Not enough Hot water from oil burner/possible bad coil?

[Mikepier]

My in-laws have an oil fired hot water baseboard system with a tankless HW heater coil rated at 5 GPM. Presently the aquastat is set for the boiler to come on when the water temp drops to 165 deg and shuts off when it hits about 185-190 deg. When someone takes a shower, the water temp initially is very hot, but then after about 5 minutes the HW starts to run out. Any ideas what could be causing this? Could the coil be dirty such that it impedes the heat transfer? The boiler heats the house with no problem, its just the HW part that has an issue.

[RBM]

The coil could be coated with mineral deposits, insulating it, or you could have a bad mixing valve .

[Mike rock]

What mixing valve are you referring to, the one on the boiler? I see a mixing valve, but it is shut off, which means I'm assuming 100% hot water out of the coil. If you are referring to the shower mixing valve, I should point out the same problem happens when washing clothes,dishes etc.

[Marilyn & Bob]

Is this a problem in the summer as well (when the boiler is not also used to heat the house)?

[RBM]

What mixing valve are you referring to, the one on the boiler? I see a mixing valve, but it is shut off, which means I'm assuming 100% hot water out of the coil. If you are referring to the shower mixing valve, I should point out the same problem happens when washing clothes,dishes etc.

You have a cold water pipe feeding into the coil on the boiler, and a hot water pipe exiting the coil. Somewhere at that location should be some type of mixing valve, so you don't get 180 degree water flowing through your domestic pipes. If it is off and not allowing any cold water to mix, the problem is with the coil.

[Mike rock]

There is a tie valve between the cold input and hot output of the coil, I am assuming this is the mixing valve to mix in the cold water. This valve right now is off, why I don't know, but in any event why would the coil be the problem if the valve is off? Are you saying maybe the oil company turned off the mixing valve to compensate for the coil being dirty? Regardless, if I'm getting "full strength" hot water out ( mixing valve off) theoretically I should have more HW than if the valve was on, so that's why I suspect a bad coil.

[RBM]

I suspect the valve was turned off when the coil stopped producing hot water. You should be getting full temperature hot water, unless the coil is dirty

[Ed Pawlowski]

Good chance it is the coil clogged or nearly so.

If the boiler is more than 15 years old, before dropping hundreds to replace the coil, consider replacing the boiler it with a more efficient boiler and a water tank that is indirect fired. I changed mine last December to a System 2000 and my oil use so far is down 32%. I don'[t have summer use yet, but I expect hat to be dramatic also. Ask the in-laws how often that boiler kicks in during the summer just to maintain the hot water at temperature. Quite often from my experience. With no heat on, I don't hear mine go on at all because the 40 gallon tank holds it at temperature for days if not used. Mine is from

If they opt for a new boiler, be sure to check out state funded rebates and

0% financing available through the state.

[Marilyn & Bob]

Before buying a whole new system just because the coil is failing, I would suggest buying a separate water heater. Install the water heater in series with the output of the tankless coil so that hot (or at least tempered) water goes to the heater and thus requires much less energy to bring the water in the heater to the necessary temperature. This also has the added benefit of letting one turn off the boiler during the months that it is not needed to supply heat to the baseboard radiators since, when not heating the house, a boiler is a very inefficient way to produce domestic hot water.

[Ed Pawlowski]

Separate heater may be a good idea, but not in series with a clogged coil that can restrict flow.

As for replacing the entire system, that depends on payback and present efficiency. In the right conditions, the systems is free of cash outlay. My interest free loan is $67 a month, but my savings in oil is bout $70.

[Van Chocstraw]

They are using the hot water too fast. Put a flow restricter on the shower head.