Gas flow question.

volvomanx · 28 Sep 2018

I am thinking of getting a Worcester Bosch 36 cdi combi.
I am also having some building improvements carried out and when finished the plan is that the boiler will be fitted in the new extension. This will place it about 12 metres from the meter.
In assessing flow rates for this combi would I add the DHW 36kw rate to the CH 24kw rate to decide the overall rate or is the higher 36kw rate the one to go with.
I had a gas engineer round earlier today who said it couldn't be done and I'm just getting a second opinion really and secretly hoping it was wrong information.
I also have a range cooker rated at 23.3kw.
Thanks for reading and hopefully giving some advice.

muggles · 28 Sep 2018

volvomanx said:
I am thinking of getting a Worcester Bosch 36 cdi combi.

Why would you inflict such a horror upon yourself?

volvomanx said:
I am also having some building improvements carried out and when finished the plan is that the boiler will be fitted in the new extension. This will place it about 12 metres from the meter.
In assessing flow rates for this combi would I add the DHW 36kw rate to the CH 24kw rate to decide the overall rate or is the higher 36kw rate the one to go with.

It's done from the higher flow rate, but remember if a combi is in one end of your house it's going to take a long time for the hot water to get to the other end.

volvomanx said:
I had a gas engineer round earlier today who said it couldn't be done and I'm just getting a second opinion really and secretly hoping it was wrong information.
.

Well he's seen the job, and we can't see anything from here, but he might be wrong.

ServiceNumpty · 28 Sep 2018

Your hot water is based on the 36kw, so therefore hot water flow rates are around 14lpm. Personally if you have the space go for the Worcester Bosch CLASSIC 38CDI. Its bigger and heavier than the 36cdi compact and bullet proof. Obviously flow rates dependent upon cold mains flow and pressure being up to it.

FiremanT · 28 Sep 2018

volvomanx said:
I am thinking of getting a Worcester Bosch 36 cdi combi.
I am also having some building improvements carried out and when finished the plan is that the boiler will be fitted in the new extension. This will place it about 12 metres from the meter.
In assessing flow rates for this combi would I add the DHW 36kw rate to the CH 24kw rate to decide the overall rate or is the higher 36kw rate the one to go with.
I had a gas engineer round earlier today who said it couldn't be done and I'm just getting a second opinion really and secretly hoping it was wrong information.
I also have a range cooker rated at 23.3kw.
Thanks for reading and hopefully giving some advice.

A comb runs EITHER the HW or the CH. It cannot do both at the same time, and priority is obviously HW. So use the HW rate for calcs. Note that the 36Kw will refer to output. The input will be slightly higher.

Is your RGI saying it cannot be done because of the gas rate? Is HE adding the HE and the CH together ?
And take note of the previous comments regarding distance to draw off points

volvomanx · 30 Sep 2018

Thanks Muggles, MikeCip and FiremanT.

MikeCip, the Classic 38CDI sounds like a good option. I'm all for bullet proof and the larger size would not be a problem.

Looking at the maximum 64.1kw of appliance heat that the meter will safely supply, it looks as if I'll just about be within this figure with the proposed combi boiler, 40kw, and existing gas cooker, 23.3kw combined. Is it feasible to ask my gas supplier to increase my supply pressure.

About two years ago the supplier moved my meter from just outside my front door, half way along the side of the building, to the front of the house, almost doubling the distance from the meter to the appliances. They then run 22mm copper pipe back to where the meter had been and reconnected.
I'm wondering if the pipe size should have increased, but hoping they made the right call.

Also, it was mentioned that a long pipe run means waiting longer for the hot water. Is the waiting time the only reason this was mentioned or are combi's poor at pushing hot water. My cold water pressure and flow is good. Just wondering.

Thanks again for reading.

dilalio · 30 Sep 2018

volvomanx said:
Thanks Muggles, MikeCip and FiremanT.

MikeCip, the Classic 38CDI sounds like a good option. I'm all for bullet proof and the larger size would not be a problem.

Looking at the maximum 64.1kw of appliance heat that the meter will safely supply, it looks as if I'll just about be within this figure with the proposed combi boiler, 40kw, and existing gas cooker, 23.3kw combined. Is it feasible to ask my gas supplier to increase my supply pressure.

About two years ago the supplier moved my meter from just outside my front door, half way along the side of the building, to the front of the house, almost doubling the distance from the meter to the appliances. They then run 22mm copper pipe back to where the meter had been and reconnected.
I'm wondering if the pipe size should have increased, but hoping they made the right call.

Also, it was mentioned that a long pipe run means waiting longer for the hot water. Is the waiting time the only reason this was mentioned or are combi's poor at pushing hot water. My cold water pressure and flow is good. Just wondering.

Thanks again for reading.

Going back to the drawing board for a moment...

What boiler and system do you currently have?
How many showers will you have in the property?
No matter what size of combi you put in, pressure and flow at one outlet will always be affected by opening another - hot or cold.
Why did grid previously move your meter?
The 22mm pipe they ran back down to the house supplies may well have been suitable for the existing installation but obviously needs revisiting for the proposed modifications.
Long runs of pipe from the combi to various outlets will mean that whole length of pipe will need to be voided of cold water before hot starts to flow and this could yield long wait times at various locales... Think waiting while washing hands after going to the loo!

vulcancontinental · 30 Sep 2018

Whether a boiler is bullet proof depends on calibre, in this case not the calibre of a firearm but of the installer and service regime.

fixitflav · 30 Sep 2018

volvomanx said:
I had a gas engineer round earlier today who said it couldn't be done

As others have asked, what reason did he give?
With everything running, I make gas flow just over 6m3/h, and pressure loss in 12m of 22mm pipe + 8 off 90° bends about 3mb. That's assuming the whole flow goes through the 12m, which won't be quite right. I've heard of 1mb maximum being allowed between meter and boiler inlet, but nobody seems quite clear whether that is mandatory, advisory or irrelevant provided the pressure at the boiler meets the requirement. What does your boiler need?
The installation manual for my my boiler says it needs 20mb at the boiler, which it clearly won't get as the meter governor gives 20mb. I wonder if they say that to give themselves a get-out-of-jail card. The specified pressure at the burner is 8.6mb, and the gas valve data sheet says < 3mb pressure drop, so it could cope with much lower pressure.
Depending on pressure needed by your boiler, I think in practice in practice you'd be OK. Also it's unlikely you'll have everything on at max very often.

FiremanT · 30 Sep 2018

fixitflav said:
I've heard of 1mb maximum being allowed between meter and boiler inlet, but nobody seems quite clear whether that is mandatory, advisory or irrelevant provided the pressure at the boiler meets the requirement. What does your boiler need?

It is mandatory. Presumably you are not a RGI?

fixitflav said:
Depending on pressure needed by your boiler, I think in practice in practice you'd be OK. Also it's unlikely you'll have everything on at max very often.

As you are presumably not a RGI, you are not qualified to comment.

lemon_1 · 30 Sep 2018

It sounds to me like your problem is that the pipes are undersized.

You need to work out the appropriate pipe sizing for your house. It's relatively simple to do but you need to know the rating of each aplliance and the length of run + Joints. If you don't know how to do it get someone out

I suspect your main pipe run needs to be 28mm.

The pipe rework will be expensive

fixitflav · 30 Sep 2018

FiremanT said:
It is mandatory.

Thanks for that, first time I've seen it stated definitely. I looked in a few places on the web but couldn't find anything definite. For my interest, can you point me to where it's stated?

FiremanT said:
Presumably you are not a RGI?

No, I'm not, I was looking at it from a practical point of view. The OP doesn't have to take any notice.
When I was working, as a process engineer, we did several effluent treatment plants producing biogas, typically 100-200m3/h, which was fed to boilers. The burner supplier told us the pressure requirement at his inlet, and using that, calculated pipe losses and biogas buffer pressure, and a bit of safety, I specified the pressure rise for the booster. Always worked fine. And I wasn't RGI then either!

nwgs · 30 Sep 2018

60kw, 12m in a straight run would need to be 28mm minimum

nwgs · 30 Sep 2018

Unless the boiler is a Navien

fixitflav · 30 Sep 2018

nwgs said:
Unless the boiler is a Navien

But even a Navien wouldn't meet the maximum 1mb pressure drop, would it?

nwgs · 30 Sep 2018

fixitflav said:
But even a Navien wouldn't meet the maximum 1mb pressure drop, would it?

It was a joke!

You got the 6m3/h correct.

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Gas flow question.

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