Pressure Drop Differences bet new panel rads & old convectors

D107

We'll be replacing four old fan-assisted hot water heating units with some Buderus 22 series panel rads. I would assume (?) the old units have a higher pressure drop than the new ones, but most of the other units on this 1950s mono-flo system are standard convectors, probably with the lowest pressure drop of all. (The fan units came in the 70s. To compensate for pressure drop then the loop was split twice right at two of the units.)



Heat loss for this loop is about 40KBtu; gain is twice that. So can I assume that if the house heated well with the old fan units, there would be no problem with mixing four panel rads (of lesser rated outputs than the old units) with six old convectors?

CMadatMe

Btu is a Btu

And the pressure drop has zero to do with it. If your question is, can I deliver the required gpm needed to move the required btu's then mine is, what is your head loss and which pump did you choose for the panel rad zones? You shouldn't be combining these on the same zone as your convectors, you have two different masses..

D107

convectors/rads

Thanks. The one-inch monoflo has a run of about 150 ft which I think came out to about 8ft head, 4gpm. Runs with a Taco 007. The installer didn't go through these calcs at the time, 007 is sort of generic around here. Has worked pretty well.



Original zone was all convectors, then in the '70s four fan convectors were put in. Probably at that time they added a return right after one of them; and ran a 3/4" supply directly to the last unit of the run (also a fan convector). Three of these units were Wenig Selectronic rated at 8500btu; one was a 'Weather Twin' rated at 7500btu. All of were combo heating /AC units.



So I figure the addition of the fan units added to the pressure drop, which is why they split the loop in two places. I questioned the installer --who is very knowledgeable--about any problems mixing in four panel rads with the convectors and he said no. So you seem to be saying that adding fan units to the convector system was a different thing than adding the mass of four new panel rads. The other alternative we're working on is just replacing the one unit. We definitely do NOT want to be adding another zone; it's an old house that will probably be sold in a few years.

D107

Think I understand now

So the steel panel rads will heat up more slowly and stay hot longer; convectors are the opposite. So, I guess the fan only increased the output. The issue came up in the first place because the old weather twin was starting to tilt a bit towards the floor and seemed a little loose and the resident reported some unusual noises from it. Don't know if these things can be maintained after 40 years. (See photo). So it seems like best solution would be to either service it/shore it up or replace with a convector.

Paul Pollets

WPRads

If the space between the monoflo tees is different than the width of the radiator, there may be problems. Using TRV's on the new rads will also not work, as it would lower the temp of all the radiators downstream. I never recommend adding wall panel rads on a convector zone, unless the heating core is steel, rather than copper.

D107

Rads/Convectors

Thanks Paul. I wouldn't have thought the TRVs on a monoflo system would affect the downstream temps--series piping I could understand, but I'll certainly take your word for it. I'll have to check if the Buderus panel rads core is steel or copper.



I thought perhaps a better alternative would be the Myson panel convectors, though I'd have to see how much more mass and water volume it has than the standard. In searching the wall on this subject, there was a thread that indicated that one might get away with mixing even CI rads and convectors IF there was constant circulation. From Brad, though I don't know if this applies in my context: "...Another exception, one that would allow mixing CI and fin-tube, is the use of continuous circulation with TRV's on each radiator. Of course, this would not be, could not be, a series loop. The radiators would need to be piped in parallel..."



If this was my house I'd remove all the recessed convectors--imagine how much loss there is from the next-to-no insulation at the unit's rear. I was thinking about the 007 electric costs on constant circ with the room sensor, at about .5amp I think that's about 60 watts--that might add up to around $15 if circ is constant during night setback too. Have to count all the extra electric charges above just the KW charge.