Hi Jon,
Thank you for the continued interest and suggestions.
Background (lengthy):
It's a complicated system -- much more than the usual installations. In brief, the system of propane-fired boiler, indirect domestic HW, and house heat that I am using was not the primary system for the original builder/owner. There were two additional sources of heat. One was an indoor woodburner, which I never saw, and the other was a conventional outside woodburner. The latter was in working condition when I bought the house. The original builder relied on the inside woodburner for most of his heat, but probably used the propane boiler for domestic HW in the warmer months.
The outside wood burner had its own circulator and is an open system. That is, the wood burning boiler has a water jacket. That heated water came into the house and transferred its heat to the house system with a heat exchanger. The heat exchanger was in series with the return flow in the house and fed the house system through that crossover mentioned in my original post. Thus, when the propane boiler was off, the high resistance of that forced most of the hot water to the house system pump. The is also a manual ball valve that can isolate the return from the propane boiler so that resistance was not relied upon.
Since I no longer use the outside boiler, my initial question was whether that crossover was still needed, as its high flow of return water contributed to my cold house.
Back to the subject:
Both pumps work, and the new pump in the boiler, which has a slightly high capacity helped, but was not enough with 20°F outside temperatures. The boiler seems OK. There are 5 zones for the house proper and one zone for domestic HW. There is another zonie for the whole house that must be open for the individual zones to get full heat. When the domestic HW zone valve is open, the main house zone valve is (was) closed. Having that priority seems logical. The return line form the domestic HW gets quite warm and feeds the main return line directly. The problem that I discovered yesterday is that the domestic HW zone valve almost never closed -- even when I was not using hot water. Apparently, its temperature was set high enough that during cold spells, the stay warm setting was never reached.
As a test, I disconnected power to the main house zone valve and manually opened it. I got much better heat last night. Every room was at its set temperature. It's too warm today to do much more, but this morning, I lowered the temp setting on the domestic HW, and the system is acting more as I would expect. The circulating pumps at least turn off, and heated water to the house zones is much warmer. As a temporary measure, I may leave it like that or play with the domestic HW setting and reattach power to the whole-house zone valve.
I have not liked the indirectly heated HW since I moved here. In warm months (most of the year in my area), it seems foolish to run a 105,000 btu boiler just to take a bath or wash dishes. I have toyed with the idea of replacing it with a conventional hot water heater. This episode may be enough to get me over my reluctance.
The answer to your "SOMETHING" appears to have been the zone valve to the whole house.