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[kayakerski]

Unless you have a lot of 90s in the black iron it shouldn't be much worse than 1" pex. It is usually over 3/4" inside and 1" pex is only 7/8"

It sounds like it is just the large volume of cold water that is the problem. Is there any way you can modify the return pipes from the radiators? If you can do that, putting a zone valve in it with a small bypass around the zone valve would keep it from dumping all the cold water in so fast. A strap on aquastat could be used to open the zone valve once the returns are hot.

Another option that would probably be kind of complicated would be to get some type of a pulse controller so the pumps just run for a few seconds on and off till the cold water is all flushed into the system.

Probably the easiest is if you have an unused supply port on the boiler, run a pipe from it to the existing return port and put a tee onto it for the existing return and another pump (probably taco 007). Then put a strap on aquastat on the return pipe that turns on the new pump and have it come on whenever the return water is under 150 or 160. If you use large enough pipe you can easily get 2-3 times the GPM that is returning from the house. So if you are getting 6gpm from the house at 110° and 12 gpm from the new loop at 180° you will  have return into the boiler about 155° to 160°.

Wow! I have alot to think about here. This is all starting to get frustating. I guess I should look at increasing flow in other ways first?

[kayakerski]

How much 3/4" iron pipe is there in the system and is it sch 40 or 80?

The iron pipe from the owb to oil boiler is schedule 40 and there isnt much of it. Mainly a few fitting to get in an out of the owb and a few more to get into the oil boiler(I do have a 12" & 4" nipple on the supply though.

Also, the ports where the supply and returns enter both boilers are 3/4", if that matters. The owb has some unused 1-1/4" ports on it but the only other ports on the oil boiler are the 1-1/2" supply and return ports that feed the radiators.

Gregg

[kayakerski]

How much 3/4" iron pipe is there in the system and is it sch 40 or 80?

you think they would use expensive schedule 80 in a house boiler application (low pressure)

but on that same note (restriction) i would almost bet if they are old cast iron rads that  the pipes (and/or) the rads have a lot of scale in them and might be reduced  a whole dang bunch?

Some of my radiator supply lines may even be 1/2" but I'll need to check again.

How do you tell if pipe is schedule 80? Are the fittings thicker?

Gregg

[kayakerski]

My untrained mind thinks its the large amount of cold water dumping at once that causes the problems, but then again increased hot water supply should help clear it faster.

As I've said before, when its real cold out everything runs great and the oil burner doesn't come on for days. I guess that's because the water doesn't get time to cool in the radiators.

Gregg

[slimjim]

Yes that is what I would expect, again the oil burner can be controlled by the strap on aquastat that I suggested.

[kayakerski]

Yes that is what I would expect, again the oil burner can be controlled by the strap on aquastat that I suggested.

Yup, thanks slim. That project is in the works in the next few weeks.

Gregg