Sizing my new combi boiler

[Triggertrevor]

Hi all wondered if someone could help me out with my question.

For a wile now I've had problems getting heat to the last radiator on the system at my house.
All the rads are now cast iron,in total I have 7 rads and a Myson plinth heater in the kitchen.

Being a plumber by trade I've tried everything that springs to mind to sort this out.
1. Tried throttling all the rads back to allow heat to the last rad which worked,it got hot but not banging like some of the others on the system.
2. I've spent hours balancing all the rads which works fine for all the other rads but after trying this the last rad goes cold again.
3. The whole central heating system was only installed 12 years ago so there's no valves that could be half shut or stuck on the last leg
4. I've power flushed the whole system every time I added a new rad so the systems clean.
5. I have traditional looking valves which are all working fine wheel head & lock shield.

I used to have a Radiant 28kw boiler only a cheap make then I swooped it out for an early Worcester Bosch greenstar 25 he the one that was discontinued in 2003 I think but this was old stock that was left on the shelf or a wile.
I had similar problems with my last boiler with the last rad not getting as hot as it should so I thought the replacement boiler would sort this but I'm thinking because I'm now running cast rads this is where the problem lies.

Ive totalled my but output of all the cast rads inc the plinth heater and it comes to 46000 btu which I thought the Bosch boiler would have been able to handle.?

Can someone advise me if it's just a case of a under powered boiler trying to run all these rads.

Thanks again all

Kev

 

[TBServices]

46000 btu is only around 14kw. The size of the boiler isn't the problem.

 

[Triggertrevor]

46000 btu is only around 14kw. The size of the boiler isn't the problem.

Well just to add a little more info.

The last radiator on the landing before the affected radiator seems to air lock now annd again which in turn cuts the flow off to the affected radiaror abd I dont know why.

Cheers kev

 

[Rickster123]

How many rads do you have running off of 15mm pipe work?

 

[Triggertrevor]

How many rads do you have running off of 15mm pipe work?

Cherrs for the prompt reply.

All the rads are running off 15mm copper. The pipework hasn't been altered since a complete new install 12 years ago before that the house never had gas central heating just electric wall heaters.

Cheers

 

[Howsie]

There you go. Can't get enough heat in 15mm to service all those rads properly.

 

[Triggertrevor]

There you go. Can't get enough heat in 15mm to service all those rads properly.

If that is the case would I have to replace all pipework or just the main flow and return that runs to the affected last rad.

Cheers for your prompt reply.

Cheers

 

[Howsie]

Ideally you'll want 22mm f&r. Depending on the runs you may be able to get away replacing some and not all of the 15mm f&r.

 

[Triggertrevor]

Ideally you'll want 22mm f&r. Depending on the runs you may be able to get away replacing some and not all of the 15mm f&r.

I would think the run from the boiler to the last rad upstairs would be around 15mtrs long but I be right in saying it would be better to replace all the flow nd return pipework with 22mm.

Cheers

 

[Howsie]

Yes you would.

 

[Triggertrevor]

Yes you would.

And would I be right to say the current boiler I have would be fine running 22mm pipework.

Thanks agin for your imput.

 

[TBServices]

And would I be right to say the current boiler I have would be fine running 22mm pipework.

Thanks agin for your imput.

Yes, you really need 22 with individual 15 branches to rads. I've seen boilers really badly damaged by insufficient flow, so best get it sorted sooner rather than later.

 

[Triggertrevor]

Yes, you really need 22 with individual 15 branches to rads. I've seen boilers really badly damaged by insufficient flow, so best get it sorted sooner rather than later.

Hi so can I still run the flow and returns to the rads off the 22mm in 15mm as the longest run will only be about 15ft and it's the 3rd rad off the boiler and about 3ft on the first floor.

Cheers

 

[Rickster123]

Yes that would be ideal. I don't tend to run any more than 3 radiators off of 15mm copper depending on rad sizes. If you start from the boiler in 22mm and pipe as far as poss in 22mm then reduce down for the last couple. You should find the extra flow will resolve your problem.

 

[Howsie]

And would I be right to say the current boiler I have would be fine running 22mm pipework.

Thanks agin for your imput.

Yes again

 

[Triggertrevor]

Cheers guys for all your advise......just one more question. Would be best if I run the main flow and return pipe in 22mm all the way to the last rad then Reduce to 15mm for the valves. Then just tap off the 22mm and connect upto my existing 15mm legs to the rest of the rads.


Cheers

 

[Howsie]

You'll be fine with the last rad on 15 f&r. It's basically a long tail.

 

[AlexGas]

Have you flushed your cast iron rads before installing them?

 

[Triggertrevor]

Have you flushed your cast iron rads before installing them?

Yeah all the rads have been flushed along with the whole system once the rads were connected up

Cheer s kev

 

[Triggertrevor]

You'll be fine with the last rad on 15 f&r. It's basically a long tail.

Cheers for that info that makes them easier for me to pipe up.

Cheers

 

[stevetheplumber]

we always work on 10000 btus max on 15mm pipe usually less