Traditional radiators and low-temp supply

Andruid_2

I'm designing an air-to-water heat pump system to heat an old farmhouse. The homeowner wants options. I'm offering him these choices: Smith H2 baseboards, Buderus panel radiators, Jaga fan-coils (his request), and (maybe) traditional cast iron radiators for that old farmhouse feel. I'm looking at the output chart for traditional cast iron radiators and wondering, can I extrapolate down from 150 degrees F? Is there a point where traditional cast iron radiators won't be functional anymore? Has anyone tried to heat with traditional cast iron radiators using 120 degree F water?

mattmia2

CI radiators in old gravity systems frequently do well with low temp systems.

EdTheHeaterMan

You can also design the piping to add a small backup boiler. It could be installed now or later. There are inexpensive electric boilers available or you could use a Tankless GAs or LP boiler. It will only go to high-temperature boiler operation when the heat pump can't keep up. (Assuming you employ the proper control logic)

Otherwise, you may need to use 2 times as many radiators to get the required results... I would think that the cast iron would do fine 90% of the winter on a heat pump. The boiler would kick in when the outdoor temperature is near design conditions (TRANSLATION: When it is really really cold outside)

Ironman

Have you done a manual J load calc to determine how much heat is needed? That’s the foundation of all design work. Make sure you start there.

All of the radiators that you mentioned above have specific output tables from their manufacturer.

Here’s one for CI rads:

Jamie Hall

To answer the extrapolation question. It's not exact, but you can. Divide the output at 150 by 80, the temperature difference between the know output and nominal space temperature, then multiply by the difference in temperature between your planned water temperature and 70. For instance, if the output at 150 is known to be 120 BTUh/EDR, then the output at 120 will be about 85 BTUh.

psb75

The farmhouse owner is obligated to take ALL measures to first air-seal and then insulate the building to the best degree possible. Only then, can you consider this ATW heat-pump and low temp radiation using (potentially) cast iron, or other low-temp compliant radiators.
I have done it, using a Viessmann boiler and 20 c. i. original and salvaged, some panel rads. and towel bar radiators. Most of the MI winter can be done with 120F and less supply water temps.
You'd best have a back-up mod/con boiler with that heat-pump.

Jamie Hall

Obligated? Well, not a word I like to use for people. Perhaps a phrase like recommended, or advised might be better. If you told me I was obligated to do something to my house, I would escort you off the property.

Now that out of the way. Yes, doing the best one can to reduce infiltration and improve insulation is a very good idea -- to the extent that it is feasible.

Alan (California Radiant) Forbes

If it's straight math, just show the ratios, cross multiply and solve for x. But output may not be linear.

mattmia2

Isn't the delta t in the air temp also a factor and doesn't it become linear if you add that in?

Jamie Hall

mattmia2 said:

Isn't the delta t in the air temp also a factor and doesn't it become linear if you add that in?

It would, or come very close, if radiation weren't a factor. But... radiation is a factor, and radiant heat transfer Sadly, radiant heat transfer is related to the fourth power of the absolute temperature of the radiating surface and the emissivity of the surface -- and the temperature of the enclosing space matters, too, as all the objects, plus the enclosure itself, are similarly radiating. So the math gets -- to put if very mildly -- very messy very fast. Fortunately for us, convective heat transfer (linear with respect to temperature differential) dominates...

hot_rod

some additional info and ideas here.

https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf

Andruid_2

Wow! You are all awesome! This really is an amazing place to hang out. For what it's worth, the heat loss is around 26k btuh for almost 2400 sq ft of home...our design day is 10 degrees F. My plan is to do a manifold on each floor each with its own Alpha pump...on all the time. Each radiator will have its own TRV. Although having a fin-tube baseboard with a TRV sticking out the end might be awkward to install.

hot_rod

If you can snake the tube down a wall, you can remotely control a TRV. Thermostatic valves are a great way to zone and control radiators.