New system & mixing microbore and 15mm

[niallmoran]

Evening guys

Looking for a spot of advice as part of some planning i'm doing on a potential new system for our house. In summary, we've got an ancient baxi open flue boiler with a standard vented system and in the near future we're hoping to do a loft conversion and as space is tight we're looking to swap to a combi. I'm not proposing to do much work myself (maybe some of the plumbing but i'm not sure how much is allowed?) but i like to have a clear idea of what we want before getting the experts in.

At present, the boiler feeds 6 rads via 2 x 6 port 8mm microbore manifolds. While i get the feeling that replacing with 15mm all round would be the preferred option, 3 of the rads are in a part of the house with a concrete floor so replacing their pipework would be pretty much impossible without a lot of work and disruption.

So, i'm thinking the options would be to retain the current microbore setup and run 22mm feed and returns from the new boiler position to the manifolds or alternatively i could pretty easily replace the pipework to 3 of the heaters to 15mm and leave the manifolds serving the remaining 3. I've tried to sketch this to explain better (probably doesn't help though!).

So my question is this a feasible option and can you mix microbore with 15mm? Also bearing in mind that we'll need to also feed an additional 3 rads in the future upstairs - thinking another 22 feed and return be teed off at the boiler and then the new rads fed via 15mm pipes would be the solution up there? Or if retaining microbore downstairs should i install a microbore system upstairs as well?

Finally, having a wee look at boilers and the baxi platinum appeals - especially with the 10 year warranty but i'm not sure what size we'd need. Ultimately, it will be feeding 10 rads and 2 bathrooms/showers so am i best to go as large as possible (the baxi plat 40 maybe?) or does this have implications for the main gas feed? Getting a 22mm feed from the meter should be ok but i don't fancy the cost of having to upgrade the feed from the street if this is likely to be needed. We've also got a range cooker (6 burner) and gas fire to feed.

Thanks in advance guys!

Cheers
Niall

 

[Agile]

Your starting point is to calculate the heating loss of the property and add 2 kW for the hot water cylinder.

The answer is likely to be about 15 kW to 18 kW.

Massive mistakes like getting a 40 kW boiler would normally only occur when a DIYer has specified a system!

Tony

 

[Agile]

8 mm microbore is not good news and it will always need very careful balancing to enable it to give acceptable results.

The usual solution would be to drop 15 mm feeds from the ceiling!

Tony

 

[DP]

Often come across micro bore and small bore and no issues. Only couple of weeks ago plumbed additional two rads in 15 and retained micro bore.

why not let the installer guide you. What makes you think 22mm gas line will suffice when supplying boiler, fire and hob

 

[niallmoran]

Thanks for the replies guys - some extra comments below:

Your starting point is to calculate the heating loss of the property and add 2 kW for the hot water cylinder.

The answer is likely to be about 15 kW to 18 kW.

Massive mistakes like getting a 40 kW boiler would normally only occur when a DIYer has specified a system!

Tony

It's a combi system i'm trying to get my head round which i assumed bigger (within reason!) was better?

8 mm microbore is not good news and it will always need very careful balancing to enable it to give acceptable results.

The usual solution would be to drop 15 mm feeds from the ceiling!

Tony

Dropping from the ceiling isn't an option either i'm afraid as the bit of the house with concrete floors is a flat room extension with no access from above.

Often come across micro bore and small bore and no issues. Only couple of weeks ago plumbed additional two rads in 15 and retained micro bore.

why not let the installer guide you. What makes you think 22mm gas line will suffice when supplying boiler, fire and hob

The gas supply to the meter from the street worries me a little if we go for a big combi as at the moment we've got just an 18kw system boiler. Fairly easy access and runs to the proposed new location of the boiler from the meter for the installer to fit whatever size needed. The hob and fire have separate runs at the moment direct from the meter.

I'll definitely need to get professional advice and i know i'm straying into specialist areas here but i'd like to at least get my head round some of the key issues and parameters before embarking on the project

Cheers
Niall

 

[Agile]

If you are going to have a combi then you need to familiarise yourself with the shortcomings of combi boilers. As well as having the dynamic flow rate available from the mains measured first.

Its hard work but it is possible to form a channel in a concrete floor.

There are also options to conceal heating pipes in special skirting boards.

Tony

 

[niallmoran]

If you are going to have a combi then you need to familiarise yourself with the shortcomings of combi boilers. As well as having the dynamic flow rate available from the mains measured first.

Its hard work but it is possible to form a channel in a concrete floor.

There are also options to conceal heating pipes in special skirting boards.

Tony

Yeah, to be honest i'd much rather keep a normal system rather than go down the combi route but with the loft conversion planned we'd really struggle to find space for the tank(s). An unvented system would maybe be possible by sacrificing a wardrobe/cupboard downstairs but it seems to add a whole level of complexity and cost that would push it beyond us. I'll maybe ponder that some more though and in the meantime i'll measure the flow rate and see what we've got.

Going from such a simple and super reliable old baxi boiler (upgraded to a C plan system though) that is older than me but needs nothing apart from its annual check and occasional thermocouple to a complex (and fragile?) modern system doesn't really appeal and if it wasn't for the loft conversion i would be in no hurry to change it, despite the better efficiency.

Tracking through the concrete wouldn't really be a problem to be honest but destroying the tiles, slate floor and solid wood floor would be a major pain and i'd much rather keep at least the 3 rads in the extension as microbore if at all possible - the other 3 existing rads that are on microbore would be relatively easy to swap to small bore.

 

[Agile]

8 mm is really far too small for reliable heating use unless very careful water treatment is undertaken. Also the maximum heat output per radiator becomes rather limiting.

10 mm however is pretty manageable though. That can be quite easy to run round walls!

Tony

 

[niallmoran]

8 mm is really far too small for reliable heating use unless very careful water treatment is undertaken. Also the maximum heat output per radiator becomes rather limiting.

10 mm however is pretty manageable though. That can be quite easy to run round walls!

Tony

I could probably change a fair amount of the 8mm to 10mm but would there be any point if some of the 8mm remained under the floor? To be fair, the existing system (despite the age) heats the house no problem and we've not had any problem with the microbore with all heaters heating up quickly and relatively quickly.