Heat transfer

Qdoggy

Gentlemen, I recently built a hot water storage setup. It's consists of a 500 gal. tank well insulated, a copper heat exchanger coming from a wood/coal boiler and a black iron pipe heat exchanger (home made) feeding the oil boiler. I get the water temp in the tank up to 180 degrees F. but only seem to get about 150 F at the oil boiler. The piping is 1" IP and is not yet insulated. I'm sure I loose about 10 F from the pipe it self, but not sure why the oil boiler is not achieving the heat at the tank. I'm using a Taco 007 for a circ. The run is apprx. 30ft. Does anyone know if the problem lies in the black iron heat exchanger or the 007 or both...?

Thanks

Qdoggy

Gordan

Why are you "feeding" the oil boiler with the stored hot water?

If the goal here is to use the oil boiler as a backup, then the common way to do that is to circulate system water either through the storage tank or, when that cools sufficiently, through the boiler, but not both.



The biggest issue is most likely your homemade heat exchanger. I doubt that it is particularly efficient, nor is iron a good choice of material for an open system.

Qdoggy

Heat transfer

Thanks for your thoughts Gordon.  I have to agree on the IP being ineffient. I'll have to go to copper or aluminum when I break it down next summer.  As far as the boiler water... Do you mean tie in the feed and return to the rads using a zone valve operated by the thermostats? As it is now, my boiler water is being circulated thru the HWS tank. Just that it's a closed loop due to the heat exchanger. I didn't want to introduce the oxygen to the oil boiler, which is why I used heat exchangers on both ends.

Gordan

With multiple heat sources, you need something a bit more sophisticated.

A picture is worth a thousand words - post a diagram of what you're proposing to do as fodder for discussion.



If you're proposing to pipe the boiler in series with the heat exchanger in the tank, that's usually a very bad idea for many many reasons. How are you preventing the boiler from heating the water in the storage tank through that same heat exchanger, causing it to be a net drain of energy rather than a net source? How are you preventing this large thermal mass from causing your boiler to condense and corrode away its heat exchanger? Typically, the heat exchanger is treated as an alternate heat source - you circulate through it OR you circulate through the boiler, not both. That would mean separate circulators on the heat exchanger and the boiler, and hydraulic separation via closely spaced tees for each (or other means.)



Also, the reason your iron heat exchanger is probably inefficient would have to do with its effective heat transfer surface area compared to flow, and with flow through it probably being laminar, which kills heat transfer. It doesn't have nearly as much to do with the material itself. You can have all the heat in the world stored in that tank and it won't help you much if you can't get the water you pump through the exchanger to be within 20 degrees of the water in tank, at the flow rates that you need for your heating.