panel rad piping

Kevin Coppinger · December 2002

I have never installed a panel rad in my life,(mostly baseboard and radiant floor). I however have wanted to do so in my own home and now I have the chance. I have an old home(1800 ad) w/ FHA and the manifold piping approach seems the easiest way to go. My question is... I have one run to a second floor that is going to be a real challenge to get any tubing through the wall. I need to feed two rads in two bedrooms w/ 1/2" pex. Can I use a Buderus panel rad diverter on each radiator, feed in to the first rad with the supply then feed the other rad off the other end of the diverter and return to the manifold? I plan on using trv on all rads.(i hope that is clear)Is that going to work? What if any disadvantages are there? I will feed two manifolds with one up15-42 circ. and use a tekmar control w/ reset. tia kpc

Mike T., Swampeast MO · December 2002

I think

It's possible.

Install as you described.

Potential pitfalls:

1) Make CERTAIN that both can be supplied with 1/2"! Since you say that run is going to be a pain, I'd bet it's going to have the worst pressure drop in the system. The TRVs will help out during moderate weather, but you might run out of heat in really cold...

2) Panel sizing will be a bit strange, but I believe this will keep both satisfied (again provided that sufficient heat is available in the 1/2" line):

a) Calculate the percentage of heat loss of the first panels' space compared to the total loss of both panels' space.

b) IMAGINE you have set the FIRST bypass valve to this percentage.

c) Calculate the return temperature after the first panel but before the mixing valve based on it operating at the FULL supply temperature at design.

d) Compute the mixed water temperature (after the bypass, but before the 2nd panel) based on the percent setting of the bypass valve.

e) Average the full supply temperature and the temperature you get from d).

f) Re-size BOTH panels to operate at this average temperature.

g) Compute total flow required for BOTH panels.

h) Compute flow required for FIRST panel at FULL SUPPLY TEMP AND NEW PANEL SIZE.

i) Calculate percentage of total flow required by first panel.

j) Set FIRST diverter to this percentage.

k) Set SECOND diverter to the INVERSE of this percentage. i.e. if first diverter at 40%, set 2nd at 60%.

Why the strange sizing? To keep comfort levels consistent in the two areas. If you sized like a diverter t system, your second panel would be larger than the first even if the spaces have identical heat loss. If the TRVs had the same setting, the first would be significantly hotter than the second. By finding the AVERAGE supply temperature and sizing both to work with this, the TRVs will balance the flow for you. With the diverters set as above this [should] be about the flow they "want" anyway.

Of course keeping the spaces at different temperatures will upset this balance somewhat, but as long as the spaces aren't wildly imbalanced in their heat loss, there shouldn't be a problem.

Mike T., Swampeast MO · December 2002

BIG GOOF!!!!

IMPOSSIBLE WITH ONE BYPASS as the 2nd TRV could stop the flow through both!!!!!!!

Will correct above message momentarily!!!!

Corrected now!

VERY SORRY! Should have made my drawing FIRST instead of AFTER when I was trying to figure out the "perfect" setting with a single diverter!

Mike T., Swampeast MO · December 2002

To Kevin

(Tried private mail, but didn't have the address.)

If you read the FIRST version of my post where I said it could be done with one diverter you likely thought me crazy!!

You were right!!!!

Was so involved in thinking how to size the panels that I completely forgot that the 2nd TRV would limit flow through the entire system!!!!

The steps to sizing the panels (as I think would work very well) aren't as involved as they sound. The Buderus literature on designing a panel system will have the information under the "Pressure Drop for One-Pipe System with Diverter Valves." That will give you the mixed water temp, and after that everything is easy.

Sizing this way will "mess" with the flow a bit. If the rooms have identical heat loss the FIRST trv will have less flow than the SECOND trv. If the spaces are reasonably close in heat loss, there shouldn't be any problem regardless of TRV setting and comfort will be maximized.

I keep seeing a possibility that by fully cranking one TRV it might be possible to rob the other, but I think this could occur REGARDLESS of how you size them--particularly with 2 1/2" TRVs operating off a single 1/2" supply line. I believe the way I suggested setting the diverters would minimize this possibility.

Sorry for the error,
Mike

Kevin Coppinger · December 2002

piping

Hi Mike, Thanks for the input. The rads are 24"x24" Buderus'. The 2 rooms have the same heat loss @4500 btu ,although one is on the north side and the other on the south side of the house.I thought that a 1/2" line could carry 25,000 btu but that assumes a 1-4 gpm at a 20* drop in temp.The buderus lit I have doesn't get into the manifold set up at all. It is only on those 2 rads I want to do this. I have to snake up walls and into closets.the walls are all hosehair plaster and not easy to break into and repair.
On a related note should I pipe this primary/ secondary or would this be OK as a normal loop and return? the boiler is a g115-21. thanks.kpc

Steve Ebels · December 2002

Kevin

I don't think you have to use the bypass setup. You can pipe them in a series if they are serving the same space, or just reverse return that loop if you need a TRV in each room. We have a system in a home we did a couple years ago with 2 manifolds, 40 feet apart,(piped reverse return) feeding thirteen loops. Those 13 loops feed 21 rads, some piped in series and some are reverse return. One of the RR loops has 3 rads on it. The whole house is circulated with one 007. Works great!

If you go series just upsize the second rad to account for the lower water temp. Buderus has calculations for this in their rad engineering manual. With that small of a load, you could likely just go up one size on the rad and be safe. If you need 2 zones off that loop and have the room, I'd RR it.

We just did a small home this fall with 8 rads. Ran a single 5/8" pex supply down the length of the house and reversed the return. Flows just as even as you please. Each one of those rads has it's own 1/2" supply. You don't need a lot of flow to get copious amounts of heat from a rad. Just enough to warm it over it's whole surface. Any more flow than that is just wasted pump energy.

I'll work on a drawing for you and see if I can get my recalcitrant H/P scanner to send it to the right place.

Mike T., Swampeast MO · December 2002

Model 22 (4\" deep)??

Provided you're planning on a supply (NOT AVERAGE) temp of about 190° at design, this sizing (24" x 24") seems close to ideal.

Bear with me a moment. Assume that the spaces have EXACTLY the same heat loss and the EXACT same setting. How will the TRVs react? The ONLY way they can produce the exact same output is if they produce the exact same AVERAGE temperature at the panels. When you first think of this it seems impossible considering that the second panel must work with a lower temperature supply than the first. How can this be? BY CHANGING THE FLOW, thus the delta T is how.

REMEMBER: Low flow = high delta t, high flow = low delta t WHILE GIVING THE SAME OUTPUT.

The Buderus panel radiator output table just happens to show very close to where this (same average panel temp) occurs.

Assume panel one has 190° supply and is operating at 36° delta t. Go a bit further in the buderus literature and compute the GPM flow at given this and the 4,500 btu heat loss. You will get about .25 gpm. The AVERAGE panel temp will be about 172°.

Now look at the 18° delta t curve. You will see that it has the required output (2,250 btu/ft) at about 180° SUPPLY.

Plug that into the flow formula and you get .5 gpm.

Put all of this into the formula for computing the mixed temperature of flow and you find that 3/4 gpm (the total flow) with 1/4 gpm going to the first panel (return temp of 154°) mixed back into the TOTAL flow yields about 178°. This would be the supply temp available to the SECOND panel. An 18° delta t from the SECOND panel yields an average panel temp of about 169°. This is 3° away from the average temp of the first panel so this isn't exact, but it's quite close. (Sorry, but I don't have the time right now to write the computer program to compute the EXACT intersection point. It should be with a bit higher proportion of flow to the first panel at near the same total flow.)

The IMPORTANT thing to note is that for nearly the same output at nearly the same average temperature the second panel is flowing DOUBLE THE AMOUNT of the first panel!

The combined flow of 3/4 gpm is well within the range for a 1/2" line so you should be fine. Remember that since you are using reset the "perfect" reset curve would yield the same flow regardless of outdoor temp.

Given this info, you would set the first diverter valve at 33% and the second at 66%.

Now, are the diverter valves really necessary? All I can say is "maybe." If you play with equations (or even just imagine) the effect of opening one or another of the valves wide, you'll see it is possible to wind up with some wildly differing flow rates/delta t.

I think using the diverter valves set to about what the TRVs "want" while both are maintaining the same temperature to be cheap insurance. What they should do is to make the panels react somewhat more slowly (but still reasonably proportionally) than if they had individual supply lines.

Regarding primary secondary, I'll make three GENERAL statements.

It's most useful when you expect return temperatures to dip below the condensation point for extended periods.

TRVs can conspire to produce surprisingly high delta t during moderate weather--if the supply temp is high and not reset the delta t can become near fantastic.

If I've made the proper assumption regarding your supply temperature, I doubt you will have a low return temp problem unless you regularly have extended periods of weather that barely require heat. Some very simple calculations based on weather assumptions (and your reset setting) should determine the need or lack thereof.

Mike T., Swampeast MO · December 2002

To Buderus:

You provide EXCELLENT data!!!!!!

Art Pittaway · December 2002

180*F

seems a bit high for a towel warmer??? Did I miss something as usual, I was under the impression that a rad.panel on the floor or out of the way may run at 180+ but if it's a towel warmer it was more like 120-140* to keep the blisters down?? Art

Mike T., Swampeast MO · December 2002

Not a towel warmer

A panel radiator. Am going by the "typical" design specs supplied in the Buderus literature.

Remember that the panels will be equipped with TRVs, the boiler will be reset AND that heat loss calculations are designed for a "worst case" scenario.

The ACTUAL temperature of the panels (and boiler supply) will be MUCH lower in most all circumstance.

The Buderus panel rad output tables do show that panels are reasonable linear in their output with changing supply temperature. Thus the "worst-case" in a heat loss is quite safe to use for calculations. What matters more than anything is that the heat loss calculations are PROPORTIONAL among the spaces in the structure--this comes from ACCURATE and CONSISTENT measurement.

Compare the output graph of B/B to panels (or radiant floor or iron rads) to see what I'm talking about.

Art Pittaway · December 2002

OK Mike,

just hate to here about someone getting branded in their bath on a 0*F day. Put in a "panel" next to a stove in a kitchen with no room and steam heat. "Panel Rad, Co." built a unit with a dummy outer panel so you couldn't touch the heated section. Worked great. Art

panel rad piping

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