I installed a used electric 23 kw boiler in my new house last fall, and used a bronze Grunfos circulating pump from an old radiator system I had at another house. I have one zone with 7 loops about 250′ long of 1/2 "" O2 Pex.
There are 3 rpm settings on the pump, which one should I use?
Sometimes the slab seems to heat up to the touch very fast, and other times it barely gets warm, although all three [60-60-30amp] breakers are activated.
The boiler has 11 elements, and they all checked out fine after I initially installed it. The pump seems to run and pump under constant pressure. I have it set on the highest setting of 2475 rpms. the two lower options are 1700 and 2100rpms. The temp/pressure combo gauge reads 125 degrees and the pressure is between 15 and 20 lbs. The pressure drops to nil after the unit and pump shuts down of course. This is a closed system and a pressure relief valve is a 30 lbs’er.

Hmmmm…. <scratches chin>. First thing I’d check is the pump; make sure it’s not pumping sporadically. Since you’re using used parts, the impeller may be worn, and only pumping part of the time. Or, I’ve also seen old electric motors vary in speed as the bearings (or even the windings!) heat up and cool down… Stick a flow meter in line and watch it?

Next, I’d check the heating elements on the boiler. It is possible that one is burnt through, and only making "contact" with itself part of the time. Does the boiler have a temperature gauge? If it does, keep an eye on it to see if it varies a lot over time.

Hope this helps!

can a combi boiler interfere with electric shower as since it the combi was put in the shower not very?