Thunder noise when HW comes on

[Shappers]

Now the warmer weather is here, I have switched off the CH but left the HW coming on using the timer.

The problem I have is that exactly (to the minute) 17 minutes after the HW comes on, there is a really loud noise that sounds like thunder and all of the pipes start vibrating.. this lasts for approx 45 secs but is followed then by a really horrible metallicy noise coming from the loft where the expansion tank is which lasts about 15 secs. This process is then repeated, and after the second time, all is then quiet and working fine !!

I have a warmflow oil fired boiler, and 2 mechanical valves made by Honeywell. The pump for the CH is a Honeywell as well.

Any Ideas ?

 

[Softus]

Please would you confirm that your hot water flow is pumped, and take a look in the loft to confirm that there is adequate water in the feed & expansion cistern?

 

[Shappers]

Thanks for the quick reply... I have checked the loft and there does seem to be adequate water in the tanks, both are about half to thre quaters full. I don't think that it is a pumped system, there is only 1 pump and this is for the central heating.

 

[Shappers]

Also, if it helps, it will start making the noise whilst its been on a while as well. but with exactly the same pattern !!

 

[Softus]

It's quite likely that your DHW is pumped, since you have a pump and two MZVs.

Please could you confirm that the MZVs are both working, i.e shutting off when there's no demand, and opening when there is?

Also, can you determine whether or not your system has a bypass (probably in the airing cupboard)?

 

[Shappers]

Yes you're right, the pump does come on for both HW and CH.

The MV's do appear to be working ok, I'm not sure what to look for as far as the by pass is concerned, there are 2 x red headed taps, one appears to be fully open, the other fully closed.

 

[Softus]

Hm. Well, the thunder is likely to be boiling water, and the noise from the loft likely to be water being expelled into the F&E cistern, although the "metallic" sound of this is not a familiar sounding description.

Anyway, common causes of pumping over are a failing/failed pump, sludge causing blockage(s), and bypass missing or closed.

The bypass is commonly a short length of 15mm pipe with a gate valve in the middle, and short circuits the flow and return pipes, i.e. teed in after the pump and before the MZVs, and rejoining on the return from the DHW cylinder coil.

I have the following further questions in purcuit of some hunches:

Do you have a bathroom towel rail that has been turned off recently?

Do you have TRVs everywhere, and the warmer weather is causing the rads to all shut off?

Do you have any chemical inhibitor in the system?

 

[Shappers]

I can't see anything that looks like a valve, just the pump and 2 MV's in the airing cupboard.. I don't have a towel rail so no is the answer to that one.. the pump is about a year old, it is set on no. 2, would it be worth increasing to no. 3 to try ? The rads do all have TRV's, but the heating is currently off. there is no chemical inhibitor in the system as far as I know.

 

[Softus]

I'm sorry to be saying it Shappers, but whoever replaced your pump did a poor job if they didn't add inhibitor.

Scale can cause your symptoms, but, based on the information so far, my best guess is that you have a dirty system, to the extent that it's clogging either the boiler's heat exchanger or the pipework, or both. It could even be that the previous pump failed because the system was dirty!

You'll soon find out if you drain down - the water will be black.

If you don't know how to drain down, there are plenty of topics on the subject - have a look over them, and you can't suss it out then come back here.

 

[Shappers]

Cheers Softus ... I must say, your help has been great.. Thinking about it, when I bleed the Rads, the water that comes out is really black, would this be a sign of what you are saying ?

 

[Softus]

Yes indeedy! It seems that I'm on the right track.

BTW, who did fit your pump?

 

[Shappers]

A company called Ticknell Fuels, they also offer maintenance sevices for boiler and Aga.. we have already dropped their AGA service facility and most recently their boiler / heating maintenance.. they were useless !!