long loop with 3 aluminum Radiators and 1/2 pipe
Hello,
On my first floor, I am planning to put 3 aluminium radiators (not a baseboard; 3150BTU each at 160 degree design temperature) together in one loop using 1/2 PEX (or PAP) pipe. The loop is going to be about 140150 feet. Manifold is going to be at the basement close to the ceiling. Since supply and return for each radiator is only 1/2 inch anyway is this a right or wrong to think that 1/2 inch pipe for the whole loop will do the job? Thank you Regards, Vad 
Are you piping this up as a series feed to the panels, or as a parallel feed. 150' of pex won't move much water. How much water flow does each panel need.

Thank you for reply,
I am talking about series loop.
The vendor do not publish the flow rate. Here is a link with specs: http://www.wallhungboilers.com/pdf/s...rsBrochure.pdf Regards, Vad 
You won't get 3150 from those rads at 160 degree water temp.
Look at that brochure. If your room temp is 70, you will only get 2450. As a general guide. The water temp leaving the rad, will be 20°(to have only a 10° drop, water flow would need to be doubled) cooler then what its entering temp was. So piped in series, you won't get much heat from the last rad. Those rads are suppose to be piped in parallel. At 1 gallon per minute flow rate, ½" pex at 150 foot will have a pressure loss of roughly 4.8 PSIG. Add in the fitting PD's and the rad PD's. 1 GPM would be for a 20° temp drop through the rads. At 2 gallons per minute, ½" pex has a pressure drop of roughly 16.8 PSIG. Again add in the fitting and rad PDs. 2 GPM would be for a 10° temp drop through the rads. So you need more then a ½" pex to feed them in series. In parallel. You can probably get away with ½", and use the valves on the rads for balancing. 
Thank you a lot for the answer.
Correct me if I am wrong. I am thinking that since fittings on rads have only 1/2 diameter, it does not matter if the distribution pipe is bigger than 1/2 or is it is equal to 1/2  the flow in the loop will be the same. The Radiator fittings are the bottleneck.
Any way I decided to use the same Baxi radiators in a Living room that is much closer to the boiler and to use low flow baseboard (1.5 GPM; Runtal) for the long loop with 5/8 pipe. Now for baxi radiators the longest loop will be not more than 100'. If I connect just two radiators in a series loop with 1/2 inch pipe, is this sounds good assuming I do not use any oversized circulators? I guess that I just have to assume that the incoming water temperature will be 20 degree lower and my total BTU from all radiators decreas accordingly. My second concern about Runtal baseboard for a long 140/150' loop with 5/8 pipe. All together I will have 16 to 17' baseboard connected in a series loop with about 11600 BTU in total (two picies; one from 10' to 12', the other 5'6') . Runtal Tech support told me that I can use even 1/2 pipe since only 1 1/2 GPM is required. Tech also mentioned that all the fittings in there are 1/2 inch. I do not want to guess and will use 5/8 pipe (even though I am not sure if it makes a difference because of 1/2'' fittings on the baseboard). Do you think with 5/8 inch I can make it without oversizing a circulator? Thank you again, Regards, Vad P.S. Link to Runtal Baseboard. My Model is UF8; Exact Size I will decide as soon as I know if 2 picies will be connected together or piped separately. To get 160 degree output, coefficient .78 is used. http://www.runtalnorthamerica.com/re...r%20Series.pdf 
Putting rads together in series is never a good idea. Unless they are piped as a mono flow system.
Find out what the pressure drop is through those rads. The ½" valves aren't the only hold up. The pex has X pressure drop per foot per GPM, the valve has X PD per gallon per minute, and so does the rad itself. All of this has to be allowed for. Thats why its better to pipe in parallel. The rads PD is not accumulative then. 5/8" pex will have a pressure drop of 4.5 foot of head(in my other post, I was posting PSIG, it was suppose to be foot of head, not PSIG). Find out all the pressure drops of the rads at the GPM you need. 
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