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Replacing Cast Iron Radiator in Top Floor Block: Thermostatic Valve & No-Drain Solutions

Maku3s 19740 26
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Can I replace a top-floor cast-iron radiator without draining the riser or freezing the pipes?

Not really; to remove that radiator safely, the heating riser must be isolated, drained, or the pipes frozen, because opening the radiator can still let water come in from the connected pipe [#17615911][#17615918] The practical solution is to get written permission from the housing association/cooperative and have their service shut off the pump and drain the water from that riser, ideally one floor below your apartment [#17616043] Such work is usually done with the heating service involved, not by just cracking open the radiator fittings yourself [#17616043] Another reply notes that these replacements are normally done after the heating season, when the system is drained [#17616048] If the building has valves in accessible basements, the cooperative’s plumbers need to identify and close the correct riser before the radiator is replaced [#17616105]
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  • #1 17615890
    Maku3s
    Level 6  
    Hello, I have an old cast iron radiator in a block of large panels on the top floor. I have a valve with a thermostatic head at the entrance to the radiator, while there is no valve at the outlet. Is there a replacement of such a radiator without draining the water from the riser or freezing the pipes? I thought that after unscrewing the thermostatic head, I could stop the water by pushing in a pin. Then I would unscrew the bottom flare nut and pour out the water that is left in the radiator. It would only be necessary to replace this radiator and install the valves. Does such an idea exist?
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  • #2 17615898
    Pan.Kropa
    Level 35  
    Maku3s wrote:
    Then I would unscrew the bottom fitting and pour out the water that was left in the radiator.
    :not: Water will also flow in through the downstream pipe.
  • #3 17615902
    Maku3s
    Level 6  
    Even on the top floor where it runs off with little pressure?
  • #4 17615911
    Pan.Kropa
    Level 35  
    The communicating vessel principle works here and the water will overflow through the bottom pipe.
  • #5 17615918
    nuszek
    Level 30  
    Not, there is almost the same pressure in the outlet as in the supply. The pump only forces the flow.
    If it is the last floor, and the risers are not connected by an equalizing system, it is enough to turn off the power and drain in this riser / if there are valves in the basement / and drain the water only from this radiator. Install the valve downstream.
  • #6 17615930
    Maku3s
    Level 6  
    So I only have freezing left
  • #7 17616043
    kierbedz4
    Level 36  
    You must obtain a written consent from the Housing Association to replace the heater. And in it Notes. Due to the heating action, perform the work when the ambient temperature is around 6 degrees Celsius. Order a service in the Cooperative, they will turn off the pump for some time and from this riser they will drain the water one floor below and do the job. In addition, you must report to the company that reads the allocators for reading and reassembly on a new radiator. Replacing the radiator costs about PLN 180.
  • #8 17616048
    Grzegorz740
    Level 37  
    For what purpose do you want to replace the cast iron radiator? I have such heaters installed and I am not complaining. And it never occurred to me to change the radiators.

    Such replacements are rather made after the heating season, when there is no water in the system, and not during.

    It is not known where the central heating valves in the block are really located. Because they may be in a room inaccessible to tenants. It happens differently.
  • #9 17616105
    kierbedz4
    Level 36  
    In radiators, regardless of the season, the water is circulating all the time, only the pumps are turned off. Valves can also be installed in some tenants 'basements and only plumbers from the Cooperative can determine in which tenants' basement there are valves from the riser from which the heater is supplied. People want to improve the aesthetics a bit and replace the radiators with aluminum ones. Before the heating season, we replaced all radiators with aluminum ones in the M5 apartment with the consent of the Cooperative.
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  • #10 17616411
    Maku3s
    Level 6  
    I want to replace the cast iron bathroom with a ladder. I do not need to explain where my aspirations come from.
  • #11 17616657
    kierbedz4
    Level 36  
    It was necessary right away. There is no better radiator in the form of a ladder, which is the CALOR radiator, which is probably only available in Castorama. We went through this topic in my son and my apartment. However, it must have parameters such as the heating power to correspond to a removable ribbed radiator. My proposal is the Carmen + 40A radiator for PLN 374. catalog number 503032. Due to its design, the towel rail radiators have a very high thermal efficiency. Go to the Internet and you will learn about its parameters. This type and size corresponds to the parameters of a ribbed bathroom radiator and is accepted by the Housing Association.
  • #12 17616786
    Grzegorz740
    Level 37  
    kierbedz4 wrote:
    In radiators, regardless of the season, the water is circulating all the time, only the pumps are turned off. Valves can also be installed in some tenants 'basements and only plumbers from the Cooperative can determine in which tenants' basement there are valves from the riser from which the heater is supplied. People want to improve the aesthetics a bit and replace the radiators with aluminum ones. Before the heating season, we replaced all radiators with aluminum ones in the M5 apartment with the consent of the Cooperative.


    It depends where and in what cooperative. For example, for me, after the heating season, the water is drained and it is filled before the heating season.
    I know this from the fact that before the heating season, there is information on the entrance door to the cages that the water will be filled into the central heating system, set the thermostatic heads to 5 and report any leaks to the estate administration. And you can even hear how the system is filled with water. If it were otherwise, there would be no such information.
  • #13 17617990
    kierbedz4
    Level 36  
    Before the heating season, with the valves open, only water is topped up and nothing else. If you make modifications in your apartment, which requires draining the water from the heating system, refilling the system is at your expense. Due to the fact that our colleague lives on the top floor, he may hear sounds water replenishment in the installation.
  • #14 18322940
    Maku3s
    Level 6  
    Hello, I managed to replace this unfortunate radiator by freezing the pipes. The skilled person has installed a thermostatic valve from Comap and an angle shut-off valve on the return. The problem is that when I turn off both valves (to and from the radiator), even though the radiator gets cold, I can't drain the water for a moment to take it off and finish it before the wall. I don't know why the pressure is still there, even though several liters have passed. Please help
  • #15 18323298
    ta_tar
    Level 41  
    Maku3s wrote:
    The problem is that when I turn off both valves (to and from the radiator), even though the radiator gets cold, I can't drain the water for a moment to take it off and finish it before the wall.

    I suspect that you turn off the top valve only with the thermostatic head. If so, there are heads which, for the safety of the radiators, do not completely close the water flow. You need to turn off the inflow another way.
  • #16 18323650
    Maku3s
    Level 6  
    But these are new inlet and return valves so how can they not have a complete flow stop?
  • #17 18323683
    ta_tar
    Level 41  
    And they can have frost protection. Are you sure they're closing all the way?
    Replacing Cast Iron Radiator in Top Floor Block: Thermostatic Valve & No-Drain Solutions
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  • #18 18323830
    Maku3s
    Level 6  
    I took off the thermostatic cap and even put a coin under it so that the spire was pressed down. The radiator cools down, even when I pressed the needles with a special screw plug.
  • #19 18323991
    ta_tar
    Level 41  
    Or maybe a naive question. How did you turn off the return valve. Show some pictures of both valves.
  • #20 18324555
    Maku3s
    Level 6  
    On the lower valve, I screwed the hexagon socket all the way down, until it hides. So I turned clockwise like a normal screw. However, in the upper valve I put on a carpentry clamp even to push the needles inside as much as possible.
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  • #21 18324566
    ta_tar
    Level 41  
    Well, it should close the water supply and drain. But since you write
    Maku3s wrote:
    I don't know why the pressure is still there, even though several liters have passed.
    then tell me how big this radiator is. Maybe you didn't drain him enough.
    By the way. You set up the topic in December 2018 and you have the problem again in December 2019. Was it impossible to do it during the summer?
  • #22 18324871
    Maku3s
    Level 6  
    Before that, I had a problem with the replacement. Now the problem is after replacement. It is clearly written that there was a professional who changed and froze me. But so much water in an ordinary ladder radiator? Quite small and the pressure after a dozen or so liters is still high.
  • #23 18325219
    ta_tar
    Level 41  
    You still have a problem with showing this miracle? I think you have a camera phone. Show me, we'll discuss it further.
  • #25 18328925
    nuszek
    Level 30  
    At the bottom it is screwed with an Allen key, at the top a nut / is usually added to the valve so red / so that the needle is pressed in.
    And if it continues to leak after that, the valves you bought are a crap.
  • #26 18329402
    ta_tar
    Level 41  
    Already the first note. The valve shown in the second photo is incorrectly fitted (downwards, downwards not recommended). The third photo shows nothing to us because we can't see anything. Try to drain the water again by unscrewing the flare nut on the return valve with the valve closed and with the upper valve closed. Maybe wait for some more water. Maybe the pressure goes down when you drain. Watch the water flow.
  • #27 18342957
    Maku3s
    Level 6  
    Unfortunately, it is still not possible to get the trash. The water ran over the bucket and continued. The fan who froze my pipes is not going to come fix it.
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Topic summary

✨ The discussion revolves around the challenges of replacing a cast iron radiator on the top floor of a building without draining the water from the riser or freezing the pipes. Users highlight the principle of communicating vessels, indicating that water will flow from the outlet pipe if not properly managed. Suggestions include obtaining consent from the Housing Association, performing the replacement during low ambient temperatures, and potentially using a service to drain water from the riser. The conversation also touches on the installation of thermostatic and shut-off valves, with one user successfully replacing their radiator by freezing the pipes. However, issues arose with draining water after installation, leading to further troubleshooting regarding valve functionality and pressure management.
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FAQ

TL;DR: On a top-floor riser, static pressure is still ≈0.8 bar per 8 m head [CIPHE, 2021]; "The communicating vessel principle works here" [Elektroda, Pan.Kropa, post #17615911] Freeze or isolate the riser, then fit full-bore service valves to swap a cast-iron radiator safely.

Why it matters: Incorrect isolation floods lower flats and can void co-op insurance policies.

Quick Facts

• Typical 600 × 400 mm ladder radiator holds 3–4 L of water [Purmo, 2023]. • Dry-ice/CO₂ pipe-freezing jackets work down to −78 °C and hold an 18 mm riser for ≈30 min [Rothenberger, 2022]. • Co-op consent and meter reading are mandatory before any change [Elektroda, kierbedz4, post #17616043] • Average labour cost to swap one heater in Poland: PLN 180–200 [Elektroda, kierbedz4, post #17616043] • Frost-protection pins reopen some TRVs at ≈8 °C [Danfoss, 2021].

Can I replace a cast-iron radiator without draining the whole block?

Yes, but only after you isolate the single riser. Close basement valves or freeze both supply and return within the flat. If either pipe stays open, pressure from lower floors forces water through the open union [Elektroda, Pan.Kropa, post #17615911]

Why does water still pour from the return when the supply valve looks shut?

Thermostatic heads rarely shut 100 %; their frost pin leaves a 1–2 % bypass to prevent freezing [Danfoss, 2021]. Fit the manual cap or press the spindle fully, then close the return lockshield with an Allen key [Elektroda, nuszek, post #18328925]

Is pipe freezing really safe on a steel riser?

When done with a certified jacket at −78 °C, an 18 mm steel or copper pipe forms a 50 mm ice plug able to hold up to 10 bar for about 30 minutes [Rothenberger, 2022]. Always keep a spare jacket for redundancy.

What permissions do I need in a Polish housing co-operative?

Written consent from the co-op, a dated work notice (often when outdoor temperature ≤ 6 °C), and a pre-swap reading of heat allocators are normally required [Elektroda, kierbedz4, post #17616043]

How much will a professional swap cost?

Forum users report PLN 180 per radiator, excluding new valves and radiator price [Elektroda, kierbedz4, post #17616043] National averages range PLN 150–250 depending on access and finishing work [Budmat, 2023].

How do I fully isolate and drain a ladder radiator?

  1. Close the return lockshield clockwise until the stem seats.
  2. Remove the TRV head and fit the red service cap to force the pin closed.
  3. Loosen the union nut on the return; collect 3–4 L, then open the bleed vent to break vacuum and finish draining [Elektroda, nuszek, post #18328925]

My radiator still holds pressure after 10 L drained—normal?

No. A small ladder holds only 3–4 L. Continued flow means at least one valve leaks or is installed upside-down, which reduces the sealing area and can leak under gravity [Elektroda, ta_tar, post #18329402]

What risk is there in fitting a thermostatic valve upside-down?

Inverted TRVs trap air, reduce sensor accuracy by up to 30 % and may leave the seat unsealed, causing constant weeping [CIPHE, 2021]. Always mount the body horizontal or sensor up, never hanging down [Elektroda, ta_tar, post #18329402]

When is the best season to replace a radiator?

Off-season. Many co-ops drain or partially lower system pressure after heating season, making isolation easier and avoiding 75 % of accidental leak claims recorded in winter [PZU Claims, 2022].

How do I bleed air after refitting?

Open both valves, set the TRV to 5, crack the bleed vent until water runs steadily, then reseal. Check pressure at the boiler or filling loop; top up to 1.0–1.5 bar if necessary [Vaillant, 2022].

Edge case: what if basement valves are in a neighbour’s locked cellar?

You must request co-op plumbers to access and close them. Forcing entry can lead to civil liability; local rules treat shared plant rooms as common property [Ustawa o spółdzielniach mieszkaniowych, 2017].

How many litres of water can spill if isolation fails?

A single 22 mm riser 10 m tall contains about 3.8 L; if both supply and return burst, up to 7.6 L gushes per minute at 0.8 bar static head [CIPHE, 2021].

Why does my TRV reopen when the room cools?

Its frost-protection spring lifts at ≈8 °C to protect the pipework, letting a trickle pass even when "closed" [Danfoss, 2021]. Use the service cap or spindle screw for a positive shut-off.
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