Out of balance radiator caused by reduced pipe size?

Jbinckly

I have hot water radiators heating my house, there are 12 radiators of different sizes throughout the house across two floors that work without problem, getting up to roughly the same temperature within 20 minutes or each other for the longest and shortest run.

I have 1 radiator however that will not get up to temp. I have closed the valves to the other valves and when I do that, the radiator does heat up quicker (so I don't believe it's clogged), and gets closer to the supply temp but takes a long time (at least an hour)

Photos attached show pictures via an IR camera of the three rads on the loop with the system running for 10 minutes. you can see one is not heating although the supply is hot ( hard to see the reading but Max is also at the bottom of the screen). The rad not heating is also the last in the loop. (They are in three different corners of a room)

Seeing as even if I throttle all the other rads on the loop it still is not quite get up to temperature, I'm not sure how to better balance the rad. I'm thinking I could remove the 1/2 inch between the valve and the body and replace it with 1 inch or 3/4 (so I don't need to get a new valve). A little weary of taking this piece off as it looks pretty in there, but I'm not getting much out of the rad anyway and have a replacement in the basement if things really go south.

My questions are:

1. Is it possible or likely that reducing the pipe from 1 to 1/2 would cause the radiator not to heat properly by impacting the flow rate or something.
2. If that's the cause what are my options? Apart from replacing that 1/2 inch pipe, can I add some sort of valve to better balance the flow between them?
3. Other though I had was pipe the second and third radiators in series instead of parallel, so the return would supply the third - not sure if that'd force more water through it or if it'd still get stuck.

Appreciate any help.

EdTheHeaterMan

My questions are:

1. Is it possible or likely that reducing the pipe from 1 to 1/2 would cause the radiator not to heat properly by impacting the flow rate or something. YES
2. If that's the cause what are my options? Apart from replacing that 1/2 inch pipe, can I add some sort of valve to better balance the flow between them? Increase the pipe size or add a small pump to that radiator
3. Other though I had was pipe the second and third radiators in series instead of parallel, so the return would supply the third - not sure if that'd force more water through it or if it'd still get stuck. Do not do this under any circumstances. The parallel system design was working without a circulator pump 60 years ago.

By adding a pump to that system when the pipes were one inch to that radiator, that installer only messed with the design a little so many years ago. When you made the pipe size change you changed the balance of the system in relationship to all the other radiators. Think of it like closing that radiator valve from being able to move 0.8 gallons per minute down to 0.15 gallons per minute. That is a pretty big reduction in flow. If you make any two or three radiators into a series of radiators, then the restriction from that one branch to the series of radiators back to the return will be more restrictive than the parallel ladder type system you have on all the other radiators.

Appreciate any help.

NOTE: PEX comes in 1". And you must use a PEX with an oxygen barrier on a closed system with all that iron. If you used non barrier PEX, you need to replace it anyway.

DCContrarian

Do those PEX pipes go somewhere accessible? Because if you don't have to bust up the walls, replacing them with 3/4" or even 1" PEX is not hard. You also want to upsize the 3" between the valve and elbow and the radiator, but the longer a pipe is the more flow is restricted by small diameter.

Jbinckly

Thank you for the replies.

To clarify, I have recently replaced the iron pipe to 1 inch oxygen barrier pex for most of the run, but I just did the work up to the 1/2 inch that was already on the existing third radiator that's not heating - I didn't think to replace the 1/2 between the valve and the radiator.

I've added some pictures below, all the pex is easy to get to - I had replaced the valve on the third radiator as the old one was difficult to adjust. I had originally piped the new valve on the return, as I had read that's usually where it should go. After emailing the company (Dahl), they said it's recommended to go on the supply.

I'm just going to switch the lines so the supply goes into the valve, and am thinking I should just try to remove that 6 inch of 1/2 inch between the valve and the radiator and replace that with 3/4 - or 1 inch. Would also need to get a new valve to replace the new 3/4 one I have if I step up to 1 inch right to the rad, but they carry them at my local supplier so it's possible.

Hard to arrange the pictures from my phone - but this shows you going backwards from the cold radiator to where it tees for rad 2 and 3 and then the run back to the house - rad 1 is right at the wall before going into the house.

I think I will try to replace the 1/2 inch between the rad and the valve with 3/4 when I drain it to switch the supply and return.

Welcome any other feedback and appreciate the help so far 👍🏻

Jbinckly

Should add that this third radiator has never gotten up to temperature since I've owned the house - but now that I have everything replaced, i have the access and tools I need to (hopefully) solve this problem

hot_rod

That short section of 1/2" pex and fittings should still allow 10,000 btu/hr or more to be delivered.

What size circ feeds all these radiators? You might just need a bit more flow.

I've flowed cast iron radiators up to 8 gpm and they heat just fine, so don't worry about excessive flow. 2 gpm in 1/2 is acceptable.

Assure you have enough flow and spend more time balancing. It does take some time to get those radiators to steady state operation. That is when you start making the final balance adjustment. Plan on an hour or more run time, depending on how fast you are flowing them

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