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Baseboard heating loop on 1/2" pipe

wodgerwodger Member Posts: 7

I recently had a new gas boiler and HW tank installed to replace and old oil furnace. At the same time, the plumber took out the monoflow system and connected the existing baseboards in a close loop so I could more easily finish the basement in future. New boiler is a Vitodens 100 and heating loop is set to max temp of 176F.

I am finding however that the house (single family ranch) really is taking a long time to get warmed up. On a cold day in MA, it can take 3+ hours to lift the temperature of the house by 4 degrees F to 68. We have had walls and ceiling insulated by Mass Save and windows are all newish double glazed units.

I have done a heat calculation for reach room and the existing baseboard sizes seem to stack up with the required BTU output. Total BTU requirement being 43,000 and the existing rads and kickspace heaters on paper able to produce 45,500. I mentioned to the plumber and he just suggested ripping out the baseboard and using the a/c blower for heating. Not something I am keen to do.

When he replaced the monoflow, he ran 3/4" pipe to the first baseboard radiator and then dropped to the existing 1/2" which remains 1/2" for the rest of the loop through all the 1/2" baseboards back to the heat exchanger where it again returns to 3/4". There is a total of 62' of exposed baseboard element plus two KS2010 kickspace heaters in the heating loop. The two kickspace heaters are in the same loop before the first baseboard radiator and are connected to the loop using 3/4" diverter tee. After these kickspace heaters, the rest of the loop is 1/2"

Would I see much benefit in having the entire heating loop and baseboard radiators increased from 1/2" to 3/4"? Or any other suggestions to improve the heating?



  • Jamie HallJamie Hall Member Posts: 12,947
    How did he perform the conversion from monoflow? Essentially replace the old monoflow Ts with an elbow (eliminating the pipe between the Ts), then through one bit of baseboard,, back out, elbow to the next bit, and so on?

    If so... it's not wonder you're not getting the heat you want. For one thing, the baseboards at the return end of the loop will be getting cooler water. Much cooler.

    You don't need to replace the half inch baseboards. What I would recommend -- but it's a bit of a chore and others will have other ideas -- is to add a return pipe parallel to the existing feed line and pipe the whole loop as a reverse return system. Pipe size for the mains would depend on the number of BTUh you are trying to put through it.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,088

    Unfortunately you and you plumber made a big mistake. Your radiation needs 45000 btus which means it needs 4.5 gpm. !/2" pipe is only good for 1.5 gpm or 15,000 btu.

    You should have left it monoflow.

    This is what you need to do. Instead of having 1 loop,Split the piping into two loops. That way the water flow will be split in half on each loop. Make all the new piping 3/4"

    The way your running now the water is restricted and your not getting flow. Don't consider a larger pump you will get velocity noise
    Steve Minnichkcopp
  • wodgerwodger Member Posts: 7
    Thanks - @Jamie Hall yes the plumber just removed the Ts and replaced with elbows and connected the return from one radiator to the supply of the next. I questioned the throughput at the time and he said it would be fine - clearly not.

    @EBEBRATT-Ed are you saying to keep the existing 1/2" baseboard and piping but split the loop in 2 and have the common return as 3/4"? Is the split loop the only option? Would replacing all baseboard and piping with 3/4" and keeping a single loop be sufficient?
  • BillyOBillyO Member Posts: 199
    what a joke!!! where do these guys come from?

  • Jamie HallJamie Hall Member Posts: 12,947
    What @EBEBRATT-Ed is saying is make two loops -- two feeds, two returns -- and the mains all 3/4 inch pipe. You will need one new feed -- halfway along the existing loop -- and a new return at that point as well. You will still be pushing more flow through the baseboards than you really should be, but... or you could split it three ways -- make three loops -- instead, still with new 3/4 inch feeds and returns -- which would be even better and only slightly more expensive.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,088
    If money is not an issue I would run (2) 3 /4" loops and replace any baseboard that is 1/2" with 3/4".

    If your baseboard covers are in good shape you may be able to find new 3/4 fin tube you could install in them

    at 4.5 gpm you maxed out with 3/4 pipe
  • mattmia2mattmia2 Member Posts: 1,415
    Probably a good idea to make the kickspace heaters their own loop, ideally in parallel as well so you can balance things better since they will behave much differently than the baseboard.
  • BrewbeerBrewbeer Member Posts: 611
    edited February 14
    @wodger , you should read the hot water loop Q & A, lots of great info, then come back and ask more questions.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread:
    System Photo:
  • wodgerwodger Member Posts: 7
    Thanks that is helpful.

    So I think what I am seeing is that I should replace all my baseboard and piping with 3/4". Split the loop similar to this picture

    I know I would need to use a 1" common return for the two loops, however the manifold connected to the heat exchanger is only 3/4". Would I need to replace that manifold or would it be OK for the last few inches of the return to be reduced from 1" to 3/4" as it goes into the manifold?

    Alternative option is a two-pipe system. Is there any benefit of a reverse vs direct return?

    This is a small 1000sqft ranch with easy access to pipes in the basement. I think the first option of split loop would be simplest assuming the 3/4" at the end of the return is OK?

    Thanks everyone for the help.
  • Big Ed_4Big Ed_4 Member Posts: 1,357
    3/4"can handle 43K , But with a 3/4" parallel split loops run a 1" on supply and return .
    I have enough experience to know , that I dont know it all
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,088
    If you pipe it like your drawing it should be 1" from the boiler and pump then the tee where it splits should be 1 x 3/4 x 3/4.

    The return should be 1" to where it splits. Also use a 1 x 3/4 x 3/4 tee (don't bullhead the tees)

    If the connections to the heat exchanger are 3/4" just keep the 3/4 to a minimum.

    Your on the borderline between the 3/4 &1" pipe sizes for the mains so a small amount of 3/4 shouldn't be an issue

    I wouldn't bother with two pipe or reverse return. Not necessary for a 1000 sq foot ranch
  • wodgerwodger Member Posts: 7
    Thanks again - you really are incredibly helpful. Hopefully last question from me on this... When you mention the 1" from boiler and pump, my pump on the heating circuit coming out of the heat exchanger manifold is only 3/4" - same with the return manifold and valve.

    Should I replace the manifold, pump and return valves with 1" versions? Or will the 3/4 be OK if I keep the 3/4" to a minimum?

    Would mean the supply tee being 3/4 x 3/4 x 3/4, but I could tee that very close to the pump. The common return could be 1" right up to where it goes into the return manifold.

  • BrewbeerBrewbeer Member Posts: 611
    If you are thinking of replacing all baseboard and piping, you should consider performing a room by room heat loss estimate and size the lengths / outputs of the new baseboards to get that boiler down into condensing range temps. But that is a lot of work.

    Another potential option is to keep the existing 1/2 inch radiators, and home -run each baseboard back to a common manifold at the boiler. It would still be nice to see a room by room heat loss estimate and radiation survey so you can get a handle on how well each room of the house is being heated, and how hot the radiators need to be to get the job done.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread:
    System Photo:
  • Big Ed_4Big Ed_4 Member Posts: 1,357
    You can remove the 1" cap on the end and pick up 1" there ,
    I have enough experience to know , that I dont know it all
  • wodgerwodger Member Posts: 7
    Thanks brewbeer - good idea on home running each rad back to a new manifold - will give that some thought.

    I have done heat loss calcs (using and the current rads linear length in each room is within a few 100 BTU assuming they were getting the full 550 per ft at 180F. So it seems when the system was installed on the monoflow it was sized correctly. Just when my plumber replaced the monoflow the way he did is when we seem to have problems. It was all done at the same time as the new boiler so been struggling to find the actual cause.

    With all the help here - I think I am narrowing it down.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,088
    A small amount of 3/4 pipe won't be an issue. Like I mentioned above you on the border of having too much flow for 3/4 and low flow for a 1" if your main runs are 1" with a small amount of 3/4 you will be fine
  • wodgerwodger Member Posts: 7
    I have looked in to the homerun option... that would obviously save me replacing all my 1/2" baseboards.

    So I think I could get a 4 zone manifold and pipe 1/2" PEX to current rads with each zone requiring between 10,000 and 12,000BTU... Does this sound sensible?

    Alternative is the split loop with 1" common supply and return and then replace all the rads and piping with 3/4"

    I am not going to do anything until the winter is over so have some time to consider the options.
    Thanks again.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,088
    Yes, you can do manifolds
  • nicholas bonham-carternicholas bonham-carter Member Posts: 8,137
    If you have a new condensing boiler, it will be more economical, running at well below 180 degrees, so the home run layout might work better
    SlantFin also has an app for heat-loss calculations.—NBC
  • Gman66Gman66 Member Posts: 34
    Are you running ODR? If you are, and you should be, a little over 1 degree gain per hour should not be unexpected. Unfortunately setback does not play well with modern modcon boilers running ODR. The whole objective of ODR is to modulate the boiler to match the heat loss of the house such that there is little excess heat production for temperature gain. You will need to think about eliminating setback or at least dropping it back to 2 or 3 degrees. You can also shift the ODR curve to add back some recovery capacity.

    If you are not running ODR then turn the supply temperature up to 185 which should help some. Your old boiler was probably set to 180 or higher.

    Long term you will need to address the piping issues that you have.
  • wodgerwodger Member Posts: 7
    No, I had to have the plumber pull the outdoor stat when he first installed. Just couldn't get any heat in the house at all - should have realized then that something wasn't adding up.

    With the ODR installed on a cold day, the house temp actually went down while the heating was on!

    The boiler doesnt go any higher than 176F which is what it is set to now.

    I assume I can put more than one radiator on my homerun loops? Rather than having to put every rad on it's own loop? I have worked it out so I can have between 10k and 12k BTU per loop if I link up the right rads together.

    I'd hope with decent piping and balance I can get the boiler temp down?
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,088
    edited February 17
    With 10K-12K I think you might want 3/4 pex. Pex Id is usually less than pipe. You can put as many radiators on a loop that you want but the required flow rate/pipe size determines what you can do. 10-12 will be ok
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