Head Loss in Pex past Manifold and BTU Delivery in Radiant Panel Retrofit

Amovida

We are installing a constant circulation Runtal panel retrofit with a 125K Viessmann CU3A in our turn of the century home with a design temp of -2 and a heat load of 67K. I have had a professional heat load calc done and am trying to calculate head loss for sizing my pumps. I have a number of questions but the toughest two at the moment are these.

If I am accounting for frictional head losses in various parts of the system I need to know GPM flow in that piping. How do I calculate GPM for system components past the manifolds? I have 2 Uponor Stainless Manifolds (or I will) each in a home run configuration…one with 8 ports and the other with 10 ports. Each port feeds one rad.

Do I simply divide the GPM rate….6.7 GPM by 18 (the number of ports) which would give me a flow rate of .37 GPM at each radiator (assuming the system is balanced)? If I enter that into the Uponor pipe sizing calculator on the Radiant tab using AWT of 150 for roughly 914 feet of 1/2" Pex I get a Total Friction Head Loss of 5.83 Ft. (I have attached a screenshot). Am I even in the ballpark as to how I am calculating this?

uponor Pipe sizing calculator.png

My other question is regarding heat delivery. If I understand correctly I calculate the required flow rate to each radiator by using the formula: (BTUH) /DT (20°F DT X 500) = GPM. In my case for my largest radiator I will not have near the flow I need coming from the 1/2" pex from the mainfold. The heat load in that room is 9452 according to the heat load calc. I will probably solve this by using HE2 baseboard for an AWT of 150 (the only emitter that won't be a Runtal). Using the formula 9452 BTU's/ DT(20) x 500 = .94 GPM. This is far more than the .37GPM I calculated above (not sure I did that right). Assuming I did that correctly do I need a separate circuit and pump for that single large radiator? Or 3/4 tubing?

Thanks in advance….and this is a real compliment….I'm in awe for your not only being able to install these systems but do the calculations on them as well…my head is spinning. Any help here would be appreciated.

hot_rod

You balance flows at the manifold. A short run of 1/2" pex to a radiator could carry 1- 1.5 gpm or so. At a 20∆ that is 15,000 btu/hr.

Also panel rads often run a wider delta T 30- 35°, that can help with tube sizing and pump sizing.

TRVs are a great way to control panel rads. Simple non-electric proportional controls.

I would not like to see you mix fin tubes into the system if you design the panel rads properly you should not need to?

https://www.pmmag.com/articles/104489-john-siegenthaler-how-radiant-are-your-radiators

Jamie Hall

Fundamental problem in your head loss/flow calculation. It appears that you are using the total length of pipe in your entire system for the length of pipe — to get the 914 feet. Unless this place is immense…

For the head loss which you need to pump against, it is the flow for and the length of the longest loop that you are interested in, not the total length of all the loops. Flows add. Lengths don't.

Now your calculation of about 6.7 gpm total flow is about right.

Now on ports — each radiator in a home run configuration requires two ports, not one. One supply and one return. How many radiators do you have total? 18? If I take your building heat load — 67,000 BTUh — and subtract that one big room — 9,500 BTUh — and look at the rest, do I have 17 radiators left I get about 3500 BTUh for each of those remaining radiators… I hope that's not right…

Amovida

Thanks hot_rod….really enjoy your posts.

We will be using TRV's at each radiator…the non-electrics and no electrics at the manifold. I am thinking that getting a larger (30-35) delta T happens when you slow down your pumping…is that right? Is my way of calculating the flow rate beyond the manifolds correct? Dividing the GPM/number of supply ports (18) or radiators (18)?

Jamie- You don't think it's 914 feet from my boiler to my master???😀

Thanks I had also read about the longest loop and ended up a bit confused. Our longest loop will be the master. I will recalculate the head for the pex using that method. I'm guessing I need to meaure the distance for the copper from the take off at the boiler to the manifold and then calculate the one run of pex from the manifold to the furthest panel rad. Still fuzzy on flows versus lengths.

I have matched the panel radiators to the heating load calc I had done. I oversized the radiators that were rated at 180T because they will be supplied by 150AWT with design of 160SWT and 140 return. I used Runtal's multiplier of .67 to make sure the radiators fairly closely matched the individual room heat load with the lower AWT.

Not loving the sound of you saying you are "hoping that's not right". I am including a summary of the room by room heat loads. I reduced the total load from 74K to 67K as I was planning on leaving some radiators out of the unfinished basement..

Screen Shot 2025-04-04 at 12.53.42 PM.png

Jamie Hall

Well… hmm. Looks like you do have perhaps 18 radiators or thereabouts. In which case, if each one is a home run, you need a total of 36 ports. However, there may be better ways of piping the system so that some groups of spaces with similar loads can be run off one pair of ports. Provided the radiators are then piped in parallel within that group, and using TRVs on them as you seem to plan you cold simplify things quite a bit…

Amovida

Thanks Jamie…In at least one area I am planning on doubling up the panel radiators for one pair of ports and based on your comments I will look for other places that might be able to be done. The routing is fairly straight forward for us with the home runs. We have some areas of the floor open upstairs where we can run the pex. For the main floor the basement ceiling below is open so we can run the 1/2 pex lines there fairly easily to each panel radiator.

I am noting hot rods comment that mixing fin tubes into the system is not a great idea and I read the Siegenthaler article…thanks.

hot_rod

Small radiators on short runs can often get by with 3/8" pex.

In some of the old castles I have visited in Europe, panel rads are retro fit with 10 or 12 mm soft copper tube, snaking around the stone walls.

The wider ∆ also brings back colder return for higher boiler efficiency.

It is all about the design, required BTU output, selected SWT and ∆.

If you do series a few radiators be aware of temperature drop. Or use an H valve with adjustable bypass.

Amovida

Thanks Hot Rod!

I married a Spanish gal and whenever we visit Europe I marvel at the different types of hydronic heat supplies….obviously Caleffi is the best! ; )

I also grew up on concrete solar heated radiant slab floors and for years we couldn't afford the flooring…so each year my parents gave me a pack of tube socks cause the concrete quickly wore holes in our socks. That and the wood stoves and radiant heat is in my blood.

Thanks for the H valve suggestion…I'm pretty sure they won't play with Runtal's stock configuration but I will keep them in mind. At the moment my plan for having 2 rads on one home run is no more than for two small radiators in separate parts of a bathroom. Mille Grazie!