Water heater not starting up

Hi,
I bought a house last summer with oil heat (previously had gas) and within the past few weeks I've noticed the water heater isnt starting up on it's own. Usually when I turn on the hot water or run appliances that use hot water (dishwasher, washing machine) I can hear it rev up downstairs after a little while to heat up more water as I am using it.
However lately I'll be getting in the shower and the water just stays cold. If I turn the heat up on the thermostat it will turn on and heat water, but summer is coming and I don't want to be cranking on the heat when it's 80 degrees outside to do a load of laundry or take a shower. It doesn't always do this either, sometimes it works just like it normally does. What does this mean?? Is my water heater going? Any help would be appreciated, thanks!
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Sounds like hot water heater is part of your burner. There should be an internal thermostat on the unit to sense the need for more "hot water" vs. a call for heat by your wall thermostat. I be it might be ban and need replacement of the internal Thermostat unless you have turn it down (yes it should have a dial)
Dee5 wrote:

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Are you saying the hot water is connected to the furnace? This hints at that but nothing says so.
Do you have oil, gas, electric, or solar hot water?

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Yes my furnace heats the water using oil.
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Dee5 wrote:

The boiler will have an "Aquastat" to control the burner during times (summer) when the house is not calling for heat.
The Aquastat monitors domestic hot water temperature and turns the burner on when that temp falls below set limit.
Here's one discussion: http://experts.about.com/q/Heating-Air-Conditioning-696/Three-dial-aquastat.htm
It may be difficult to "find" the Aquastat as they take many forms. Some very new on es are even all-electronic.
Jim
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