Gaining certainty of gas flow before ordering/specifying new boiler ?

[Craig Bruce]

Hi,

I'm struggling to gain clarity from installers when discussing quotes for a new boiler. What I'm hearing is that they wont be able to tell whether my existing pipework will deliver an adequate gas flow until the install is underway - and at that point I potentially get a big additional bill to pull up laminate etc.

Current situation is a Greenstar 30si (Mk3) which has been measured as having a working pressure of 22mbar when running at max, dropping to 20mbar when the adjacent gas hob is running at max (all 5 rings - mixed fuel, electric ovens). There are no other gas appliances in the property. Water flow rate measured as 15l/min with 2 taps open.

I currently have 22mm pipework and understand that it would be common for 28mm pipework to be required to support my desired 35kW boiler. Considering either a Greenstar 8000 Life 35kW or a Viessman 100-W 35kW.

The measured working pressure would seem to my simple mind indicative of flow rate being adequate. Happy to be educated on why this isn't the case!

My plan B is a 32Cdi boiler which I'm advised should be a safe bet in terms of gas flow - but would like to understand if there is any way to get certainty (or 90%+ confidence) on a bigger boiler without getting halfway through the install process ?

Thanks in advance,

Craig

 

[oldbuffer]

1. You are "allowed" a drop of 1 mbar from the working pressure at the meter to the working pressure (full blast) of any appliance, with all other appliances running. On this basis, your existing installation is faulty. I say "on this basis" because its hard to know where the pressure is being measured, particularly with the boiler.
2. Given the above, plan B wouldn't be OK either.
3. The size of pipe required can be calculated provided you know the length of each pipe run, the number of bends / elbows / tees in the run, and the gas requirements of each appliance running full out.
4. You don't have to have the same size pipe all the way from meter to appliance. You could start with 28 and reduce to 22 further along any run. In fact because gas is compressible you can have the wider pipe near the appliance.

In summary, I'd plan for some upgrading of the gas pipework, but may be able to upgrade only in more accessible areas. I can't tell for sure without knowing the layout, but it is highly likely that if you replaced half the 22 with 28, you'd be OK with a 35 kW boiler.

 

[Madrab]

As suggested, they should be able to do a pipe size exercise as long as they know the equivalent length and size and the max draw of each appliance that will be on the pipe, gas installers 101. Don't see why they need to wait till half way through the process to see if there would be more or less than a 1mb drop.

 

[Craig Bruce]

Thanks - I think a combination of location (West London) and demand (winter coming plus punters hearing about the end of traditional boilers) is meaning many installers aren't interested, or if they are are either spending 5mins to quote or giving scary numbers. I can be flexible in considering different boiler options, but would like relative certainty on what I'm getting into price-wise.

I've had an email discussion with the installer who did the most thorough of the surveys, probably best if I re-engage in a conversation on a good old fashioned phone to see what more he suggests or is willing to do in terms of further pre-work.

As an aside, the primary driver for me on boiler capacity is hot water flow rate. I'm in a 4 bed Victorian house with 2 bathrooms - one shower only (attic conversion) and one bath and shower, by rights it should probably have a hot water tank/system boiler, but space has been used by previous owners and now on a combi. Filling the bath and (during winter) keeping the thermostatic shower happy about temp can be a challenge with current boiler, which may be due to age rather than specified performance....I'm conscious I could be over-egging things going 30-35kW but if I'm replacing and have the option of 35kW it seems sensible to jump to that if I can rather than get 32kW (or another 30kW) and later regret it whilst waiting for a hot enough bath to run.

 

[ianmcd]

Something else to consider is the turn down ratio if you don't have much of a heating demand and you go for a large boiler with poor rundown ratio it will constantly cycle when on heating

 

[oldbuffer]

Changing the 30si for the 8000 should knock about 1 minute off the time to fill a normal bath.
Based on 70 litres hot / bath:
a. Figures for 30si = 10.8 litres per minute if raising temperature by 40 degree. 6.3 minutes
b. For 8000 = 13 lpm for same temperature rise. 5.3 minutes

 

[Craig Bruce]

To close off on this, my selected installer (RBHM of Bagshot) catered to my upfront queries, did the required calculations and gave a steer on likelihood of all being well. I proceeded to have an 36CDi installed which successfully met all the required installation checks with the pre-existing 22mm pipework. Very happy with install work done and have the oodles of hot water I'd hoped for

 

[MeldrewsMate]

I love a happy ending. Thanks for the feedback - it's a rare and appreciated thing.