The U.S. department of energy suggests electric radiant heat can be cost effective if preheated during off peak electricity hours if installed within an appropriately massive covering.

Is there a way I can predict heat retention over the course of a winter day? In particular, I'm interested in installing it under porcelain tile. We do have an existing dual source furnace, but want to install this to avoid waking on stone cold floors in the morning.

Department of energy states:

from http://energy.gov/energysaver/articles/radiant-heating

Because of the relatively high cost of electricity, electric radiant floors are usually only cost-effective if they include a significant thermal mass such as a thick concrete floor and your electric utility company offers time-of-use rates.

Time-of-use rates allow you to "charge" the concrete floor with heat during off-peak hours (approximately 9 p.m. to 6 a.m.). If the floor's thermal mass is large enough, the heat stored in it will keep the house comfortable for eight to ten hours without any further electrical input, particularly when daytime temperatures are significantly warmer than nighttime temperatures. This saves a considerable number of energy dollars compared to heating at peak electric rates during the day.

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    What is under the tile? Wood, concrete? Is it insulated? Off-peak or not, I don't think electric resistance heat makes much sense these days, when you can easily get 3 times the heat for the same amount of electricity from a cold-climate heat pump. If floor height is not a problem you can add thermal mass by pouring a layer of gypcrete to embed radiant in, but I'd use radiant PEX tubing in that case and heat water with a heat pump water heater rather than use electric resistance. If you have an uninsulated concrete slab, you are heating the great outdoors and "storage" will be mostly a myth. – Ecnerwal Oct 13 '14 at 15:54
  • I'll update the question, but this is on a first floor remodel with basement underneath and wood subfloors. My main purpose for this heating is to ensure we don't step onto cold tile floors on the morning. We're planning porcelain type throughout the first floor. I don't want to lose significant height, so adding floor volume isn't an option. We have an existing relatively new ultra efficient dual source furnace with heat pump. – glenviewjeff Oct 13 '14 at 16:00
  • In that situation I'd put radiant tubing and heat spreaders under the subfloor (on the basement side) and run them from a heat-pump water heater. With potable-grade tubing and stainless pumps this can be done off the same water heater the rest of the house uses, and you lose no height at all. – Ecnerwal Oct 13 '14 at 16:06
  • Yes, but then there's the cost. Contractor estimates about 25k to install hydronic. – glenviewjeff Oct 13 '14 at 16:08
  • It can be done for a lot less than 25k if you go diy. Ideas here... builditsolar.com/Projects/SpaceHeating/DHWplusSpace/…. in your situation, you won't be able to heat the house as you propose (heat tile at night, have heat release during the day) without your place being crazy insulated passivehaus style. There just won't be enough thermal mass in such a small slab. But that is usually a good thing since the heat will be more responsive... Ie it won't take a day to get heat when you need it during the shoulder season. – bobfandango Oct 13 '14 at 20:11

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