Flat Panel Radiator Flow

[HeatFarmer]

So I'm trying to decide if my flat panels are operating as efficiently as they should. Knowing next to nothing about balancing hydronic systems I'm throwing this out here......

I've got an Alpha circ pump showing 2gpm feeding an 8 port manifold with 6 ports activated. I've got a total of 300 feet of 1/2" pex going to 6 varying sized flat panel radiators. Right now all radiators are emitting heat & fully purged. The heat is nice & the house is SO much warmer given that it is 5º outside and before things would be frozen in the house with all wood stoves AND propane furnace roaring....

The panels are hot to the touch, but not unpleasantly so. Incoming supply water is showing an average of 160º and output is just above 140º. All of the radiators are wide open, i.e. the non-electric thermostats are on MAX. What should my flow meters be set to? Should I up the flow rate to achieve more heat and be able to shave off supply temp? or is an average of 1.2-1.5 gpm per circuit good enough? Should the longer runs have more flow or less? Should the higher output radiators get preference?

Looking to be educated and improve performance of storage & keep my sweeties warmer.

 

[DaBackBurner]

If you lower the supply temperature you will have to raise the flow rate to achieve the same BTU output you are getting now and vice versa. I don't have the specs in front of me but just shootin' in the dark your flow rates at 2gpm/6ckts for 0.33gpm/ckt seems a bit low to me. Normal slab radiant flow rates are in the 0.6gpm/ckt range so maybe tweak them some. Your manufacturer specs should show info for your particular panel(s) like...supply temperatures at such and such flow rate will yield such and such BTU ouput. They also usually show correction factors for different supply temperatures and what you could expect for BTU output.

 

[HeatFarmer]

Bob, thanks. Your post prompted me to call the manufacturer to see what they recommended as a basis. I spoke to the salesman who sold me the rads and as we had discussed the system he could give me some recommendations.....he suggested .5gpm per radiator was a good average.... The slower the water moves through them the more heat exchange happens up to a point. He also suggested I modulate my Non-electric thermostats so they regulate the flow a bit more than they were. Small tweaks, but tweaks nonetheless. My Delta-t for the system appear to be with in the 15-20º range, so my supply & return temps are within normal.

I found, in researching my radiator literature that I screwed up...these rads are designed with a 200º supply and 160º return temp for a room average of 68º. There is a table of adjustments for varying supply & return temps which effect the sizing of the radiators....... I should have paid closer attention to these factors when I designed my system. I sort of did, but just didn't make the adjustments.....

Since things are much warmer than before and everything is humming along I'm not going to fret.

 

[flyingcow]

Heatfarmer- Glad everything is working better. But i feel your pain on not reading everything and using the info correctly. No offense, but it helps hearing these stores. :smirk:

 

[in hot water]

The radiators should be sized to meet the heat load of the room at the supply temperature you intend to send them. Most manufacturers have de-rate tables, as mentioned. 180F supply is fairly hot for a panel rad. 160F is a good temperature.

Actually the faster the flow the warmer the temperature across the entire radiator. The warmer the radiator the higher the output. Slowing the flow will reduce the output some what. It's all about the delta T, the temperature difference between the radiator surface and the room temperature.

Baseboard fin tube is often listed at 1 gpm and 4 gpm flow rates to show the output difference. Changing flow rates does not have as big an effect as changing the temperature, however. Most reset controls modulate the temperature to the heat emitter to get a change in the output.

hr

 

[tom in maine]

With 2 gpm and a 20F delta T, you are delivering about 20,000 btus/hr. assuming the panels can output that amount.
You can work the ratings numbers on the 6 panels that are operating.

I suspect a higher flow would be necessary.
Ultimately, you probably need more panels for the biggest heating load days. You might be
able to install two panels in series if they are left open.

As you say, it is better than it was. This falls under tweaking the system.
Being warm is nice.

Nice Job, this was a big one and you have done well to get to this point!

 

[woodsmaster]

I think you want the flow as low as you can get it and still get plenty of heat. That will make your storage last longer
with less mixing. I usually have about a 20 degree delta T.

 

[tom in maine]

I think you want the flow as low as you can get it and still get plenty of heat. That will make your storage last longer
with less mixing. I usually have about a 20 degree delta T.

Heat is measured in this case by flow in gpm X 8.32 X 60 X 20F Delta T.
This equals the btus/hr.
Increase the flow, the heat output will increase within the parameters of the heater.

Increase the delta T and the output increases.

We usually shoot for 20F delta T.

8.32 and 60 do not change in that equation only flow and temperature difference make the numbers change up or down.

 

[jebatty]

Heat is measured in this case by flow in gpm X 8.32 X 60 X 20F Delta T.

Simplify to gpm x 8.32 x 500 (although I use 8.33 - difference is not significant).

 

[HeatFarmer]

Tom & JT, I've tried screwing around with the crappy Rifeng flow meters and can't get them to change much. My Alpha pump stays at 2gpm no matter what I do. I have it set in Auto Adapt. I may try changing to either of the 2 middle settings and see if I gain or loose anything with more pressure or speed.

Right now, the combined sizing of my 6 radiators is 27,020btu/hr at 180º. Adjusting for my lower temp I should be getting 18,013btu/hr out of the radiators to achieve 68º The 20,000btu/hr I'm throwing at them is giving me the 60º room temp I am seeing. So I either throw higher temps at the rads or try to get more flow from my Alpha.... I'll give the second a shot first, to try & save storage.

 

[HeatFarmer]

UPDATE: I adjusted the Alpha to constant pressure setting #3 and shot up to 4GPM.....I measured the panel rad output vs where it had been before using an IR thermometer and touch. There wasn't enough difference to make it worth the extra wattage wasted. I just lowered it back to pressure setting #2 and dropped to 3GPM at 17w which may be a happy compromise.

 

[Jeff S]

I think Tom nailed it,you either have to add more rads or use higher temps.