Do I need a bypass or modulating pump?

[CharlieO]

Okay folks, here's the story so far.

My system is an Myson Orion Fanfare 40 Si with the de facto standard three port valve, single circulating pump and domestic HW storage tank - wired as Honeywell Y-plan.

Its running about 13 rads due to extensions on the house, and system was in a bit of a sorry state when I took house over. (I'm aware thats probably a little too many rads for the boiler, but in practise it rarely needs to heat up the whole house due to the TRVs and it seems happy doing the heat from cold. Its the 'keep warm' bit I'm having a few problems with.)

Since then I've flushed the system with DB X400, and refilled with DB X100 inhibiter and DB X200 in system descaler.

I've fitted TRV's to all rads, apart from the one in the hall where the main room thermostat is, and in the main bathroom.

The rads have been balanced - my what fun several hours with clip on thermometers can be!!

I've also replace the thermostat with a Drayton Digistat to give me finer control as thats worked well for me at previous houses.

So far I'm happy and the system seems to be okay, but I've still not eliminated boiler noise - you can hear the kettling and bubbling - and pipework is noisy as the TRVs shut down. When the house is cold and the rads are pulling a lot of heat from the boiler it doesn't seem to have that problem.

Originally I thought the boiler was badly sludged and scaled, but its had the full flush (not power, just hosepipe!) and had X200 in for a season, and when the house is warm and few rads are running there is a fair amount of water rush noise.

I'd hoped by leaving both the bathroom and hall rads without a TRV I would provide enough bypass, but now I'm starting to wonder.

So where do I go from here?

One thought is that boiler is still scalled and I should try a slightly more agressive descaller.

Another is that its the TRV shutdown is sending the pressure and flow rates all over the place. I guess a modern install would have a bypass between the feed and return, but I really don't want to disturb the pipes, and I've no idea of a technique to set the pressure level properly without an actual meter.

I'm rather attracted to the idea of replacing the main pump (A British Gas rebadged job "VariHead II" - no idea what it actually is) with one of the Grundfos Alpha+ units and let that regulate the pressure/flow in the system. Anybody think this is a good idea, and would it drop straight in as a replacement?

And as an aside if I've balanced the radiators up, how do you ensure the flow rate in the boiler is right?

Cheers guys.

 

[oilman]

As you seem rather determined to do a good job, sorting the boiler noise wil keep you amused for a while. The non acid cleaners for systems will not remove the tenacious scaling that gets in boilers. The best thing to do is to disconnect the boiler from the system and use a descaling pump with an acid cleaner for a couple of hours, (at least). You may need to do it more than once. Add neutraliser before you dump the cleaner into the foul drain, and make sure the pH is between 7 and 8.

 

[mave]

It works fine from cold and only has a problem when warmed up. With so many rads with TRVs the pressure and flow is changing dramatically when warmed up. It may be that a more powerfull pump, or one of these: http://www.plumbworld.co.uk/acb/showdetl.cfm?&DID=10&Product_ID=2989&CATID=46 will do the trick.

Mave

 

[oilman]

If the system is ok when cold, your pump can cope. It has the most work to do then when it's shifting most water. With a constantly open radiator you should not need a bypass unless you have 2 port valves. I would get the boiler quiet first before doing anything with the pump as no amount of pump changing will improve a scaled up heat exchanger.

 

[CharlieO]

Thanks for the thoughts so far guys!

The pump can cope fine - the reason I was suggesting the alpha was to cut the water rush noise.

I think the problem is probably scaled heat exchanger and low water flow rates.

I've opened up the lock shield a little on the main hall rad, it does unbalance the system a bit but thats the one thats on all the time and thats helped cut the noise and calm the boiler.

I've also dropped the boiler thermostat from 7 (Max) to around 5 - my only question here is that the water never seems to get above 60 c even flat out which I think might be the scale?

Unfortunately its just not practical to unhook the bolier from the system right now due to the disturbance it would make. Would running a good descaler for 24 hours in the system with the rads shut down followed by a good flush be worth trying?

 

[oilman]

You could perhaps still descale the boiler safely by removing the pump, and connecting a descaling pump between the boiler side connector and the drain cock on the boiler. You need to block up the feed and vent pipes as well.