I have a NTI-Trinity condensing hot water boiler heating three zone on three levels (basement radiant heat; first and second floor cast iron radiators and a couple of fin radiators). Its a completely new system with new piping to all radiators (radiators are original).

The problem is this. When the temperature drops below about 30 and all zones are calling for heat, the hot water flows only to the first and second floor bypassing the basement entirely. Naturally, the basement zone temp drops (to below 50 or less). When zones 1 and 2 no longer call for heat, the basement starts to get hot again. But, being that its cold outside the first and second floor call for heat again before the basement reaches 60 (by the way the basement is radiant heat in a new slab with insulation underneath). This is not an issue when the temperature outside is in the upper 30s.

The system has three pumps: an Taco pump, and a zone pump for each of the three zones. The pumps seem to be the proper size and seem to be working fine.

I've been told I can solve the problem in one of two ways.

First, by creating more head by restricting the amount of hot water flowing to zones 1 and 2. I've tried this route first. I've retricted the water going to the other zones by almost 80%, and its helped. The basement now will heat to 66-68 range. Still on exceptionally cold days with temps in the mid twenties, the problem resurfaces and the basement drops to about 60 degrees.

The second option suggested was to have a fourth pump installed.

What is going on here?

Do I need a fourth pump to force the water into the basement loop?

Shouldn't adding heat to the system by shuting down flow to zones 1 and 2 have solved this?

Do I need self-regulating valves to stop and start flow to the other zones?

Any suggestion would be much appreciated.