An outdoor reset like the one included on many of the boilers youÂre looking at reacts to the change in outdoor temperature almost as fast as an eye blink. Large mass systems take longer to react regardless of whether it is a start/stop, constant circ or setback.
A low mass, high fire boiler from a cold start fires hard with the low temp water return and the outdoor reset will help prevent sending too hot of water to rads, which tends to over-produce heat before wall stat can react resulting in space temp overshoot and uncomfortable temp swings.

You donÂt have to set your system for constant circulation however, older homes with cast iron radiation and big pipes that arenÂt changed out usually find this to work the best. If you expect to do a temp setback often, constant circ may not be for you however, outdoor reset should still be used to avoid room temp overshoots and wasted energy.

The controls are constantly on anyways waiting for a heat signal using milliamps of current, so thatÂs not a factor and I would consider a more informed hydronic person.

Depending on your system, old gravity, two pipe, series, reverse return, someone with knowledge should advise you on proper piping changes. Without tearing out mains in basements and installing a smaller and appropriate sized copper in basement in a two-pipe reverse return configuration, many times the system is best suited to be split in two and returned from a center point and use two circs to distribute. On top of that a whole additional conversation would be to add thermostatic radiator valves (TRV's) to control individual rooms and balance house nicely.

Hi thanks z1700. Thanks so much for your response.

So, if I understand correctly you are advising that the outdoor reset will be beneficial even with our large radiators.

We have a heat pump right now and we do have the two pipe return. It used to be gravity but no longer.

I understand your advice on the individual radiator thermostats. We will have to think about that. The in-place system has met our needs very well -- very responsive and even enough so we don't notice any room-to-room variance. The only complaints we have are oil bills and the noise level on the oil burner. It is like a jet taking off in the basement right below our living room.

Our thinking is that we would leave the old Timken, the pump, the oil tank and burner in place --as a complete back-up system. We can afford the space and it would be less expensive to re-pipe to the new system than it would be to tear out the old one (it is huge). And we like the idea of keeping it here and having the back-up. Any thoughts about that?

We had talked to two contractors neither of whom had ever installed TT and both of whom led with the WM Ultra. Thanks to your information on TT, later this afternoon we are meeting with a contractor whose name I got from the local TT distributor. Since he was recommended as having the most experience with TT, even before meeting him I suspect he may be the one to do this installation. We are trying to understand all the nuances so we will be prepared. I greatly appreciate the realtime education you have provided me, z1700, whomever you are, and wherever you may be! Please let me know if you have any more thoughts for us. . . .