We have at least twice as many radiators on a single boiler in a much larger house. The boiler does not need to heat all the water at the same time. It circulates the water through the radiators and heats it as it circulates through the boiler. So while it may heat only 2 gallons at a time, all the water in the system will circulate through over a period of time.

Note that this is entirely different if you're talking about hot water supply for baths/sinks--then you need a storage tank.

Thanks heimert!

No, I have a separate hot water heater for the bathtub and the sinks (2) in the house. The boiler is just for the radiators.

Thanks!

Using a modulating/condensing boiler that has outdoor reset changing the water temp based on outdoor temp is perfect for a cast iron radiator system. Before forced circulation boilers were sized by the sqauare footage of radiation and tended to hold alot of water. In your case, with proper piping, the boiler is sized by total BTU's required to meet heat loss of the house. Don't be surprised when the radiators don't get steaming hot however, before your stat is satisfied. With forced circulation the entire radiator will be warmed with heat stratifiication of the sections not happening.

The key to a good, efficient and comfortable install is the piping. You should be looking at a 2-pipe reverse return system. That means the first radiator on the supply pipe is that last radiator on the return, thus first radiator on the return is the last on the supply. This affords even heating because of distributed flow rates across all radiators. If you have manual or thermostatic valves on each radiator, the flow will not be interupted to the others allowing for individual temp control or zoning.

Hello to you all. I have been meaning to drop in and give you an update on the performance of our new Triangle Tube Solo 300 boiler. Having read all the posts, I can reaffirm that the cast iron rads will never be really hot as they were with our old 1919 steel boiler but the heating is very satisfactory. We have an 8 unit apartment building four storeys high and I am at the top of the building. I keep the thermostat which is installed in the hall of my unit at 21 Celsius which is the law here in Toronto for heat temperature of Apartment buildings. At night, thermostat goes down to 19 Celsius till I get up at six am. On the first, second and third floors, rads are hotter than on the top floor, This puzzles me but I figure that once the stat hits 21 Celsius, the pump or whatever kind of slows down so my long living room cast iron rads only get heated all the way across when it is really cold. We have the reset outdoors and the little boiler is a marvel. It sits on its little steel table in the old boiler pit and is so quiet. It vents a light warm vapour, no smell, out the side of the building. We are awaiting the first natural gas bill. I will keep you informed of savings over last year.

I just had to weigh in on this old post.

First, before sizing radiation or boiler you must know the load. Using a heat load software program an experienced hydronic designer can find the amount of energy (DOE output measured in Btu's) needed to heat the space on the coldest day of the year.

The boiler is sized accordingly, but not more than 20% over the design load.

Second, you have to choose the type of radiation, size and place according to the individual room heat loads. Cast iron radiators are sized according the potential output at a given average water temperature.

I design cast iron radiator to condensing boiler conversions and find most will operate well within the condensing range of a properly sized modulating condensing boiler using outdoor reset to increae comfort and lower fuel bills by as much as 65%!

Here is a link that might be useful: condensing boilers and cast iron radiators

badgerboilermn-

Are you still available for a reply? The link you left on this string 10 years ago is no longer available.

My son and his wife own a 68 year old, 1.5 story cape in Concord, NH that's been weatherized. The heating system includes a 35 year old standard efficiency oil boiler with cast iron radiators and 1.25" copper mono-flow distribution. It's time for at least a boiler replacement. If the masonry chimney is included in a new system it needs a liner.

Can you suggest an option or two? Stay the course and replace the boiler with a similar unit; adapt a HE boiler to the existing distribution; keep the radiators but replace the boiler and the piping; or replace everything. Any wisdom would be appreciated. Thanks!