Sizing boiler output pipes v cycling v pump speed

[BAD_BOB]

I'm trying to figure out how to make my system work better and if it has been installed properly - boiler tends to switch off before rads are really hot (system is clean)).

What's the max BTU (or Kw) you can stuff through 22mm flow and return pipes before the boiler starts cycling due to not having enough heat transported away? [Ideal system boiler is marked as 40/60 but I cant work out how many BTU's its chucking out as it references the BTU tables to different input gas pressures - could bbe as high as 60,000.]

I guess it's not this simple, but can you just increase pump speed to assist carrying the heat away - would you do that in reference to return temp? If so what difference in return temp is allowable?

Thanks if you can be asked to educate me and sorry so many questions

Bob

 

[Malc]

Here are some useful links for you:

http://www.barlo.co.uk/ - heatloss calculator. I've used the myson HL98 in the past but that doesn't seem to be available online anymore.

http://www.cda.org.uk/frontend/pubs.htm - lots of info on pipes, flow speed, power transfer, etc.

The amount of power you can get through a 22mm pipe depends on the temperature difference and the flow speed. 10/11C is the standard temperature difference. If your radiators are oversized then you can stand a lower average radiator temperature and thus a larger difference and thus get more power down 22mm for the same flow speed.

Oilman's rule of thumb is 13.5kW. The max speed on cda (1.5m/s) gives more like 16kW for a 10C difference.

 

[leafy]

If you have TRV's fitted it could just be that they're doing their job - they are operated by air temperature and radiators don't neccesarily need to be scalding hot to raise room temperature enough.

 

[Malc]

Have you balanced the system? Basically you need to adjust the lockshields so that the flow is equally distributed. After you achieve that you can then change the pump speed to get 10C across the boiler.

Have a look at http://www.diyfaq.org.uk/plumbing/rad-balance.html which explains the process.

The other question is do you have a bypass or TRVs? If you have TRVs and no bypass then the flow will reduce when the TRVs all operate and the boiler cycle. A bypass returns hot water to keep the flow going.

 

[BAD_BOB]

Fab stuff - thanks guys.

I'm going to balance the system after a flush cos I'll need to open all the valves fully for that. Sounds a tedious and hit and miss process!

Any more info you can give me on a bypass would be good - is this to help things when all TRV's are shut; I noted my mother in law's system hammering when a TRV in her living room closed...? [I havent managed to fit TRV's all round yet - tails wouldnt bloody budge on some rads!]

Thinking about it should the room stat be set lower than the lowest TRV to avoid short cycling?

D

 

[oilman]

The radiator in the room with the room stat shouldn't have a TRV.

Oilman's rule of thumb is 13.5kW

Not mine, this is from system design publications.

 

[Malc]

A bypass lets the water return when the TRVs are closed so it would help with the hammering. The other possibility is a grundfos alpha pump or similar. Unlike a normal pump when the pressure rises when the flow is reduced these don't so (they say) you don't get the hammering. The bypass achieves the same end and is cheaper than changing your pump although need more installation.

In fact I wonder if these reducing pumps are such a good idea if the flow is slowed then the return water will be lower and you run the risk of condensation (unless the boiler is a condensing one anyway).