Poor Performance from Runtals

msalb

Guys:

We have a combination of new Runtal and Myson radiators and old cast iron. The cast iron works beautifully; the new ones are lackluster. They do not get as hot or stay hot. They seem to have little water flow except when the boiler is working full bore, like when heating up the house. These new radiators run on narrower pipe than the cast iron radiators do; I'm thinking that there isn't enough pressure in the system to keep water circulating in the radiators fed with narrower pipe.

We have auto air vents that do a great job keeping air out of the system.

Does anyone have any ideas about how to improve these radiators' performance?

M.

Zman

I sounds like your lazy water (all water is lazy) is finding an easier path through the the cast iron radiators leaving the Runtals with lack of flow.
How is all this piped? Some balancing valves may be in order.

hvacfreak2

I would say that the different radiation types would have to be isolated ( piped ) and controlled differently from one another. Example : the cast iron zones may do better with warmer water and run for a shorter duration , and the Runtal may do better with cooler water for a longer duration. The Runtal catalogs seem to have plenty of design data to work with . One thing is clear to me as I get older , when it gets cold outside there isn't anything that can compete with cast iron , notta nothin.

Jamie Hall

If I assume that the Runtals -- with their high flow resistance and narrower pipe -- are on the same zone as the cast iron -- with low flow resistance and bigger pipe -- it's no wonder they don't work well. As @Zman said -- water is lazy, and it will go where it's easiest. Balancing valves might help by throttling the flow to the cast iron radiators, but keep in mind that that will raise the head loss in the whole loop, and you may be disappointed by the resulting low flow. It would be better to split them into their own zone.