Bath water super hot for a few seconds then cold

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Old 03-31-14, 07:12 AM
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Bath water super hot for a few seconds then cold

Here's what I have. Oil fired burner, no hot water tank. When we use hot water anywhere in the house, the burner fires up because it is trying to maintain 180 degrees. All the hot water in my house works fine. My bath tub has two separate faucets; one for hot, one for cold.

So when I go to fill the tub with water, it takes the normal few seconds to get hot, stays hot for a few seconds, then tapers off to cold and stays that way. If I turn off the water and wait a few, the same thing will happen all over again.

I've researched and searched but found no info on my particular issue. Most of the time is a mixer valve issue of some kind which I'm assuming only exists in a single knob faucet.

I'm losing my mind.
 
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Old 03-31-14, 07:52 AM
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This is pretty typical of a " thankless coil " water heater. To put it bluntly, they 5uck. The only worse way of producing domestic hot water is a kettle on a wood stove.

Even when brand new they have a limited ability to produce copious amounts of hot water. As they age, the get 'limed' up with mineral deposits and this limits their capacity.

They can't keep up with higher flows which is typical of a bath or shower. A faucet tap may not flow enough to 'overwhelm' the coil.

The 'mixer' that you see spoken of is usually installed AT THE BOILER. It will be a tee-shaped valve. They do go bad, but if this was your problem I suspect the trouble would be with the sink faucets as well.

What happens at the tub if you turn the flow rate down? I know it's going to take forever to fill the tub but it's probably the only way.

What are the settings of your aquastat on the boiler? The HIGH , LOW , DIFF dials?
 
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Old 03-31-14, 10:59 AM
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You mean at the bath faucet itself? I believe I've tried that but I'll give it a shot again when I get home
 
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Old 03-31-14, 03:50 PM
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Yes, turn the flow down to maybe a gallon and a half a minute and see if the water stays hot.

Do let us know the settings on the aquastat also.

And look for a mixing valve on the domestic pipes coming out of the boiler and let us know what you've got there... pictures might be helpful.
 
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Old 04-06-14, 05:25 PM
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After reading your replies, reading some other things on the internet. Played around with the flow rate and that's exactly the problem. And apparently my wife has figured that out already as she fills the tub every night for my son. Go figure
 
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