Apex Plumbing and Heating

Specialists in hot water and heating systems

Unvented FAQs

  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 is the market leading brand of 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, along 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, notification should happen automatically following the installation, 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 will ideally need 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, so in some locations may require a sump and pump. (top)

6. What are the purpose of the safety controls?

Safety controls are required to protect the householder and prevent the temperature of the water in the unvented system exceeding 99°C. These are absolutely essential and they must be in good working order. They usually comprise the following:- Control thermostat which is usually set between 60 and 65°C. Energy cut out device with manual re-set, usually set between 85 and 89°C. Temperature and Pressure Relief Valve set to 90 – 95°C. (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 would relieve this pressure by automatically opening at a pre-set rating, thereby allowing water to escape from the system and the pressure to reduce. However, UK water regulations/byelaws prohibit the regular wastage of water, so an expansion vessel is required to absorb the increase in volume. Note: Megaflo cylinders are designed with an internal air bubble, and do not need a separate expansion vessel. Either method allows the incompressible water to compress the air until an equalisation of pressure occurs. (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, and inspecting the overall system. For the annual service we 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, which creates an air-gap in the discharge pipe. This 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

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 will vary according to many 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?

An accumulator is a vessel that holds water under pressure. It is typically installed on the incoming mains supply and provides a buffer store of pressurised water. This is 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 any 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 air bubble will be absorbed into the water and the air bubble will need recharging. This is done on 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. (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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