EFM 520 decreasing hot water supply,

I'm new to this but have been using coal stokers my entire life. Twelve years ago I purchased a new EFM 520 coal/oil furnace and it's always worked perfectly providing all the warmth and hot water we could use. We just had two new bathrooms installed 6 months ago, now just recently we noticed we have very little hot water available to use for our grandkids to take baths. it only last a few minutes at hot the rapidly gets cold. It never did that before and we never were able to turn the shower on full blast hot but now we do to finish our showers. Now the question, is my hot water coil going bad or is there something else I should be looking at? The plumber who installed my furnace worked for a local oil supply company and has retired and they have young service guys who are unfamiliar with this furnace. Any help or suggestions would be greatly appreciated..

The coil could have lime scale building up on tbe inside of the tube. It can be removed with an acid wash but best to have someone who knows what he's doing.

Welcome. Assuming you have not changed any of the settings on the stoker or aquastat, I think you have a mixing valve that is stuck and needs to be cleaned, and/or a coil that needs to be cleaned.

If the coil is more then ten years old, just change it out with a new one.

hard water, soft water?

assuming the stoker is otherwise working fine, putting out heat as per usual, right?

can you move the mixing valve barrel/dial? most of them are serviceable in the sense that the adjustment dial screws on and has a washer or few in there. I now 'exercise' mine every month or few - give it a full-range back and forth and then back to the setting as before. This is a result of having to rebuild it once when the hard water locked it up and it started leaking.

replacing the coil is a fairly straightforward that requires no coal-specific knowledge, it's basic plumbing. that said, you do want to inspect the rear of the boiler, where the coil slides in to make sure the gasket is sealing properly.

If you do have hot water for a while, and then it gets cold. First check the boiler temp , (in this cold weather, over 180 deg.) should work. If its been a long long time sence the boiler has been cleaned, that's something too .
Both of these are no cost issues to look at. If you see the boiler temp is a bit low , this cold snap could be putting the boiler on the edge , depending on the stoker settings, and coal quality.

Mixing valve would be the first place to check. Let the hot water run and put one hand on the hot feed line to the valve and the other on the hot out line. You'll know in a hurry if it's the valve.

With the coil producing satisfactory hot water for a few minutes of tub fill or most of a shower, I don't think the coil or mixing valve should be the main suspects. It takes a lot of btu's to make DHW and it sounds like you are using btu's faster than you are making them. To run even a single 2.5 gpm shower in real time, especially in winter, you probably need your stoker running on at least 5 teeth of feed. Other things to check include making sure the low limit is set to at least 160, and that the heating circulator shuts off below the low limit (to prioritize DHW). Of course, making sure you are running a clean boiler with good coal and ash ring is always good practice.

Mike