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# How to size pump for panel radiator system

Member Posts: 4
I'm installing a panel radiator system in an old house. I'm planning a homerun system, running TRVs and a differential pressure bypass valve. There will be 12 radiators total between the upstairs and downstairs. How should I calculate the head loss for the pump? Is it the head loss for each individual circuit + individual circuit+ individual circuit... or is it the head loss for the longest circuit? Or is there a formula to figure this out?

Also, any recommendations for best brand of boiler to look at? I'm looking for a combi- boiler, direct vent.

I attached a screenshot of my calculations, if they make any sense to anyone else. Feel free to poke at any glaring errors you see.

• Member Posts: 78
edited November 2021
I am working on a similar setup.

PANEL RAD BTU / DELTA T(SWT-RWT) * 500 = GPM

0.05(a WATER TEMP) * .71213 (1/2" PEX) * TOTAL FT OF PEX TO RAD(SUPPLY AND RETURN)
*
GPM^2
=

The value of 0.05 was for a SWT of 120, that value would change depending on your SWT. I forget where I found the chart.

The problem im facing with my setup is low velocity/turbulence with my planned home runs.
• Member Posts: 15,510
You calculate the head loss of each circuit at the flow for that circuit only. Then take the circuit with the most resistance and use that #. It's usually the longest circuit but not always. You have to do the supply and return and the radiator itself

Then add the portion of the common piping (headers etc.) that are common to all circuits (at the total flow rate) and add that in.

As far as the boiler and near boiler piping and accessories it depends, if you are using the same pump to pump through the boiler then add the calcs for that. If you have a boiler pump (primary secondary or low loss header ) then don't add it.

So at the end you will come up with the total GPM of all circuits and you head will be the loop with the highest resistance + the common piping resistance
• Member Posts: 22,103
looks like you are a numbers cruncher.This journal takes you through all the steps and has plenty of good installation tips.
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_16_na_0.pdf
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 78

You calculate the head loss of each circuit at the flow for that circuit only. Then take the circuit with the most resistance and use that #. It's usually the longest circuit but not always. You have to do the supply and return and the radiator itself

So, even if you have 12 home runs/panel rads on a single manifold/zone, you just use a single home run's headloss?

• Member Posts: 2,766
GPM is cumulative , head loss to be used is the greatest circuit head loss
You didn't get what you didn't pay for and it will never be what you thought it would .
Langans Plumbing & Heating LLC
732-751-1560
Serving most of New Jersey, Eastern Pa .
Consultation, Design & Installation anywhere
Rich McGrath 732-581-3833
• Member Posts: 2,245
Generally boiler likes flow range between one third and three thirds of some maximum. So when TRVs are fully open choose pump for two thirds. That allows for miscalculation. And you want to be certain that bypass allows that one third.
• Member Posts: 2,779
edited November 2021
A smart pump would be my choice with no bypass needed. With TRV the load will change all day long , the circulator will adjust per load . I normally control the circulator with a outdoor thermostat if the boiler does not control the heat circulator . And control the boiler temperature with a outdoor reset .. This would flatten the heat curve close to a constant circulation . You will have home comfort and maximum efficiency .

Boiler Bosch condensing boiler would shine in this application . Being a serviceman and called in to fix them them all.....

Tip ..... Yes do a heat loss per room for a minimum . But it is ok to oversize radiation with TRV . I find to fit the space not to make it look awkward it may be oversized which will help the system in whole ..

No thermostat is needed , but if a room sensor is optional install one in the room with the largest load .

I have enough experience to know , that I dont know it all
• Member Posts: 22,103
TRVs, mod cons and delta p circulator are a match made in hydronic heaven. Full system cruise control, and near perfect heat delivery, when properly installed and dialed in
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 2,245
@Big Ed_4 What does a smart pump do with no bypass and all valves closed?

While we're on subject of bypass doesn't every pump want some pressure to work against?
• Member Posts: 23,270
jumper said:

@Big Ed_4 What does a smart pump do with no bypass and all valves closed?

While we're on subject of bypass doesn't every pump want some pressure to work against?

Most smart pumps will just shut off, or at worst reduce to a very low power state. No problem...

A pump doesn't need pressure to work against. Most of the pumps we play with in this business are centrifugals, and the flow through the pump is inversely related to the pressure difference between the inlet and the outlet (that is to say, more pressure difference, less flow). If the pressure difference is very small, you will get the maximum flow the pump is capable of (efficiency will be low, but that's another issue). If the pressure difference is large, there will be less flow. If all the valves are closed so there can be no flow, you get the maximum possible pressure difference between the inlet and the outlet. Stupid pumps don't do well under that condition as they will overheat. Smart pumps though, are usually fine.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 2,245
>>(efficiency will be low, but that's another issue)<<...>>Stupid pumps don't do well under that condition as they will overheat.<<

For me it's a big issue. Left to run wild that inefficiency cooks the pump. That is why I design bypass with enough resistance to limit flow. Intelligent pumps improve control but why take chances?
• Member Posts: 23,270
jumper said:

>>(efficiency will be low, but that's another issue)<<...>>Stupid pumps don't do well under that condition as they will overheat.<<

For me it's a big issue. Left to run wild that inefficiency cooks the pump. That is why I design bypass with enough resistance to limit flow. Intelligent pumps improve control but why take chances?</p>

Oh quite.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 2,779
edited November 2021
You can go either way , use a pressure bypass (the old way) with a standard pump .Or use a smart pump , no bypass needed . ... All could be calling for heat or just a few ...It is what the smart pump was designed for ...
I have enough experience to know , that I dont know it all
• Member Posts: 4
Thank you all for the help! I will be researching this whole smart pump option, it sounds like just the ticket.
• Member Posts: 2,779
Home run system with TRV's oh yeah
I have enough experience to know , that I dont know it all