Unvented FAQs

1. What is an Unvented or Megaflo System?

A Megaflo or Unvented System provides high pressure hot water. The flow rates are generally very much better than traditional open vented systems, which rely on gravity and the height of the water tank. Additionally, the cold water outlets are also supplied directly from the mains, giving high pressure potable water throughout the property. As a rough guide, with an unvented system, the hot and cold outlets, will operate at a pressure similar to the garden tap!

Megaflo is the (market leading) brand name for unvented cylinders made by Heatrae Sadias. Nowadays, most major manufacturers have similar products offering similar performance. For more information see Unvented Hot Water Systems. (top)

2. At what pressures do Megaflo or Unvented systems operate?

Inlet pressures typically range from 1.5 bar to 3.5 bar. A pressure control valve ensures it does not exceed the maximum pressure. (top)

3. Who can install a Megaflo or Unvented hot water system?

Unvented hot water systems should only be installed and serviced by a qualified and competent person holding a current G3 Unvented Qualification. This is a legal requirement, which helps ensure the strict safety regulations are followed. The system must be correctly installed with all mandatory safety controls. Furthermore, all installations must be notified to Building Control. If the installer is a member of a Competent Persons Scheme, they will process the notification and the customer will receive a Building Regulation Compliance certificate. (top)

4. Is my mains water supply suitable?

Ideally, the mains water pressure at peak times should be a minimum of 1.5 bar dynamic pressure and 20 l/m flow. Whilst all unvented systems can operate at levels below this, the performance may not be acceptable!

Clearly properties with multiple bathrooms will require higher flow rates to ensure water supply and demand are correctly matched. Ideally the size of the incoming main should be at least 25mm MDPE or 22mm copper, to provide adequate flow. Apex Plumbing and Heating will check and assess these parameters as part of our free on-site survey. (top)

5. Where can an Unvented cylinder be installed?

As an Unvented cylinder is pressurised, it can be installed in almost any location. This may be a loft, garage, upstairs, downstairs, out building or even a basement. The cylinder does have a safety discharge pipe, so in some locations may require a sump and pump. (top)

6. What are the purpose of the safety controls?

Safety controls guard against dangerous levels of temperature and pressure. The water in the unvented system must not exceeding 100°C, and the pressure must be within the designed limits. Safety controls are absolutely essential and they must be in good working order. They usually include the following:- Control thermostat, usually set between 60 and 65°C. High level energy cut-out with manual re-set, usually set between 85 and 89°C. Expansion relief valve, typically pre-set at 8 bar. Temperature and Pressure Relief Valve set to 90 – 95°C and 10 bar pressure. The controls and settings will vary; they are specifically designed (and tested) by each manufacturer for their own product. (top)

7. What is the expansion vessel for?

Given that water expands when heated, the pressure also increases (as it’s a sealed system). The expansion vessel absorbs this increase in volume by compressing the air within the expansion vessel. Crucially, this prevents the pressure rising to dangerously high levels. It’s important to realise that if there is no air in the expansion vessel, it will not serve it’s purpose! Note: Megaflo cylinders are designed with an internal air bubble, and do not normally need a separate expansion vessel. Either method allows the incompressible water to expand and compress the air until an equalisation of pressure occurs. (top)

8. Does my Megaflo unvented System need to be serviced?

Yes, all unvented cylinder manufacturers specify an annual service. This typically involves cleaning filters, checking and testing the safety components, and inspecting the overall system. For the annual service we offer a fixed price service (plus parts, if required).

The Hot Water Association (HWA) publishes a generic Service Guide describing the steps involved in servicing an unvented hot water cylinder. This aims to specify an agreed “best practice” approach to hot water cylinder servicing: Servicing Guide 2018.pdf The HWA (Hot Water Association) publish a homeowners guide which gives an insight to the servicing procedure. (top)

9. What is a Tundish for?

All Unvented hot water systems should have a tundish fitted. This creates an air-gap in the discharge pipe, and makes it easier to detect warning drips from any of the safety devices. The required installation and positioning of the Tundish is controlled and documented in the Building Regulation G3 Approved Documents. (top)

10. Dripping water in the Tundish

Water dripping through the tundish may indicate a problem with the hot water system. It may be a fault with one of the safety devices: expansion vessel (or air bubble), pressure reducing valve, expansion relief valve, temperature and pressure valve or another safety device. Additionally, if a sealed heating system is plumbed into the same tundish, it may indicate a problem with the heating system rather than the unvented hot water cylinder. We would strongly recommend having a G3 qualified Unvented Engineer diagnose and rectify the problem. (top)

11. Hot water from a cold tap (unvented cylinder)

In a Megaflo or Unvented system, this may be an installation/design issue or possibly a missing check valve. If the cold water is only warm for a short period of time, before returning to a normal cold supply, this is more of an inconvenience than a major problem. Generally it is fairly straightforward to rectify. Unfortnately, this is a “feature” on some older Megaflo cylinders! It can be rectified by replacing some parts, but (as we know), genuine Megaflo parts aren’t cheap!

In a traditional vented system, this situation may indicate a more serious and potentially dangerous problem. If the cold tap continues running with hot water, we would recommend switching off the hot water system and having it investigated as soon as possible. If the hot water is circulating with the cold water storage cisten (in the loft) or if water is boiling in the cylinder, this needs immediate attention. If this only happens with one tap, then it may be an issue with that particular outlet, but get it checked! (top)

12. Megaflo Installation Costs

Every Megaflo installation is different and costs vary according to many factors. Two significant considerations are, one, the size and location of the mains water pipe within the house. And two, the proximity of the Megaflo Cylinder to a safety discharge outlet (e.g. an external wall).
Electric (Direct) systems are generally cheaper to install, as they are not linked to a heating system.

Megaflo Unvented always carry out a full on-site survey, and our estimates allow for a professionally installed system with correctly sized pipes. (top)

13. What does an Accumulator do?

An accumulator is a vessel that stores water under pressure. It’s typically installed on the incoming mains supply and provides a buffer store of pressurised water. This offers a simple and effective solution to address flow rate issues found in properties with multiple bathrooms. Accumulators are ideal for properties with high pressure, but poor flow. For example, a property with 10 bar pressure but only 10l/m flow would benefit hugely with one or more accumulators. (top)

14. What is the Megaflo Air Bubble?

Megaflo cylinders are designed with an internal air bubble. This absorbs the expansion as the water is heated. Other unvented cylinders may use an external expansion vessel, which serves the same purpose. This is also mentioned in question No. 7 – What is the expansion vessel for?.
However, over time the air bubble is absorbed into the water stored within the cylinder. Once this happens, the air bubble needs re-creating, and is normally done on an annual service. However, any competent person can reinstate the air bubble by following the instructions on the side of the cylinder. Even the homeowner/user can do this, but if in any doubt, please refer to a G3 qualified engineer. (top)

15. Why is my hot water cloudy?

As the mains water is heated under pressure, tiny carbon dioxide bubbles may form within the water. This happens as calcium bi-carbonate present in the water changes to calcium carbonate. When drawing off the water, into normal atmospheric pressure, these bubbles are released giving the appearance of cloudy water. It is completely harmless and normal, and may be more noticeable in the winter months when the incoming water is cooler. As long as it clears without leaving any sediment, there is nothing to worry about. (top)

16. What are balanced hot and cold supplies?

In this context it refers to the hot and cold water supplies. Generally, showers and mixer taps work better with balanced supplies. Where possible, we recommend installing balanced supplies to bathrooms and mixer taps. This is even more important is you have very high water pressure. (top)

17. Poor Hot Water Pressure

If the hot water pressure has deteriorated over a period of time, there are a number of potential causes. It may be the inlet filter is blocked, or with older systems, the PRV (Pressure Reducing Valve) cartridge may have failed such that it restricts the flow. The problem may be more noticeable with higher outlets; for instance, a shower in the loft may suffer more than a shower on the ground floor or basement. (top)

18. Is the Pressure Reducing Valve adjustable?

The Megaflo Pressure Reducing Valve is not adjustable. This is also true for other brands of unvented cylinder, and is typically set at 3.0 or 3.5 bar. It’s important to note that the valves have different pressure ratings, so it’s extremely important to use the correct one. All manufacturers design and test their cylinders as a package, using specific components. These exact same components are necessary to maintain the integrity, safety and performance of the system. (top)

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