CI Radiator retro

warmAK

Hi first time here.  I’m a mechanical contractor in Alaska and I have come across a project that I don’t do every day and wondering if some alternative experience can help me out here. 

We are remodeling a 9 unit 1940’s building in Fairbanks, down to studs, nothing is staying except all those old cast iron rads.  I have worked on plenty of systems with CI rads but never designed one.  

8 of 9 units are 500 sq ft with 2 rads per unit.  With all of our energy upgrades to the building (windows, insulation, vapor barrier) they are for sure oversized, but that doesn’t concern me as they will be quicker to satisfy and I will have a modulating outdoor reset setpoint. 

My question is…. Can I run the 2 rads in series with 1/2” pex?  I cannot seem to get any info on desired flow rates.  In my experience wether it be baseboard or slab heat 1/2-1 gpm will satisfy most properly piped terminal units or heat loops, I don’t think these would be any different.  That being said once it is roughed in and buried behind painted walls, it’s not a mistake you ever want to make.

the heat system- Is 2ea Lochinvar knight 199s 10ea pumped zones with grundfos upse pumps.  I have a dirt/mag spirotherm seperator and stainer to protect boilers from whatever crap these things send back. 

There is also a 2 bed unit 1200sq ft with 4 rads in it and a house loop for laundry and entryways.  Those will each be fed with a reverse return 3/4” loop with trv’s on each rad and may add a few buderus panels where we are short some old cast iron units. 

On the 500 sq ft units I also plan on running a couple of staple up loops just under the bathroom floors to warm up the tile. 4’x12’ max on the way back to the boiler.  And those will be run in 1/2” either way.  So if I’m going to neck down to 1/2” on the way back is there any advantage to supplying the rads with 3/4”?  And will it work? 

Also a couple of other questions-

i plan on just brushing the grease and grimes off these things and putting them back into service, don’t have time for stripping or sanding, and they are in good shape, but would you recommend painting them? 

For the units that are going to have trv’s, why can’t we buy these in America? Every site in Europe has nice looking trv sets for 27 eu, but no one will ship to me.  Here all I can find is chrome with a white head for 100 a set if I’m lucky. Like no one ever used these in America?

Flushing, I was going to flush each one individually before install but with almost 30 rads I decided that it would save me a lot of time to wait and just hook them up in a finished building and just TSP and drain the whole system in one foul swoop.  Also it seems maybe the more you disturb the more trouble you get.  The system was fully functional when it was demoed. 

Anyways I know that’s a mouthful I will take and post some pictures to help make sense of it.  Appreciate your time in reading and replying. 

warmAK

So I have decided to run a test with a temp heat zone and will see what the outputs are.

Jamie Hall

Those are nice radiators. Bit bigger than we see around here, but "here" isn't Alaska.

You will have a lot more flexibility -- particularly if you are planning on using TRVs -- if you run the radiators in parallel, reverse return, rather than in series. However, there would be no harm to running the radiators in a single space, controlled by a single TRV, in series.

On flow, the same old hydronics formula will give you the flow rate you need for each space. Those radiators are big enough, though, that you will be able to run them at relatively low input temperatures, so you may be able to run them at a lower delta T with a slightly higher flow rate. But the radiator size, input temperature, delta T and BTUh demand all interact.

Will half inch PEX be adequate? Probably, but I'd have to have the design flow rates...

psb75

Those cast iron rads + panel rads will not be oversized. ODR will take care of that. Make sure to "dial in" those boilers at the initial set-up.
Those studs look like 2" x 4" rough-cut. Minimal wall cavity.

warmAK

2x6 existing framing with minimal insulation and no vapor barrier.  With adding a vapor barrier and blown in fiberglass I’m feeling pretty good about having these rads running at some lower temps.  

I hooked a couple temp zones up one per floor, ran 1/2” pex to 2 rads in series no trvs and results were excellent.  As long as the smaller rad is first in the loop there is minimal temp differential between the two.  Bigger rad first does work but takes a bulk of the first 5-10 mins of the heat call. The gpm I’m producing? My guess is 1-2 is definitely sufficient for my application. 

4 rads and a unit heater is heating the whole place without the building envelope closed up. No new insulation or vapor barrier in the walls at -20 *f. 

Baby’s gonna puurr once it’s closed up. 

PS all series loops will be run off an independent tstat, I do have a house loop with 2 old rads and 3 panel radiators which will be reverse return and trvs