Hi

My heat loss calculation is 7.5kW

I have a complete design for a 035 Altherma and radiant floor system. The system has a built in 3kW electric booster heater. It can easily handle my load so don't let that influence your answers.

This is new construction so I am thinking of adding a few baseboard electric heaters in the basement for BACKUP in case something fails. These would be on proper thermostats and set basically to keep the pipes from freezing.

*What percentage of the 7.5kW heat loss would you consider also having on backup BBE? Keep in mind this is primarily to protect the pipes from freezing, not maintain full heat.

Thanks in advance for any advice.

John

Assuming the electrical service still functions and just the heating plant goes down, the solution could probably be a couple 1500 W milk shed heaters to keep the place from freezing.

It will be hard to say much about it because you haven't mentioned your location or the type of construction.

if all you care about is pipes freezing, you could just put heat tape on your pipes.

Rob

I've got 3800' of radiant tubing alone so heat trace isnt practical.

Its a "code +50%" insulated shell in a 18 F design temp location.

thanks

John

What happens when the power goes out?

could just glycol if it's the radiant pipes you are worried about. I don't like glycol, but if this is an actual concern it's better than freezing. I assumed you were talking about exposed mechanical pipes, not the entire building. If you are code +50 in that kind of location, house freezing is probably an unlikely event.

But, regardless building as a whole kind of options:

1. duct heater in your ventilation/AC ductwork
2. fireplaces/woodstoves
3. electric or gas boiler backup
4. electric baseboard as you have noted.

What happens when the power goes out?

could just glycol if it's the radiant pipes you are worried about. I don't like glycol, but if this is an actual concern it's better than freezing. I assumed you were talking about exposed mechanical pipes, not the entire building. If you are code +50 in that kind of location, house freezing is probably an unlikely event.

But, regardless building as a whole kind of options:

1. duct heater in your ventilation/AC ductwork
2. fireplaces/woodstoves
3. electric or gas boiler backup
4. electric baseboard as you have noted.

Posted By John7 on 20 Oct 2011 10:27 PM
Rob

I've got 3800' of radiant tubing alone so heat trace isnt practical.

Its a "code +50%" insulated shell in a 18 F design temp location.

thanks

John

If  plumbing (not heating system plumbing) freeze-up is the issue, you only need to calculate the heat loss at 32F indoor temps of the rooms with the plumbing at design temp to come up with something that'll keep things from freezing.  The heat loss at 32F indoors 18F outside is about 1/4 what it is at 70F indoors, 18F outside. If well distributed  2kw of electric baseboard could keep the the house from freezing up even if the AVERAGE outdoor temp was 18F.  If you left all the lights on + the TV you're probably already above 2kw.

In practice most places with 18F design temps with all of the plumbing within conditioned space needs no backup heat to prevent freeze-up, even at code-min insulation & window specs.  All houses have thermal mass, most have at least some solar gain, and even on design day the design condition does not persist for more than a few hours. Even if the heating & power were off for weeks the odds of any room dropping below 32F long enough to cause the pipes to freeze are slim unless you left the windows open, or the place has the highest air leakage on the planet.  A tight house @ code + 50% insulation with decent windows and even modest solar gains could coast through just about any weather event without actual freeze-up.

A few years ago my sub-code-R 1920s bungalow was out of power & heat for 10 days due to an ice storm taking out much of the local grid.  The average daily temps were in the 20s,  and the overnight lows hit 10F & lower for three days in a row. The 3-day high for that stretch hit 21F.  Only in the upstairs bathroom dormer on the north side (with little to no wall insulation and ~ R20 in the roof) did the indoor temp drop below 36F.  The basement stayed in the 50s, the first floor barely dropped below 40,  and that was on the third night. When the daily highs got above freezing the interior temps began to rise.  YMMV.

I agree - but at -20F with nobody home one will have problems.

FWIW It got down to -2F here once and that was pretty much a record. The next day it was about +10F.

John