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Old 07-07-2011, 04:59 PM   #1
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No Hot Water when Central Air is On


I bought a house in 2005 that was built in 2000.

All Fall, Winter and Spring the heat and hot water work fine. When we start to use the central air conditioning in the summer we no longer get hot water. I've noticed that whenever there's no hot water (for showers, etc) the pilot light on the furnace is out (Utica MGB150).

I also have a hot water heater in my crawlspace.

When I relight the pilot light on the furnace we can take hot showers again.

Last weekend I investigated this problem and determined that I would replace the thermal couple unit. I replaced it and it ran perfectly for three days. This morning I woke up and there was no hot water and -- sure enough -- the pilot light was out. I relit it before leaving for work and it was still lit when I called home at 5pm.

Two questions:

1. why would the pilot light go out only while the central air is operating?

2. why don't I get hot water (for showers, etc.) when the pilot light for the furnace is off? What's one got to do with the other?

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Old 07-07-2011, 06:08 PM   #2
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No Hot Water when Central Air is On


Is your air handler in the crawlspace too? Is there ductwork in the craw space? If you answer yes to either question your pilot light could be blowing out from ductwork or air pressure from the A/C unit.

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Old 07-07-2011, 07:02 PM   #3
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No Hot Water when Central Air is On


What type of water heater is it? What type of furnace do you have? Are the furnace and air conditioner the same unit?
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Old 07-07-2011, 07:07 PM   #4
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No Hot Water when Central Air is On


Yes, there's a lot of ductwork in the crawlspace.

However, the pilot light (in the furnace) that keeps going out is in a small room on the first floor -- not in the crawl space. The only ductwork in the room with the furnace is the duct that comes out of the top of the furnace and goes into the ceiling.
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Old 07-07-2011, 07:12 PM   #5
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No Hot Water when Central Air is On


>>What type of water heater is it? What type of furnace do you have?
>>Are the furnace and air conditioner the same unit?

Bradford White PHCC water heater (in crawlspace).

Utica MGB150 furnace (in utility room on first floor).

The two Bryant air conditioning units (outside the house).
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Old 07-07-2011, 07:29 PM   #6
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No Hot Water when Central Air is On


open return pulling air thru the flue.or supply air going up it..work the fan only to see if the flame bounces as somebody is cycling the supply fan
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Old 07-08-2011, 06:38 AM   #7
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No Hot Water when Central Air is On


It definitely sounds like something from the A/C is snuffing out the pilot either a return suction draft or a supply air movement past the thermocouple. I know this sounds silly but it will give you a good indication of air movement near the thermocouple Take a small strip of toilet paper or tissue and tape it near the opening where you light the pilot and watch the paper when the A/C comes on or you turn the fan on manual by the thermostat.
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Old 07-08-2011, 06:00 PM   #8
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No Hot Water when Central Air is On


I'll try the toilet paper test.

But why would I no longer get hot water (showers, etc) when the pilot light of my furnace goes out if I have a separate hot water heater?
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Old 07-08-2011, 08:40 PM   #9
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No Hot Water when Central Air is On


Do you also use a in-furnace water heating tankless coil? Is this in operation all year 'round?

If the stand alone water heater acts as a preheater and then the water gogoes nnin into a furnace coil than if the furnace is shut off the furnace coil will suck all the heat out of the water before the water goes up to the faucets.
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Old 07-08-2011, 10:22 PM   #10
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No Hot Water when Central Air is On


Looking up the model for the heater only yeilded that it is a BW proffessional series, is the water heater a high efficiency unit, where-by the vent pipe is PVC?

The furnace and hot water heater should have no correlation, for some reason the HWT is not lighting off when the AC is running, likely cause if the HW tank is a high efficiency unit is combustion air proven switch.

But this is all supposition without knowing all the facts.
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Old 07-10-2011, 12:41 PM   #11
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No Hot Water when Central Air is On


Sorry I misread your 1st post I thought the pilot on the water heater went out.
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Old 07-12-2011, 01:12 PM   #12
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No Hot Water when Central Air is On


Jackofall1 wrote:

"Looking up the model for the heater only yeilded that it is a BW professional series, is the water heater a high efficiency unit, where-by the vent pipe is PVC?

The furnace and hot water heater should have no correlation, for some reason the HWT is not lighting off when the AC is running, likely cause if the HW tank is a high efficiency unit is combustion air proven switch.

But this is all supposition without knowing all the facts."

It's now been over two weeks since replacing the thermal couple unit. Three days after replacing tcu, the Central Air Conditioning and hot water were fine. after three days we woke up to no hot water. The furnace pilot light had gone out. I relit it and the pilot light and the hot water (along with CAC) have worked fine since (week and a half).

I would assume that the problem was fixed except for the one morning where the old problem returned.

What do you need me to tell you for you to formulate an opinion as to what's happening?

Thanks.
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Old 07-13-2011, 10:40 AM   #13
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No Hot Water when Central Air is On


Perhaps buy the electronic ignition kit for your furnace and get rid of the standing pilot presently used?
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Old 07-13-2011, 11:46 AM   #14
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No Hot Water when Central Air is On


Just to throw this out there,could it be caused by some "creative " wire work at the air con. or water heater? Are they on the same circuit? Maybe at the thermostat? I have seen alot of "very creative" things. Good Luck
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Old 07-13-2011, 08:59 PM   #15
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No Hot Water when Central Air is On


When the hot water went out did the water heater pilot light also go out?

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