Steel,copper or plastic pex, for 30 ft run between oil & coa

Good point. It's all a question of what you accept as performance. All boilers have a stated BTU production capability, rated in BTU/hr. This tells you how much heat the boiler can deliver in one hour of time when running full out. Your home has a heat loss also measured in BTU/hr. The calculation is based on the coldest day in your area. The assumption is you will need the entire capability of your boiler to heat your home on the coldest day of the year. So on the coldest day of the year your boiler piping must move that amount of BTU's. It's that simple.

Now, the coldest day of is a rare event and it would be even rarer to need to heat that home from cold start on the coldest day. Furthermore as a heating season progress you have warmed up the insulated structure part of your home day by day. So the demands on outdoor boilers and the underground piping are much less than the rated capacity of the boilers. There is no question that 1 inch PEX-AL-PEX tubing CANNOT transfer the rated BTU of the boilers they are connected to. They transfer a fraction of the boilers capability. If there is an extended cold snap, one that lasts days at the coldest day of the year design temperatures the home will get cold. It has to. If that's acceptable performance to you fine.

My point is that you can design a piping system that will transfer all or almost all of a boilers BTU to the desired location. It requires selection the right type and size of pipe and the right pump. You can get more BTU's through a small pipe with a larger pump. As I see it most underground outdoor boilers systems use 1 inch PEX because that's what's available not because there was any conscious engineering decision to use it. In principle a system designed to transfer the full BTU capability of your properly sized outdoor boiler could heat your home from a cold start in one hour on the coldest day of the year. That's my personal design criteria.

Freddy wrote:Lowes sells 1 1/4" black iron for $1.76 a foot by the 10 foot lengths. That's cheaper than I can buy it at the wholesale place! They'll even cut & thread it for free.

Hey Freddy,

Will they cut & thread it into smaller lengths than 10' for free? As in you walk in with a list of the lengths you need for your project?
I can get it for $1.45 a foot but no free cutting or threading.

Thanks,

Mitch

So 1 1/4 inch copper or black pipe is what I should use? should I go smaller or larger? thanks

I-1/4 Copper or Steel will work fine. Base your choice on cost and your plumbing skills. If using copper type M is preferred, no reason for the heavier wall and more expensive grades. Pipe should be insulated. The best insulation will be the rubber like insulation used on A/C lines. Be sure you get the correct ID insulation to match the OD of the pipe. The insulation can be purchased slit lengthwise or not. If not it needs to be put on while assembling the pipe. More difficult to do. With copper it will bend around els easily and stay on better than the slit and stick glue type. Steel pipe els will need a larger ID size insulation than the pipe. I find the slit and glue type will not stay on long term on els. You need to add plastic tye raps.

efo141 wrote:All the outside boilers use the Pex for long runs and it works. Why wouldent it work for a 30' run?

It does "work", but there are many outdoor wood boilers that have undersized tubing. On the other hand, 1" pex is probably sufficient for many of the outdoor wood boilers since many of the barrel-type designs only have 30-40% net efficiency. 200k btu's (advertised output capability) x 35% (net efficiency) = 70k btu/hr