When building a hot water heating system the plumber has connected one of the hot water radiators backwards, i.e., the loop comes in at the bottom of the radiator and leaves the radiator on the other side at the top. Unfortunately, this has been discovered only weeks after the installation and it's not easy to fix (there is no access to the pipes making up the loop anymore). What is the impact of such a backward installation? Does it affect the heating, efficiency or maintenance?

2 Answers 2


I can't say I'm familiar with your setup, but only with big old radiators & baseboards of the same principal & setup. Your problem is no problem in those systems, they are just to circulate water in one direction of a daisy-chain. The only definition of inlet was which side a flow management valve was installed on.

But, definitely get the plumber back out to inspect his work with you & explain how the system works. In your case I would assume it also doesn't matter at all because your system is actually zoned (it better be), meaning that this radiator might be its own loop & in is out & out is in regardless of what end & what pipe is where.

Don't prejudge & stay calm, just ask him back out to briefly explain the system. You'll need him friendly when the system goes active to adjust the zones for your maximum comfort & overall house balance.

Due to your response indicating this is indeed the start of a zone, the best fix would be to just swap the pipes at the manifold in the basement (or wherever) as the other radiators after the incorrectly installed unit are just flow-through units. Proven by their working perfectly with the first unit being essentially bypassed almost entirely.

No cause for panic nor messy permanent repairs, like replacing the entire zone. The plumber may have initially intended the incorrect unit to be a flow-through, but had to change his plan if a zone was added after the fact (& he/she forgot about the reversed plumbing). I don't know the entire situation, I'm just explaining & not excusing, a proper repair is absolutely necessary.

  • The system is a 3-zone system and this radiator in on a loop with other 2 following it on that zone. I definitely see one difference: when heating up the loop the outflow water temperature is much lower than on "correctly" hooked-up radiators. As a consequence the following radiators take longer to heat up. This apparently due to the fact that in the correct setup not all cold water has to leave the radiator since the outlet is on the bottom, but in this "backwards" case the radiator has to entirely "fill up" before the hot water gets out. But I don't know what other effects this has.
    – user
    Commented Jan 8, 2016 at 19:57
  • Sorry, just getting back from a suspension. Oh, well that is worrisome, I think you're right to be concerned. That radiator is mostly just getting osmosis heating. However, the plumber should be able to just swap the pipes in the basement at the manifold to get everything back to flowing right. It isn't causing any damage its just inefficient.
    – Iggy
    Commented Jan 16, 2016 at 2:03
  • swapping the pipes in the basement would work if it was the only unit on the zone loop. If swapped would not the two correct units then be incorrect?
    – Alaska Man
    Commented Oct 3, 2016 at 18:10
  • Nope, the Asker indicates in their reply that this is the start of the daisy-chain. So, all units after it are just flow-through units that wouldn't be, at all, affected by feed changes at the start of the run...thus they work perfectly even though the first unit is screwy. The alternatives would be to easily flip & reinstall the erred unit correctly or have a correcting pipe behind or above it to accomplish the same correction. Nothing dire, just inefficient.
    – Iggy
    Commented Oct 4, 2016 at 13:25

I do not know if the piping has been corrected however let me make a few comments: you cannot daisy chain cast iron radiators and expect them to work/heat properly. Each radiator must have its own supply and return piping. Also you cannot series connect radiators. As to the radiator being piped backwards it will only affect the operation a small amount and probably not enough to notice. I read other answers that mention flow through and osmosis which are terms I am not familiar with for this type system. Each radiator should have been sized for the heat loss of the room it is installed in, meaning that if everything was done properly there is almost no need for controls on each radiator. Good luck

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