T Nation

Need Help from a Plumber - No Water Circulation in CH


I know this is a weird place to post this, but 2 other plumbers websites gave me no response, and this section seems pretty active/versatile :slightly_smiling: Any feedback would be greatly appreciated...

My central heating system is vented and very simple (pretty old setup, probably 20 years old) - no valves (multi-fuel boiler heats DHW through a gravity pipe leading up to coil in tank), and the CH is pumped from the boiler too (4 connectors on the multi-fuel boiler). I manually turn the pump on when there's enough heat.

There is only one feed, and one return pipe for the radiators.

Recently, I fitted a bathroom with underfloor heating (I have experience with fitting radiators etc). The floor is less than 2 square meters. I concluded that since the piping for underfloor heating was short, I could test it out on the central heating circuit (after all, I thought the pump could handle it since the demand would be small and would be equivalent to adding another few radiators). There is only 5 radiators in the house. The pump is a grundfos 15-50.

I drained the system, and connected the underfloor pipes to the central heating return pipe (in series). Refilled the system (released air in radiators), put the fire on, switched pump on but none of the piping heated up (including the feed pipe). Pump SOUNDS like it's working. Gravity hot water supply is working perfectly fine as before.

I returned everything back to the way it was (took the underfloor piping out of the circuit). Refilled, put the fire on, switched pump on but still NONE of the pipes heated up (water not flowing). Pipes before and after pump aren't hot, same for feed pipe (until convection happens). Since the gravity domestic hot water system works fine, this makes me feel that there's no problem with trapped air (since it runs on the same circuit)?

Each time I refilled, I bleed the radiators to release air. I have refilled 3-4 times now. Each time, there's plenty water in there and water is pretty clear (used to be sludgy). I have bleeded the pump (a little air comes out, not much) a few times. At one point, I tapped on the pump and surrounding pipes and the pump and the pipe going into it got hot all of a sudden then went cold again. The motor's working fine on the pump (tried stopping the spinning with a screwdriver and it didn't stop so there's enough force there).

Through some research I've found it's:

1) The pump? (e.g. impellers worn?)
2) Blocked? (some pipes blocked especially around pump?)
3) Airlocked? (have bleed the system and pump, however, - not much air comes out)

Any feedback would be great, thanks in advance!


Mandatory plumber's crack pic is mandatory.


Well, at this this ^^^ is something

Cheers! :slightly_smiling:


I'll read this again when I'm not hungover. PM me your number.


i speed read your post but it sounds like the pump to me. if it's hot in some areas but not others, it's usually the pump. this is coming from an electrician though...


Yeah I posted this question in another forumn (3rd one so far) and feedback says that it's likely the pump. I just wonder why it only died on me when I connected the UFH? It's only like 4 meters long tube (probably less than you'd need for an extenstion/extra radiator...and I only had 5 radiators attached at the time?)

Maybe all the "messing about" dislodged something into the pump which was the "final straw" and killed it?


We talking about the pump? It's as satisfying to me as, uh, coming is, you know? As, ah, having sex with a woman and coming.


^^^ is everyone pissed tonight or what?



It is a Saturday you know.

I think Wideguy on here is also a plumber, might be able to help too.


Start with taking the casing for the pump apart. If the water was sludgy you might just have a blockage. After that it's a quick step to see if the impeller is worn or the shaft is bent/broken. I don't think you're air locked since the air bled off and water did start coming out.

As far as the heat goes, make sure the water heater/radiator/whatever pipes aren't completely constricted. I don't how old the pipes are or what the quality of water there is, but I've seen pipes rust completely shut.

The main thing about trouble shooting is to start with the easiest/cheapest solution first. Then move on to the bigger pain in the ass.


I thought I was the resident plumber. Th gauntlet is thrown!


I look forward to the duel.

grabs popcorn


Thanks! I'll take off the pump tomorrow and have a look (since lighting/access is difficult).

Piping is probably 20+ years old and has a mixture of copper/plastic, and joints everywhere. First few drains, the water was very sludgy. In the near future I'll be upgrading the piping layout anyway (only the old single loop just now) to the two loop version and will probably have the primary feed/return as 22mm piping? That should help prevent blockages...


I'll resort to him if Mad HORSE's advice doesn't work :wink:


It looks like the Grundfos is similar to any circulator in the US. First of all does the motor run? You should be able to put a wooden dowel to it and hear it run. Is there any way to gross bleed it at the boiler? With the pump running, there should be an outlet somewhere at the boiler to get air out of the system.

If you have a fast-feed valve where the water comes into the boiler (also called a dual-valve) it should have a trip handle on it so you can fill the water at a greater pressure. This can force an air bubble through.

Like mad horse suggests, you may have washed out the pump impeller with all you've been doing. See if you can get another one or a re-build kit.



I'm 99% sure it's the pump now - the swirling sound has stopped completely (the sound of water moving) and you can just feel/hear a gentle hum of the motor running (vibration's not as intense)...so it's as if the impellers slipping/grounded down (motors moving but not the impeller).

As soon as I get the motivation (lol) I'll be going under the floorboards again to take the pump out. When I put the new one in, I'll flush out the system first with mains cold water.

Thanks for the help guys


Did someone call?


I've got some proper heavy plans for the future, so I'll probably be asking loads of questions for all you plumbers/engineers/DIY'ers lol

Will be installing my own home-made heat store (~800litres) and solar panels. May not be the most efficient but they'll save me hundreds and you can't complain if it's 'free'.

These will be integrated with my multi-fuel boiler.


That pic, and Uncle Sam telling me so, is what got me into plumbing. Unfortunately it's never like the movies.