Radiant heat and aged wiring in hot joist bay

jim lockard

That also Randy. I would think the thermoplastic insulation with a 40 degree C rise would be ok after all the attic in summer becomes quite hot 130 F plus. My concern would be the older organic insulations with natural rubber and a cotton wrap or binder. Provided that the surface tempture of the floor would be no higher then 75/80F max. Bay tempture may be 120/130 F seems about the same as the attic in summer. Will these temps break down electrical wire insulation over time? lot of old wiring in attics across this land. Best wishes J.Lockard

Randy_7

Radiant heat and aged wiring in joist bay.

My daughter (with 3 young grandchildren) recently had radiant heat installed within the joist bays of the 11' x 25' kitchen of her 85 year old home. The tubing is suspended within the joist bay and thick foil (about 3/16") has been stapled to the bottom of the joists - thus the air is trapped and heats the entire bay as it heats the floor. Her kitchen is very comfortable.

My concern is that the old (and energized) wiring and packed full splice boxes that are common to the basement of an 85 year old home still remain within the heated joist bays. My gut tells me that either the wiring should have been removed (a major project) or the tubing installed directly to the underside of the sub-floor. Is my concern unfounded? Your comments will be appreciated.

Randy

Joe Brix

As long as

the wiring is secure and not moved you should be OK. Down the road, any wire modifications or repairs would probably require the whole section of wiring to be replced since the brittle wire would probably short if moved.

Dean_7

Old Wiring

From personal experience when we upgraded our electrical service and rewired the house (102 years old) a few years ago. We found that all the old wiring near the heating system and in the attic that was in anyway affected by heat to be severly degraded. Some of this was 90+ year old knob and tube but most was romex from a rewire in the late 1940's. On the surface and undisturbed it looked fine but if you moved it or flexed it the fabric, paper, and rubber insulation just crumbled.

EBEBRATT-Ed

I have a copy of 1937 "Electrical Code Hand Book" It states thet "rubber covered wire shall not be used where the temperature exceeds 120 deg F" Don't forget that as a wire carries current it heats up and must dissipate the heat to the surrounding air. The temperatur the wire operates at is a combination of the surrounding "ambient" condition and the dissipation of heat from the wire. If the wire is in a high ambient location dissipation of heat is reduced.

New wiring with "Romex" although the wire is rated at 90 deg. centegrade (197 F) It must be installed so that no combination of ambient temp and heat from carring current will allow the wire to become warmer than 60C (140F).

If new wiring is installed in an ambient warmer than 30C 86F the current carrying capacity of the wire must be reduced (smaller fuses or breakers or larger wire used)

up to 95 F amperage reduced by .96
" 104 " .91
" 113 .87
" 122 .82
" 131 .76
ETC

You concerns over the old rubber covered wiring is well founded. It should be replaced.

Ed-not an inspector-just an electrician

Constantin

Precisely why....

.... I specified 12/2 wiring for all electrical outlets and 14/2 for lighting circuits. Given that the rooms are on individual circuit breakers for the outlets and the lighting circuits are shared only across 2 rooms at a time, I could be reasonably certain that the wiring will be fine even when it is run through areas with Corbond or Icynene in it...

... I was inspired to do this by the stories from older homes where ill-fated attempts to insulate resulted in knob+tube wiring getting covered, followed by ignition at a later point in time. The larger the conductor, the lower the resistive losses, so I hope that 12/2 for up to a 15A load in an insulated environment will be fine. Well, at least the electrician and the inspector thought so...

Randy_7

Old wiring in heated joist bay

Certainly appreciate the responses. My daughter lives in Wisconsin and local electrician she is using also expressed considerable concern when he saw the "history of wiring" contained within the heated joist bays. She has knob & tube as well as the heavy black Romex, and the silver colored Romex.

Thanks,

Randy