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Megaflo and Unvented FAQ

    1. What is an Unvented or Megaflo System?
    2. At what pressures do Unvented or Megaflo systems operate?
    3. Who can install a Megaflo or Unvented hot water system?
    4. Is my mains water supply suitable?
    5. Where can an Unvented cylinder be installed?
    6. What are the purpose of the safety controls?
    7. What is the expansion vessel for?
    8. Does my Megaflo unvented System need to be serviced?
    9. What is a Tundish for?
    10. Dripping water in the Tundish
    11. Hot water from a cold tap!
    12. Megaflo Installation Costs
    13. What does an Accumulator do?
    14. What is the Megaflo Air Bubble?
    15. Why is my hot water cloudy?
    16. What are balanced hot and cold supplies?
    17. Poor hot water pressure

 

1. What is an Unvented or Megaflo System?

A Megaflo or Unvented System provides high pressure hot water. The hot water flow rates are very much better than traditional open vented systems which rely on gravity and the height of the water tank. Additionally, all cold water outlets are supplied directly from the mains, giving high pressure fresh water throughout the property. As a rough guide, the hot and cold water outlets on an unvented system, will operate at a pressure similar to the outdoor garden tap!
Megaflo are the market leading brand of domestic unvented cylinders. For more information see Unvented Hot Water Systems. (top)

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

Inlet pressures typically range from 1.5 bar to 3.5 bar. A pressure control valve is used to ensure the maximum is not exceeded. (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. It is extremely important that the system is correctly installed, together with all essential safety controls. Building Regulations require all installations of unvented hot water cylinders to be notified to Building Control. If the installer is a member of a Competent Persons Scheme, this will happen automatically, 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, although all unvented systems will operate at pressures as low as 1 bar dynamic. The size of the incoming main ideally needs to be at least 25mm MDPE or 22mm copper, in order to provide adequate flow. Megaflo Unvented 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 can be a loft, garage, upstairs, downstairs or even in a basement. The cylinder does require a safety discharge pipe, and therefore in some basement locations, a sump and pump may be required. (top)

6. What are the purpose of the safety controls?

Safety controls protect the householder (or users), and ensures the water temperature never exceeds 99C. These are absolutely essential and they must be in good working order. They usually comprise the following:- Control thermostat which is typically set at between 60 and 65C. High level, energy cut out device with manual re-set, usually set between 85 and 89C. Temperature and Pressure Relief Valve set to 90 - 95C or 10 bar pressure. Expansion relief valve set to 8 bar pressure. Additionally, the two electrical thermostats (control and high level) and wired into a motorised valve such that any indirect heat source will be shut-off. (top)

7. What is the expansion vessel for?

As water is heated its volume increases, and because water cannot be compressed, its pressure will increase. An expansion relief valve can relieve this pressure by automatically opening at a pre-set rating, thereby allowing water to escape and the pressure to reduce. However, UK water regulations/byelaws prohibit the regular wastage of water, so an expansion vessel is required to absorb this increase in volume. Note: Megaflo cylinders are designed with an internal air bubble, and do not generally need a separate expansion vessel. Either method allows the incompressible water to compress the air until an equalisation of pressure occurs. If water regularly drips through the tundish whilst the water is being heated, this may indicate faulty or undersized expansion volume. (top)

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

Yes, most unvented cylinder manufacturers specify an annual service. This typically involves cleaning filters, checking and testing the safety components, valves and thermostats. We also check the thermostat wiring and general performance of the overall system. For the annual service we currently charge a fixed price of £89 (plus parts, if required). (top)

9. What is a Tundish for?

All Unvented hot water systems should have a tundish fitted. This provides an air-gap in the discharge pipe, and also enables any discharge from the safety devices to be seen, as it runs through the pipe. 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

Dripping water in the tundish may indicate a problem with the hot water system. It may a safety device, the expansion vessel (or air bubble), or the pressure reducing valve. Additionally, if a sealed heating system is plumbed into the same tundish, it may indicate a problem with the heating system. It would be advisable to have a qualified Unvented Engineer diagnose and rectify the problem. (top)

11. Hot water from a cold tap

In a Megaflo or Unvented system, this may be an installation/design fault or missing check valve. If the cold water is warm for a short period of time, before returning to a normal cold supply; this is more of an inconvenience than a major problem, but generally straightforward to rectify!
In a traditional vented system, this situation may indicate a more serious and potentially dangerous problem. If the cold tap continues to run with hot water, we would recommend switching off the hot water 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, these need immediate attention. (top)

12. Megaflo Installation Costs

Every Megaflo installation is different and costs vary according to many different factors. Two significant considerations are the size and location of the mains water pipe within the house, and also 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?

Unvented system require sufficient incoming pressure and flow to be able to service all outlets in a property. If there is insufficient flow, then an accumulator may offer a suitable solution. An accumulator is a vessel that holds water under pressure, and is typically installed on the incoming mains supply to provide a buffer store of pressurised water. This offers a simple and effective solution to address flow rate issues found in properties with multiple bathrooms. (top)

14. What is the Megaflo Air Bubble?

The Megaflo cylinder is designed with an internal air bubble, to take up 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?.
Over time the Megaflo air bubble may be absorbed into the water, in which case the air bubble will need recharging. This is done as part of the annual service, but any competent person can reinstate the air bubble by following the instructions on the side of the cylinder. 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 bubbles may form within the water, and this gives 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, and where possible, we generally recommend installing balanced supplies to bathrooms and mixer taps.
In areas with high water pressure, say greater than 5 bar, it is more important to consider having balanced hot and cold supplies. (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)

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