Possible to use old cast iron radiators for low temp?

weil_fail

Are there any calculations for fans/airflow you need to get equivalent heat out of a cast iron radiator with water heated to 170F? For different water temps around 120F? Is it possible?

EBEBRATT-Ed

If you are talking about cast iron radiators yes there are charts that show the output at lower water temps but fans have nothing to do with that. When you say "cast iron water heater" what do you mean?

PC7060

No need for active convection.  Just need enough radiators and tight enough envelope. I run a low temp system (116F SWT @ 11F outside temp) using original radiators in a 100 yo home.  

weil_fail

EBEBRATT-Ed, sorry for the confusing wording. I edited it. 

I'm trying to how much fans add to the heat output of a radiator. Like, is it possible to add fans and lower water temp and get the same btu output 

PC7060, I'm thinking of about the idea of lowering the temp in an existing system

PC7060

weil_fail said:

PC7060, I'm thinking of about the idea of lowering the temp in an existing system

Have you assessed the heat loss/gain of your house?  Determining the max BTU required on coldest day is the first step. 

What type of boiler is currently in the system?  Condensing boilers work well in this application but cast iron boiler may require tempering of the return water to prevent corrosive condensation of exhaust gases in boiler or flue. 

The changes in my system were limited to new modcon and additions of TRV for each radiator. 

Jamie Hall

weil_fail said:

EBEBRATT-Ed, sorry for the confusing wording. I edited it. 

I'm trying to how much fans add to the heat output of a radiator. Like, is it possible to add fans and lower water temp and get the same btu output 

PC7060, I'm thinking of about the idea of lowering the temp in an existing system

Well, yes it is -- but you have to direct the airflow over the radiator with shrouds to get any significant increase, and the work would be experimental. I know of no standard equations, never mind tables or designs. A fan blowing at an unshrouded radiator will have minimal impact.

weil_fail

PC7060, I'm just thinking generally about replacing high temp water boiler with lower temp heat pump

Jamie Hall, unfortunate that there aren't guidelines 

jesmed1

How do you know how much heat output you need? Just because the previous boiler ran high water temps doesn't necessarily mean you need to match that heat output. Many old houses with cast iron rads only need 120 degree water or less, because there's so much installed radiation. I live in one such old house where we can heat with sub-120 water throught the original cast iron rads. So do a heat loss calculation first.

hot_rod

Some ideas for you to look at here

https://www.caleffi.com/sites/default/files/media/external-file/Idronics_25_NA_Lowering water temperature in existing hydronic heating systems.pdf

flat_twin

https://www.expressradiant.ca/pdfs/product_classic_sizing_how_to.pdf