Furnace heats house OK, hot water sometimes ?


My brother-inlaw has problems with his hot water supply. The furnace heats the house OK but the hot water, supplied by the furnace, seems to work best only when the furnace is also heating the house. It seems like the furnace has the wrong control, one for a furnace that only supplies home heat. Assuming that the furnace has the proper control does it seem that the control has failed in some way such that the furnace will cool all the way down when there is no demand for home heat and thereby allowing the heat exchanger coils to no longer heat the domestic hot water?
Thank you for any ideas, andy
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There should be an aquastat somewhere on the boiler. It's basically a thermostat for the hot water. There should be a dial on it for you to adjust, normally hot water is set to about 150. Either it's set to low, or the aquastat and/or the wiring is not working right. that's why you only get hot water when the boiler comes on to heat the house.
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If it's a fairly modern boiler, with a domestic coil, it will probably have a triple aquastat relay mounted right on the front of the unit. It'll probably say Honeywell on it. If you have one like this, it will have two adjustable dials inside, One for high temperature cut out, and one for low temperature cut in. This type of boiler is designed to maintain a tank of hot water for both domestic and heat. If the low temp cut in is set to low, it won't fire up unless the heat thermostat is raised. Check to see that the high temp cut out is set for 180 and the low temp cut in is set for 150

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