Heat won't turn on - what to do: how to diagnose & fix "no heat" problems in buildings. This article describes what to do if you can't get your building heat to turn on.

We give a simple sequence of steps in the order that is most likely to succeed, starting right at the wall or room thermostat and continuing through making sure there is electrical power and fuel.

For more complex no-heat problem diagnosis we link to in-depth articles on heating boiler, warm air furnace, heat pump and electric heat repair procedures.

No-Heat Diagnosis & Quick Repair:

If your heating system won't turn on at all, first check the following things in this order:

• Is the thermostat working at all - is it on?
  
  Check that the thermostat has power: if your thermostat is a programmable model or has an LCD or electronic display and the display is blank (red arrow in our photos) then the thermostat has no power - its main power source is off and its backup batteries are dead..

On an older analog dial type thermostat you can't determine if the thermostat is working just by looking.

But turning the thermostat UP well above room temperature should turn the heating system on. Thermostats are usually powered by a low-voltage system wired from a transformer near or right on the heater.

If the thermostat has no power, check that its wires are connected, intact and that power is turned on at the heating system since power off there will often also turn off the transformer that powers the thermostat. Then
see LOW VOLTAGE TRANSFORMER TEST

• Check that the Heating Thermostat is set to "HEAT (on)". If a room thermostat is in the OFF position or is set to a temperature that does not call for heating (or cooling) then the heating or cooling system is just not going to run.
  
  For the room thermostat shown at above left, push the clear plastic switch to the right towards HEAT as far as it will go.

• Check that the Heating Thermostat FAN switch is set to AUTO  if the switch is present on your thermostat.
  If your thermostat has a FAN control switch but your building only has hot water or steam heat, most likely this switch does absolutely nothing.
• For forced warm air heating systems, AUTO will let the fan turn on and off when the heating furnace runs while ON forces the fan to stay on constantly.

See FAN AUTO ON THERMOSTAT SWITCH for details about what turns the blower fan on and off on forced warm air heating systems or on air conditioning systems.

Separately, a FAN LIMIT SWITCH, we describe how this additional control, located on the heating furnace itself, should turn the forced warm air heating blower on or off. Don't bother with that switch yet.

• Check that the thermostat is not in "HOLD" or "Standby" mode.
  
  If the set temperature is down lower than room temperature and the thermostat is in HOLD or STAND-BY mode (a feature present on most electronic thermostats) the device will not turn on heat until room temperature is below that HOLD number
  or
  until you return the thermo sat to normal or RUN mode.
  
  In our photo my pencil is pointing to the "Hold" button and the red arrow shows that the display is telling me the thermostat is in HOLD mode.
  
  Press the "Run" button (below my pencil point) to go back to normal operating mode for this thermostat - that is to get out of "HOLD" mode.

• Check that that the set temperature is at least 5 degrees above the room temperature. Later you can set the temperature lower if needed. of how to set all types of room thermostats are
  
  at THERMOSTATS, HEATING / COOLING.

Watch out: on some heating or cooling thermostats like the one shown at left, the device is so simple that the meaning of the temperature shown in the display may confuse you.

Left untouched and in HEAT mode, and controlling just a hot water heating boiler in this installation, the 67 degrees shown on the thermostat is the present room temperature.

So we cannot tell the SET temperature of this thermostat just by walking up and taking a look. So we don't know if the thermostat should be calling for heat or not, nor if it has been SET above the present room temperature.

Luckily and without even reading the instructions (a last resort for many of us) it's easy enough to figure out what's going on.,

The heating boiler was not running - I'd already checked that, and I'd also noticed that the radiators in the building were tepid or cool.

Just pressing either the "down" button (blue arrow pointing down in the photo above) or the "up" button (red arrow pointing up in both photos) will change the display to show me the present SET temperature.

Notice that the SET temperature is all the way down to 56 degrees? This thermostat has been told "don't turn on the heat until temperature in this room drops below 54F! No wonder the heat was not running. The room was already much warmer than that.

What if the thermostat doesn't turn on the heating (or cooling) system?

Now if I press the "up" button repeatedly until the SET temperature is well above 67F then the heat should turn on. It did.

If the thermostat does not cause the heating (or cooling) system to run when it should, there could be any of a number of problems (as we outline here) but among them you may need to check the thermostat itself or its wiring - starting

at THERMOSTATS, HEATING / COOLING.

Watch out: to avoid embarrassment, don't forget to check electrical power switches including service switches that someone may have set OFF. For example

see ELECTRICAL POWER SWITCH FOR HEAT

• Check that the heating system is now running. If it is not, Check that electrical power to the heating system is turned on and for oil or LP gas check that there is fuel in the tank.
  
  Just take a walk over to your boiler or furnace room and listen - you should hear the equipment running, and for oil or gas fired equipment the metal "stack pipe" (properly, the flue vent connector between the heater and the chimney) will be hot.

• If none of these steps fixes the no-heat problem, depending on what kind of heat you have, see the explanation of no-heat troubleshooting in the articles listed below.

If you are confident that the no-heat problem is not due to electrical power having been turned off and not due to a thermostat not properly set, then you'll want to choose a no-heat diagnostic procedure depending on the type of heating system your building uses - listed just below.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-11-29 - by (mod) - cleaning up the gas flame igniter did not fix it so time to replace the assembly

Yes I'd replace the unit as you suggest. Keep me posted.
And
stay warm if you can.

On 2020-11-29 by jim

Pulled the whole unit. sanded down all the parts to clean off any discoloration. Middle rod was slightly bent to the side. Straightened it out. put the whole thing back in.

Did not light on the first try, but eventually, after about 20 minutes of cycling, it did light. I think I'm going to by the entire unit and replace and see if it lights more regularly after that. will call supply house tomorrow morning to make sure I get the right unit.

The most I'll need to pay is 75, and possibly only 25. Therefore worth swapping out. Won't have until end of the week, so that is next weekend's project. I'll let you know how it goes

On 2020-11-29 by jim

Thanks - will try that today. I'll let you know.

On 2020-11-28 - by (mod) -

Check the thermocouple end at the gas valve is snugly tightened,


Clean the sensors,

and if that doesn't fix it you may need to replace the part

On 2020-11-28 by jim

thanks dan - looking at my setup, that is exactly what I have (see pictures). to add a wrinkle, the burners now lit. This is the most pain in the but part of my problem, is that it is intermittent. It seems as if when the system is really cold (very cold last night - Colorado mountains 8,000 ft) the burners often don't lights and when the system warms up it will start lighting.

On 2020-11-28 - by (mod) -

THERMOCOUPLE REPAIR / REPLACEMENT https://inspectapedia.com/heat/Thermocouple_Replacement.php

may also be helpful though until I update it based on your query it's focused on older type thermocouples.

The following example flame igniter and sensor checks are from Weil McLain's boiler manual page 102 - that I'll cite below:

Check ignition wiring

1. Check ignition cable electrical resistance. A good cable will have resistance between 900 and 1000 ohms. Replace if not acceptable.

2. Inspect boiler ground wire from ignition electrode to ground terminal strip.

3. Verify all wiring is in good condition and securely attached.

4. Check ground continuity of wiring using continuity meter.

5. Replace ground wires if results are not satisfactory.

Inspect flame sense assembly

1. Remove the flame sensor assembly from the boiler heat exchanger piping end section. See image at right. Flame sensor insertion length is shorter than the ignition electrode (see above) and has a pink ceramic insulator.

2. Remove any white oxides accumulated on the flame sense rods using steel wool. If the flame sense rods cannot be cleaned satisfactorily, replace flame sense assembly with a new one.

3. Reinstall flame sense assembly, making sure gasket is in good condition and correctly positioned.

4. Replace gasket if necessary.

5. Inspect the flame sense harness and connections. Replace if neces- sary. See WARNING above about flame sensor location.

Weil-McLain SlimFit BOILER MANUAL [PDF] Series 1 Models 1000 - 2000 condensing gas fired boilers

- this will not, of course, necessarily be your boiler - so look for the equivalent instructions in your own boiler manual or if you don't have the manual see or web page giving boiler manuals

MANUALS & PARTS GUIDES - HVAC - home - Master Index to All Brand Names & HVAC manuals, wiring diagrams, installation and repair guides

and also

MANUALS for HEATING & A/C SYSTEM CONTROLS

On 2020-11-28 - by (mod) -

Unfortunately it's a more-expensive part (around $100.) than a traditional gas flame standing pilot thermocouple.

On 2020-11-28 - by (mod) -

Jim; Thanks for that added detail - More proof of the limitations of guess-diagnosis from remote.

But take a closer look at your burner and follow the tubes and wires:

A gas boiler that does not use a continuous or standing pilot indeed is turned on by an igniter, but generally there will still be a flame-proving thermocouple needed for safety.

The thermocouple in this case is not confirming that a pilot is lit, rather it is confirming that the full burner flame ignites when the gas valve opens. An example thermocouple for an intermittent burner on a Weil McLain gas boiler is shown below.

That larger "hooked" wire with ceramic insulator is a "main gas flame sensor".

Notice the aluminum tube and brass ferrule and tip? That connects to the gas valve. If that sensor has failed the burner won't stay on because the sensor will shut the gas valve.

On 2020-11-28 by Anonymous

I didn't think I had a thermocoupler on an intermittent, non-standing, pilot light?

On 2020-11-28 by Anonymous

thanks for the reply Dan. How would I test for that? Could it be the relay?

On 2020-11-28 - by (mod) -

Jim

We usually go for the simplest explanation which means we look back at the work that was just done on the system for a loose wire or bad connection.

However I must add that what you are describing sounds like a failure of the gas valve to open which could simply be a bad thermocouple.

On 2020-11-28 by Jim

Weil McLain CG-4 boiler - mid 90's. Just replaced zone valve the other day. Worked fine for 2 days -now will not fire. Intermittent pilot.

Zones open, damper opens, system goes through nokral starting sounds, pilot lights and goes for a few seconds trying to warm up igniter (?) burners never light. system shuts down.

Stays shut down. If I turn switch off and then on, system goes back through this same process and does not light - ideas?

On 2020-02-04 - by (mod) -

Scott

Post a photo of "the box", one of the "box" with cover off, one of any labels or markings, and one from more distance so I can see its location and wiring; You can only post one photo per comment so you'll need to comment multiple times

On 2020-02-04 by Scott

Thank you for this, the box is located on the back side of the furnace about 2 feet above the burner. I agree, it sounds exactly like a loose wire/bad connection, but when I took the cover of the box, there appears to be only one wire which connects to that unit and goes into the side of the furnace.

I am sure this is the box that controls the starting of the furnace once the thermostat drops to the appropriate temperature, I just don't know the "technical" name for it. It looks like a simple replacement, I just don't know what to call it when searching for one.

On 2020-02-03 - by (mod) -

Scott

That sounds like a bad heater control unit, limit control, or if the "black box" is right on the oil burner, it may be a failing transformer.

Ask your heating service company to look again; a bad room or wall thermostat - mounted in the rooms you occupy, not at the oil burner - would not have the slightest idea that somebody was tapping on the furnace controls.

However something as simple as a loose wire connection might respond to tapping. It'd be a shame to start replacing parts before we even know where the problem lies.

Perahps you can use the "add image" button to show me the part about which you ask. One image per comment.

On 2020-02-03 by Scott

I have a Gemini Oil fired forced air furnace about 20 years old. There is a small box on the back of the furnace about 1 1/2 inches wide, 1 1/2 inches thick and about 6 inches long. The 2 wires from my thermostat connect to the bottom of the box.

Lately, about half of the time you want the furnace to come on you need to tap this box with your finger and the furnace usually starts right away and seems to work fine. Had our oil provider in to look at it, they cleaned the furnace and said the problem was the thermostat, put on a new thermostat and still have the issue.

The furnace service tech says they no longer make the box that I believe needs to be replaced. Do you know the proper name of this box and is it still available? Thank you

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Continue reading at NO HEAT - BOILER or at one of the articles listed just below depending on the type of heating systerm your building uses, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see HEAT WON'T TURN ON FAQs - questions and answers about no-heat calls & problems posted originally at this page.

Or see these no-heat procedures:

No Heat from a Forced Hot Water Heating Boiler or Steam Heating Boiler

See NO HEAT - BOILER

No Heat from a Forced or Gravity Warm Air Heating Furnace

See NO HEAT - FURNACE

No Heat from Heat Pumps and Air Conditioners:

See AIR CONDITIONER WON'T START (also for heat pumps)

Suggested citation for this web page

HEAT WON'T TURN ON at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING SYSTEMS

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Citations & References

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