boiler circulation problem

Hi,

Forgive me if I do not get all the technical language correct here and the long post

Last year we replaced our old oil fired boiler with a new gas fired boiler. Our home has the original cast iron radiators and I believe it was originally a gravity system. The old boiler worked fine, it was just old and oil was getting too expensive. Upon installing the new gas boiler, nothing was changed, with the exception of a 3 way thermostatic (mechanical) mixing valve plumbed into the return line on the boiler, done according to specs from slantfin, to prevent cold water from entering the boiler.

Now for the problems. Immediately after the new boiler was installed, none of the radiators in the basement were getting any hot water, there was also one radiator on the second floor that was only getting warm. Everyone thought ok this is an air problem, so we began the tedious task of bleeding the system. After a lot of bleeding, nothing changed, so our heating contractor looked at the situation and thought it all worked great before the change and the "only" real difference was this mixing valve. He decided to bypass the mixing valve and this immediately fixed the problem, as all the radiators in the house were now getting hot, but we had the problem of the return water to the boiler was below the recommended temperatures and the manufacturer insists we need the mixing valve for warranty on the boiler. At this point the heating season was done, and a new mixing valve was placed back in the system. Now we are again in the heating season and are experiencing the exact same problem. Our boiler technician is at his wit's end, Slantfin has not been of any help and I am hoping someone here can maybe offer some advice?

You may wonder why only one radiator in our top floor is only warm, while the rest are hot. The only difference with this radiator is that it is fed from basically the same lines that feed the radiators in our basement. The whole thing really does not make much sense as we have had a few different people come in to offer advice, including reps from the manufacturers and such, biggest problem with most of these people as they are not used to these old style heating systems.

Thanks,
Chuck

Get rid of the thermostatic mixing valve and install a by-pass valve instead.

If I understand the logic, a bypass valve would send some of the hot water back to the boiler, making sure it never got too cold. Not as dynamic as a thermostatically controlled valve, but sounds good to me. Do they come with any adjustments so you can tweak the amount of bypass? (or is that necessary?)

Jim, I just use a ball valve. the problem with the thermostatic valve is they tend to over or under correct.

Thanks for the replies!
So why would the mixing valve cause me grief in my particular situation and cause the water basically not to flow through this one particular loop (not sure if that is the correct term)? In the basement, the rads are not heated all from the same lines, there is a number of branches that go and feed different rads. One set of the basement rads is also on the same line that goes to our second floor to feed the one bedroom that is not getting heat. Then I have another bedroom that is on the same line as one rad in the basement, the basement rad in this case gets no heat (backfeeds a bit on the return side as that pipe is warm) but the rad in the second floor bedroom gets hot. Like I said, when we removed the mixing valve, everything worked fine.
Thanks,
Chuck

Just a guess, but maybe the valve is restricting the flow just enough so that some of the radiators are fine but others further down the line aren't.

It's not so much a restriction of flow as it is a bypass of too much hot water problem. You can leave the tempering valve in place and put a ball valve in the mix line return to throttle it back some. Remember that water will always find the path of least resistance. If the resistance (head) of the radiators is greater than that of the by-pass, then that's where the water will go.