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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22012
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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my furnace wont turn on my thermostat is set to 80 the temp

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my furnace wont turn on my thermostat is set to 80 the temp is down to 60



What furnace and model number?

Customer: replied 6 years ago.
MGB5OHD Utica Boiler, Gas, forced hot water

Is circulating pump running?

Is system on zone valves?

Is boiler cold?

Customer: replied 6 years ago.
it dosent sound like anything is running their is a fan that turns on when either the boiler or the water heater turns on the water heater was just on and the fan was on. There are two zones and both pipes above the zone valves are cold below the zone valves the pipes are warm. Both thermostats are up high.
Ok, it sounds as though the zone valves are not even opening. Can you check power from the transformers to see if themostats have power?
Customer: replied 6 years ago.
do i just take multimeter leads to the top of thermostats where the wires are located one has two wires other has three but one of the three is jumping over to the other valve.

This can be as simple as following the wires. The power for the thermostat originate somewhere. Track back to where this power starts.


Do the zone valves have little levers or dial on the side or end for manual open?

The boiler control power can be itting at the valve waiting to make contact through a end switch when valve opened. The power for the stats can simply work the zone valves, and the fact the valve opens turns on the boiler.

Customer: replied 6 years ago.
both valves do have manuel levers but when i pushe them up it dosent feel like anything happens other than the lever moves loosely
Great that just saved time. It means the valves are open. Now do you know where the aquastat is?
Customer: replied 6 years ago.
got it its an aquastat relay by honeywell type L8148E
Ok we want 24 volts on terminals T and TV. Also this unit if wired into a vent damper can be held up if the vent damper does not open all the way making yet another end switch. Check for 120v at L1 and L2 and 24 volt atT and TV

Edited by Billy on 12/24/2009 at 2:43 AM EST
Customer: replied 6 years ago.
at l1 and l2 there is 120vac and at t and tv nothing...... at the valves there is 24vac
Excellent Now T an W.....Now is where we pay attention to if the high limit was tripped, low water (if equipped). The T and W will verify the 24 v transformer on the control. After that we will be zero'd in
Customer: replied 6 years ago.
At T and W i have 24VAC

Now we know transformer on aquastat is good, transformer for zone valves good, the zone valves are open, the stats work etc.


Now is there a vent damper on the vent pipe that opens everytime boiler fires?

Customer: replied 6 years ago.
there is it has a 24vac motor
Is it opened fully? and can you tell if it has 24 volts to it?
Customer: replied 6 years ago.
its definately closed not 100% percent sure on voltage...... i removed the cover and exposed a circuit board and motor there are 2 wires coming off the board and into a litle motor didnt remove those 2 wires from board but checked for voltage on its soldered comections and no VAC there.....out of the damper there is a wire with 4 wires in it going to the aquastat relay didnt check the end in the relay for voltage on any of the 4 its kinda bunched in there and didnt want to disturb


Back to aquastat, Terminals R and B in the center. That is a normally closed switch . We now check B to ground for 24v , R to ground for 24v, and Rto B for 24 (if open)

Customer: replied 6 years ago.
B to GND 4VAC R to GND 0VAC R to B 0VAC and damper is closed
please verify...was B to ground 4 v ac or 24 v ac
Customer: replied 6 years ago.
B to GND was definately 4VAC


Ok last test use a short wire or jumper and briefly touch T to TV on the aquastat. See if relay clicks to bring on circulator pump

Customer: replied 6 years ago.
nothing happens when jumping the two



Bad K1 relay in aquastat. 24v available and sent direct by jumping yet circulator relay doesnt pull in.


Looks like you need another aquastat

Customer: replied 6 years ago.
one last thing boiler just turned on briefly maybe 5 mins but pipes above valves are still cold......
if K1 relay not pulling in there is no circulating pump. No circ pump means stalled water in boiler which runs on limit very quick. No pump = warm boiler, cold pipes
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