Questions about a Heater Replacement

Old 11-07-20, 06:18 PM
Thread Starter
Join Date: Nov 2020
Posts: 1
Received 0 Votes on 0 Posts
Questions about a Heater Replacement

Hello and thanks for your attention. I'm an amateur DiYer, and just starting to tackle electrical work, so please bear with me!

I'm trying to replace an old drop-in (wired) heater, and ran into a problem: The new heater has 2 black wire receptacle leads, but the supply leads are black & white, yet seem to be 240V.

The old heater had the typical white-->white, black-->black setup, and I've disconnected+reconnected it before to figure out what to buy (old one discontinued in the 70s), couldn't be simpler! But the new model having two black wires is making me want to confirm I know what I'm doing.

Information I have right now:
  • Old heater appears to have been 240V (but it was very hard to tell, it was 50+ years old & very faded/rusted)
  • New heater is definitely 240V, specifically a Marley FDI1504
  • Old heater had a white & a black receptacle, connected to white & black supply
  • New heater has two black receptacles which appear completely identical in all regards
  • With a voltage meter, supply wires read 240V (black+white), 120V (black+ground), & 120V (white+ground), ergo I assume it is 240V supply (prior to 2011 code changes requiring neutral white)???
  • Wire gauge appears to be between 12 or 14 gauge... can't really tell.
  • Supply comes from breakers 18 & 19 at the main panel. Both are single-pole breakers, but connected by a handle tie. 18 is white & 19 is black.
  • Thermostat control shared with 3 other heaters also on breakers 18 & 19.


1) Is the hot white wire just a pre-2011 relic, but generally safe & normal in older construction?

2) Is this white+black = 240V indeed a 240V supply, or did I buy the wrong heater, and need a 120V model?

3) Assuming I'm okay on 1 & 2, do I just "pretend" the hot white wire is black, and connect hot white supply to either identical black receptacle randomly? (Manufacturer instructions are to connect 2 black receptacles leads to 2 black supply leads.. but I appear to have 1 black + 1 white supply leads instead?)

4) There doesn't appear to be a neutral supply (just a ground), nor a neutral receptacle (just a ground). This sound right/typical?

5) Any more arcane tricks I need to be careful about or rookie tips I should know?

Thanks again!
Old 11-07-20, 06:53 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Posts: 68,804
Received 2,677 Votes on 2,429 Posts
Welcome to the forums.

When NM cable is used for 240v..... there is no choice but to use the white as a hot leg.
Not a relic.... it's still done today.
Correct..... there is no neutral. There are two hot legs and ground.
If you want to be pro-active..... tape the white wire red.

You should know the wire size, the breaker size and the total current draw of the circuit.
Your three heaters cannot approach the limits of the wire size.
Beginneratwork voted this post useful.
Old 11-08-20, 05:21 AM
Forum Topic Moderator
Join Date: Sep 2005
Location: USA
Posts: 6,211
Received 298 Votes on 266 Posts
If you want to be pro-active..... tape the white wire red.
For what it's worth, it's actually part of the NEC to re-mark the white wire when it's used as a hot wire. A lot of electricians skip this step, and it results in confusion down the road - as you've noticed!

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: