using electric heat cables in the slab, not for the full heat, but enough to counter the losses through the slab. But it was too late and I did not have the time to research it (and no internet ). Also I was leary of anything in the slab. I only allowed for drain pipes and the heat ducts. Also any kind of water based systems where very rare here and radianet heat never more so.Also I ask about geothermal heatpumps (but did not have the name for them) and no one knew what I was talking about in this area.In a few years I will need to start replacing equipment. With two separate systems it is hard to justify some of the better options. So I am look at the concept of using hydro heat exchangers in air handlers. Then I have have heat pumps and/or gas hot water heater to supply both areas heating and domestic hot water.
*Bill,Where I am currently, in central valley California, electric is .126 for "baseline" (what you are apparently supposed to be able to, but cannot, get by on) and .143 above that. These prices, they point out gratuitously, include such additional charges as Transmission, Distribution, Public Purpose Programs, Nuclear Decommissioning, Competition Transition Charge, Trust Transfer Amount, and Energy Surcharges. I'm not sure where, exactly, they hide the Yacht For CEO and Cronies Surcharge.Ask me, if you are weak-brained, if I'll be happy to move to Oregon.
*Jim,I have both the mat and the cable system installed in my house. Love them both. I heat the space with a propane-fired forced air furnace. I keep the temps low, because thats the way we like and saves $$. The electric resistance provides warm feet only, though in one bathroom with my "envelop" walls in place, the 800W provides more than enough heat on all but the coldest days ( in N. IL).On the "off the shelf" variety, there are two ways to control the floor. One of my systems has a thermocouple imbedded in the floor that monitors floor temperature and it internally adjustable, the same controller also senses air temps to start and stop the heat and is externally adjustable like any thermostat. This system, called "Infloor Heating System" is produced and sold by the Maxxon Corp in Hamel Minnesota. This is a cable system. I installed it over 1/2" decking and 1/2" Durock in a mortar bed under ceramic tile.The other system, by the name of NuHeat, is the mat system. This came with (as an option) a programmable controller that controls via an imbedded RTD, and controls solely on floor temp, but has the flexibility to contain 4 "modes" plus manual. This was installed directly over a slab on grade in a layer of thinset under quarry tile.
* I use alcatel (from orbit mfg) elec cable...it is in gypcrete which is over 4in concrete and 2in foamboard...ten zones...house is a pleasure..
*Dan,Thanks for info. I ran across "alcatel" somewhere on net, and emailed appliedradiant.com for prices, etc. about a week ago, but have not heard back. I just wrote orbit and asked for closest distributor.Do you remember about what your cable and controls cost?
*cost was about five thousand for three thousand feet of cable and ten thermostats...need 220v..heaats about 2000sq ft of structural insulated panel house...cable is from nexans of norway who have been using it for about fifty years..
*Thanks. I'm still having trouble getting in touch w/ distributor. The email I sent to company listed at Orbit website for western US (H-S-C) was bounced. But I'll keep looking At least I now know approx. costs.
*Bill I looked up my variable rate...winter...9-5 .1785-8 .13and night.. .085regular rate...11 with $10 per month charge...taxes..blah blahSo like I have said...Nowhere are the rates higher...And nowhere should they be lower!We have tons of hydro up here!Oh well... Politicians... Keeping NYC lit...and out of bankruptsy.near the stream that leads to NYC,aj
*Northwest mechanical in seattle...