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Is there a counterpart to the advice given in this video except for late 1940s-era convector-style radiators which do not have a valve on their return and have only an air-bleed-valve? Basically a pipe with fins arranged around it. The convector farthest from our boiler is staying cold though all others are getting warm. All of the air has been bled from the system and this rad doesn't sputter when its bleed valve is opened but shoots out a thin stream of (cold) water.

These convectors are not daisy-chained; each one has its own supply to the main 1.5" NPT black iron supply and its own return to a 1.5" NPT black iron return.

EDIT1: Last night I installed a 3-speed TACO circulating pump yesterday (Model 0010-MSF1-IFC), refilled the system and carefully purged it of air one radiator at a time. Both sides of the house get warm, all radiators, except for the cold one, which shoots out a stream of cold water from its bleed valve, and this happens even when the pump is on "high" setting. So I don't think it's an issue of not having enough pump.

EDIT2: I'm going to make an edit to this abstract diagram in case it's significant in terms of the diagnosis: each zone has a return stop valve. The 1/2" supply and return pipes for all radiators run verticaly from the basement except that with RAD5 (the cold one) the 1/2 pipes turn 90 degrees when then reach that room and then run horizontally for about 12 feet between the floor joists.

                        FRONT ZONE
           
==================================supply============================
||     |        |              |             |
||     RAD1       RAD2          RAD3          RAD4
||         |         |             |             |                        
||        ============================= return======================
||       /
||       x front zone return stop valve
||      /
  BOILER
||      \ 
||       x  rear zone return stop valve           
||       ​\                                REAR ZONE
||       ================================return===========\
||         |          |              |            |        \________
||        RAD1       RAD2          RAD3          RAD4            RAD5
||            |          |              |            |   ___________|
================================== supply==============/

I'm thinking the only way to solve this, absent a way to restrict the return flow at each radiator, is to pump the water into the supply line with greater force, so the radiators first in line simply cannot consume all the water and some will get past them to supply radiators farther downstream. Or possibly close the return stop-valve at the boiler and then back it open just a little so the return is a "bottleneck" which will force water further along the supply line.

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  • If Rad4 is cold (not just colder, or insufficiently hot, but actually cold) you probably have a different problem not balancing. Turn the thermostat all the way up so the system runs hot for at least an hour. If Rad 4 then heats up then yes it's a balance problem that may be exacerbated by oversized boiler or oversized rads 1, 2, 3 or the thermostat being too near one of those rads. But if rad4 stays cold after this test you may have an air constriction or other flow problem.
    – jay613
    Nov 5, 2021 at 12:48
  • @jay613: Farthest radiator is cold, not tepid. The thermostat is indeed placed in a bad spot, as you suggested. There are two radiators near it and that area gets toasty rather quickly. But I've been able to get the farthest rad to heat up by playing with the return valves in the basement (sort of like putting your thumb on a hose end when you don't have a nozzle but want the spray to go farther). But that doesn't really work until winter when the boiler stays on for a long time, and it's inexact, hit-and-miss.
    – mr blint
    Nov 5, 2021 at 15:00
  • What return valves are you playing with? You say the radiators don't have valves, but then you say you are playing with "return valves in the basement" - are those 1 per radiator? Then you do have valves. If they are not per radiator, then please explain or update your diagram. Fundamentally, as diagrammed, your system is poorly designed, as the return to the boiler should come from rad4 if the supply goes to rad1 first, so that the resistance to each radiator is balanced, not less on 1 to most on 4 as diagrammed here. This "saves pipe" but guarantees troublesome performance.
    – Ecnerwal
    Nov 5, 2021 at 16:55
  • Ok if it goes from cold to tepid on the coldest days and playing with valves helps, then you do indeed have a balance issue. Do none of your rads have any valve on either side? A cheap temporary solution would be to "unbleed" (partly fill with air) rads 1, 2, and 3. If your system overall is then able to heat the entire home sufficiently, that's good enough. On the coldest days you may find the entire home is balanced but can't heat up enough. You may also find the boiler fast-cycling, which may not be good for it, not sure about that. I stress, temporary.
    – jay613
    Nov 5, 2021 at 20:34
  • @jay613: There is one return valve per zone. Two return valves in all.
    – mr blint
    Nov 6, 2021 at 11:12

5 Answers 5

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Rads 1, 2 and 3 should have a flow valve as well as an isolating valve fitted, one each side of the radiator. Some systems have one of the radiators to be uncontrolled as a heat dump in case the control system / boiler has an issue, this was often the bathroom radiator.

The presence of both valves allows 1 radiator to be taken out of service while the others continue to work.

The flow control valve is there to make sure each radiator gets a share of the available flow and does not hog too much. If they are not fitted then what happens is the last radiator in the string stays cold.

Of course sufficient flow should be available to overcome the total pressure drop of the system and the flow should also be sufficient to deliver the correct amount of heat to each radiator or room. The total heating load needs to be known so that the flow rate and temperature change can be established. At least that was how we approached the distributed heating system we designed so each area got the correct amount of heat.

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  • I'm thinking of upgrading the pump on the boiler to a beefier one. Retrofitting the radiators with valves would be problematic as much would have to be changed even to fit the new hardware into the space. The convectors are partially inset into the wall-cavity not hanging on the interior wall, and the supply and return pipes emerge through the floor. It would be a major undertaking, time-wise.
    – mr blint
    Nov 5, 2021 at 14:52
  • A better solution if you want to pump your way out of the problem is to add a booster pump to the supply between rad3 & rad 4, inline in the basement.
    – Ecnerwal
    Nov 5, 2021 at 23:05
  • ...now rad4 & 5 since you changed the numbering with the diagram update.
    – Ecnerwal
    Nov 6, 2021 at 17:54
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Considering all you've said, especially that none of your radiators have any control valves, I recommend that you

  1. install a thermostatic valve on every radiator
  2. move the two thermostats to the coldest room in each zone
  3. also install a shutoff valve on the other side of every radiator to make it easier to service. This isn't critical but if you'll be doing all the work to install valves you may as well do it.

This has the added benefit that bedrooms can be at different temperatures as their occupants wish, and unoccupied rooms can be set to receive no heat.

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The water at the last radiator is cold because all the heat has been extracted by the efficiency of the other radiators up the line. They radiate too well. If you can box in the nearest (hottest) radiators, more hot water will pass along to the next ones.

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  • I don't think that's true for the situation OP has, where all the radiators are in parallel between the supply and return lines.
    – SteveSh
    Nov 5, 2021 at 12:29
  • I have been able to play around with the return valves in the basement (there are two heating zones, front-of-house and rear-of-house) in years past and get the farthest radiator to heat up, but it's trial and error. I've marked the stop valve handles and counted the number of turns, but it's more trial-and-error than easily repeatable from year to year. I think convection is involved to some extent, and that doesn't really kick in until the boiler is constantly on, on a cold winter day.
    – mr blint
    Nov 5, 2021 at 14:42
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Do you have any valves at your radiators at all? (If you do not have valves, then not much can be done).

Start by closing/partially closing the valves on radiators 1-3 and making sure the valve on 4 is open.
Be careful, as old valves may sprout a leak when moved (they don't like being disturbed)
Wait a bit and verify the radiator 4 is warm now.
Go to second farthest radiator (3?) and open the valve partially - quarter turn-half turn. Wait couple minutes to see if the radiator warms up. If it doesn't, open by another small turn.
Repeat the process until you get heat at the radiator, then move to next one.
This process should balance the flows and ensure all radiators have flow.

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  • The only valve on the convectors is a bleed-valve.
    – mr blint
    Nov 5, 2021 at 14:38
  • And there are two zones (front of house, rear of house) and each of those zones has a regulating stop valve in the basement.
    – mr blint
    Nov 6, 2021 at 11:39
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So, the updated diagram not only changes the original cold "rad4" to "rad5" but reveals that rad5 is tacked on with ~12 feet of 1/2" pipe each way.

That takes out the option of fixing the flow by making the return be from the end of the cold radiator, since it's got that long small pipe section (which is also a direct cause of it getting no flow in this setup.)

A booster pump on the 1/2" line to rad5 seems like the least work that should solve this. Putting in all the valves that are missing has its benefits, but is going to be a lot more work.

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