Pressure Drop on a Multi-Tenant Retail Strip
A single 50 mm NB steel gas main serves four tenancies in a retail strip. Tenancy A has a 120 MJ/h ducted heater, B a 90 MJ/h cooktop, C a 60 MJ/h hot water unit, and D a new 200 MJ/h bakery oven being added. The main was sized years ago for A, B and C. The meter regulator is set to deliver 1.1 kPa at the outlet, and the longest run from meter to Tenancy D is 38 m with six elbows and a tee. When D's oven was trial-lit, its inlet pressure fell to 0.6 kPa and the flame was soft and yellow. The other tenants report no change. The installer wants to simply increase the regulator setting to 1.5 kPa.
1. Explain why raising the regulator setting is not an acceptable fix, and what the correct approach is.
Read the worked answer
Raising the regulator outlet pressure does not create capacity that the pipework lacks; it just shifts the problem and can over-pressure the appliances already connected.
Why it fails:
- The pressure drop from 1.1 kPa to 0.6 kPa at D under load shows the main and branch are undersized for the added 200 MJ/h. Increasing the set point raises pressure everywhere, including at A, B and C, whose appliances are rated for the original pressure. Over-pressure causes incomplete combustion, sooting and unsafe operation.
- The other tenants reporting no change is misleading: they may not have been running at full load simultaneously. A diversity check at peak demand would show further drop.
- Regulator adjustment does not address the six elbows and tee on a 38 m run, which add equivalent length and pressure loss.
Correct approach:
- Calculate total connected load (120 + 90 + 60 + 200 = 470 MJ/h) and apply the appropriate diversity factor for the tenancy mix.
- Size the main and the D branch for that load over the actual equivalent length, using the correct pipe sizing method and gas properties.
- Upsize the main and/or the D branch, or provide a dedicated supply to D.
- Re-set the regulator to the correct outlet pressure, re-test pressures at each appliance under simultaneous full load, and leak-test all altered joints.
- Issue a compliance certificate for the altered installation.
2. What tests and evidence would you require before certifying the altered installation?
Read the worked answer
Certification requires demonstrated performance, not just a working flame.
Tests:
- Strength and leak test on all new and altered pipework before commissioning, at the appropriate test pressure, with the result recorded.
- Operating pressure test at each appliance inlet with all appliances running at maximum simultaneously, confirming each is within the manufacturer's rating.
- Flame appearance and combustion check at each appliance after pressure verification; soft or yellow flames mean re-check.
- Check regulator lock-up and operating pressure, and confirm the regulator is rated for the new total load.
- Verify ventilation for each tenancy is adequate for the appliances present, particularly the bakery oven.
Evidence to retain:
- Pipe sizing calculation showing the main and branches suit 470 MJ/h (or diversified load) over the actual equivalent lengths.
- Leak and pressure test records with readings and dates.
- Appliance data plate details and inlet pressure readings.
- Compliance certificate covering the alteration, plus notification to the network operator or retailer if the meter or regulator capacity changes.
Without simultaneous-load pressure results, you cannot show the installation is safe at peak demand, and certification should be withheld.