HVAC experts--I'd appreciate your help since we have paid the contractor, who agreed to fix the problem but I don't think he will have unlimited patience to do it.
Background: My house is heated with steam radiators. An addition at the back is heated with 2 Sunrad radiators off the main steam pipe. Because the long steam line and higher heat loss of the addition, it is always cold. The 2 radiators do have enough BTU to heat the space. The problem is if I crank up the heat the main part of the house would be too hot--too much waste.
Problem: We hired a contractor to convert the 2 radiators to hot water on a separate zone. The radiators have 2 pipe connections, he ran about 50 feet one-way of 3/4 copper pipes from my unused water coil in the steam boiler to the 2 radiators (in a series connection), with a circulation pump at the return end of near the boiler. But the result is a minor disaster. The first radiator is fine. But after the first radiator, the water is already cooled down so the second radiator doesn't get really hot. As a result the room is 10 degrees colder than when we had them on steam.
Again, the 2 radiators do have enough heating power for the room because on steam they were able to heat up the room in the coldest days. After the hot water conversion only 1 is working 100% and other at most 20% since there isn't much convection going on. The contractor came back once to increase the hotwater temperature and insulate the copper pipes, that only improved the heating slightly.
The boiler can produce enough hot water, and the radiators can radiate enough heat for the room. From the simple physics, obviously not enough volume of hot water is sent to the radiators. Is the size of the copper pipe 3/4 inch too small? Is the water pump too small (the contractor said that is the same pump for the whole house)? Can I add one more pump to push the water to the radiators (in addition to the current one that sucks the water from the radiators)?