Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

Mixing 3/4 and 1/2 inch pipe on a hot water system?

JoeDoh Member Posts: 6
Two questions if i may about my hot water radiator system

1.  First floor mud room radiator has 1/2 inch while all others in house have 3/4 inch.

Meaning,   a 3/4 to  1/2 reducer has been installed in basement plumbing leading

to the mudroom radiator.

Could that impact overall system performance?

Could that impact mud room radiator performance / efficiency?

The mud room gets very cold but I attribute that to poor insulation in that room.

We are installing insulation and under floor heat mats soon

2.  Second floor hotter than first floor.  (as much as 6 or 7 degrees)

Have tried adjusting 2nd floor valves with minimal success.

We have no door leading to 2nd floor so that solution is out.

Am thinking first floor insulation issues causing thermostat to trigger boiler

longer thereby making 2nd floor hotter.

My guess anyway but very much appreciate your thoughts




  • JoeDoh
    JoeDoh Member Posts: 6
    edited May 2013
    Addl info..

    Boiler is Weil Mclain Model CG
  • SWEI
    SWEI Member Posts: 7,356
    As usual, it depends

    are the radiators piped in, parallel, series, or some combination of the two?  Any diverter tees?
  • JoeDoh
    JoeDoh Member Posts: 6
    Follow up

    Believe it is parallel.

    Assume way to determine if serial is to shut off valve to one rad and see if others are impacted,  right?

    No idea what "piped in" means...

    Would a pic of boiler and plumbing around provide any help ?
  • SWEI
    SWEI Member Posts: 7,356
    If you could post photos

    of the boiler and surrounding pipes, valves, and such plus a radiator or two with their associated valve(s) and pipes it will help a lot.   Even better if you can show us where the pipes entering and leaving a radiator join the system mains.
  • CMadatMe
    CMadatMe Member Posts: 3,085
    Back To Basics

    Your chasing a ghost without doing a room by room heat loss, calculating each radiators capable btu/hr out put at a specific water temp and comparing that to each room's heat loss.

    I have tons of rad's on 3/8" pex so highly doubt it's pipe size.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • JoeDoh
    JoeDoh Member Posts: 6
    edited May 2013

    Coming off boiler the feed splits into 2. (see Boiler_two pic)

    Thinking feed on right is for first floor with left feed being for 2nd floor.

    About 8 feet off boiler and before hitting any rads

     convert to 1/2 inch  (see to_half.pic)

    Approx 24 foot run to mud room rad with 1/2 inch  (see to_rad).

    Note on boiler says return pipes have tape as seen in to_rad pic

    Out of mud room rad convert back to 3/4 feeding other 1st floor rads

    (see back to 3/4 pic)

    Return at boiler (see Return_to_boiler)

    MANY....many thanks for your input

    UPDATE after posting...  darn,  thought it would add them as attachments with names.

    Hopefully my narrative will help with pics?
  • JoeDoh
    JoeDoh Member Posts: 6
    edited May 2013
    Back to basics...thanks

    thank you for your response.

    excellent comment but at this point was looking for help in basic hot water system plumbing, if you will,  around   "mixing 1/2 and 3/4"....

    our handyman dude  pointed out the 1/2 inch into the mudroom rads

    whereas all other rads had 3/4.    he suggested we may need to replace all of

    that 1/2 inch piping with 3/4 but I am not sure of value add of that.

    he confesses  to not being  a hot water system expert

    hence...i reached out to this forum.
  • Gordy
    Gordy Member Posts: 9,514
    edited May 2013
    Heat loss

    Like Chris said first step.

    Then determine if there is enough base board in each room to cover the heat loss in each room. So you will need to know type of base board, and its output per Lin. Foot at its rated water temp.

    Like Chris said 1/2" , 3/8". Can do the job if it can carry the btus needed for the space.

    1/2" can carry 15000 btus depending on gpm which is quite a bit for a mud room.

    3/8" can carry 7500 btus

    3/4" can carry 40,000 btus at 4gpm
  • CMadatMe
    CMadatMe Member Posts: 3,085
    Pipe Size

    Is just a highway that carries btu/hr at a particular flow rate. I have 1/2" pipe running to and from my entire first floor baseboard and it's 3/4" High Capacity. Running 160 at my design day temp and heats the entire 1,200 sqft to a 70 degree set point just fine.

    Your chasing a ghost without a room by room heat loss and emitter measurement for capable btu/hr ouput at a 180 degree water temp. From that you can then determined the proper pipe size needed. To calculate gpm needed to move the required btu/hr the formula is:

    gpm = btu/hr divided by (delta-t x 500)

    Delta-T is the temp difference between the boiler supply water temp and the returning water temp from the zone. A typical heating system is designed for a 20 degree delta-t. So, if the heat loss of a particular zone was 10,000 btu/hr you would need to move 1gpm of flow through that zone. 1/2" pipe would do the job..
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
This discussion has been closed.