Question About Expansion Tank

[ltgrady]

We're moving into an old converted barn. It had an ancient oil burning hot water furnace that was completely shot. So we ended up getting our hands on a Thermo Dynamics Boiler Company HT Series I boiler, it's about 10 years old and in really good shape. It was serviced every year and was working flawlessly but the old owner was doing a geo-thermal install and didn't need it anymore.

There were a couple of differences. Unlike my old furnace, the recirculated hot water from the baseboards and the supply line both flow into the same place right before the circulator that feeds into the furnace. We have an electric hot water heater and we're just bypassing the hot water heater in the furnace, capping the in/out pipes to the hot water heater.

The big difference and where we're stuck is the expansion tank. On the old furnace we had a classic expansion tank. Rubber diaphram tank hanging off the baseboard lines. However, the new furnace didn't have this. WHen we picked it up it was already cut out of the plumbing. There was a large metal tank above the furnace which had been cut out of the pluming system. It doesn't appear to be anything but a tank with a 1/2" pipe feeding in and a drain valve on the bottom. The HT furnace has a specific expansion tank pipe coming up out of the top of the furnace with an air vent above it.

I'm not sure what to do here. Is this read tank a form of an expansion tank? Do I just hang it to the ceiling like it was on the old furnace and sweat it onto that expansion tank pipe? I don't think it was installed with an air vent but since it was the old one I'm really not sure.

Can anyone give me a little direction? Thanks.

 

[Jadnashua]

If I was doing this, I'd trash the old tank and put in a diaphram tank properly sized for your application. The size of the tank depends on how much water is in your system and the temperatures you run it at.

It sounds like the boiler has a domestic water heater. As a result, the controls probably try to maintain a constant, fairly high temperature so you can get hot water when you want. This isn't necessary if you've bypassed it. Whether you can save some money by tweaking it to allow it to cool off some when your dwelling isn't calling for heat would take some investigation. A boiler often tries to maintain a temp between the high/low point that could be 30-40 degress or more. WHen it is trying to maintain hot water for your shower, the temp range is often much more tightly controlled, and always on.

Depending on your energy costs, you'd probably save some money using the boiler and an indirect WH, then feed that to the electric tank. You could use it to heat the water prior to the electric, and then the electric would only be used to take care of standby losses and you could shut the boiler off in the summer and then revert to only using the electric.