Hello. My wife and I are hoping that someone here can help us determine the cause of our fluctuating temperature of hot water.
We do have a winter summer hook up and when we first turn on the hot water it is really hot, we don't have a mixing valve on it, but then it gets, for the most part, cold and then it will warm up again.
I have replaced the aquastat and the problem persisted so I then put di-electric gel on the sensor but still the problem persists. I did put in a bottle of Hercules Boiler Liquid prior to all this but it clearly states that "will not clog or coat submerged hot water heating coils,controls, vents or valves". I did drain the boiler after adding this stuff and it seemed to help but I don't know that for sure and really I don't know for sure if the problem existed before that because we didn't have a tub then as we do now and we don't have a problem showering. I did check the flow out of the tub faucet and it is about 2 or 2 1/2 gallons per minute.
Also, sometimes I can hear the boiler kick on and then quickly off in about 20 or 30 seconds and I don't think the burner heated the water form 160 to 180 that fast! Strange. Anybody think they know what is going on here?
Thank you man. (or women)
Jay

Having your hotwater run off your boiler is the worst way of getting hotwater. The main problem is that it will change in temperture. You get it so its perfect, then it goes colder and colder. The problem is that the water doesn't go slow enough through the coil for it to heat up enough to make you(or anyone) happy. Some people get used to it and it isn't a problem. Others want the water really hot and don't like the coil. You can try slowing down the water that goes through the coil so it has more time to heat up but it will lower the water pressure a lot! The only real soultion(other than changing the coil because over the years it builds up junk around it so it doesn't absorb heat too well) is to either go with a indirect water heater(your best option) or a electric weater heater(cheaper than a indirect).

The indirect water heater runs like a baseboard zone in your house. The zone runs through a coil(like your current one) and heats the water very quickly. This gives you constant hot water and good water pressure too. You would have to tap into your boiler to run a seperate zone from it to the water heater. All it requirs is either a zone valve or a circulator and a relay. The other benifit to having a indirect hot water heater is that the furnace will run all year round which is much better for it than for it to sit and rust during the summer.

The electric hot water heater will require a 220v line running to where ever you want it. This will also give you constant hotwater at a good water pressure.

You didn't say if you had gas or oil but you could also do a gas water heater(which would be better than the electric water heater) or a oil fired water heater. The oil fired is MUCH more expensive than the other options and isn't suggested unless you need a lot of water.

Adam, I just wanted to say thank you for your post on this subject. I previously didn't know of indirect water heaters. I guess my only perplexity is that the water doesn't just cool off but it ultimately gets warm again after it is on for a while and we get a good chill. The burner is sssslow in turning on but once it does it will return the water to comfortable tempurature. For what its worth.
Also, the boiler does burn oil.
Thank you again and I hope that you have a good day.
Best,
Jay

The temepature difference between high and low is about 20 degrees. So the low might be 160 and the hot might be 180. It will turn on when the temp gets to 160 and turn back off around 180. Some people like to have it at 180 and 200, but if you do it that high, just be very carefull the first few times because it will be a shocker.

We do a ton of indirect water heaters,I tell the home owner to stand by the boiler,I go upstairs and turn on sink and in a few seconds his 100,000 btu boiler fires to let me wash my hands. With the indirect tank you can go almost all day and NEVER fire the boiler.SWEET!!!!

I've heated my domestic hot water for 25 years with a boiler and have only experienced the problem you are having one time. It was caused by partial clogging of the domestic coil in the furnace. If you have a hard water problem, you may have a build-up of calcium or minerals inside the coil which would act as almost as an insulator. I solved the problem by pumping acid through the coil to disolve the minerals. Problem solved. You should probably make sure the the high and low temperature limits on your boiler are not set too far apart. It could be that the boiler is cooling off too much before it kicks back on.