Mechanical ventilation with heat recovery (MVHR) has been around for over 50 years but it is still a relatively new technology to the Irish market. For some reason when heat recovery units (HRVs) first emerged in any kind of volume on the Irish market in 2005, they were not taken on by the traditional ventilation industry.

Heat recovery – the debate continues

Ciaron King December 18, 2012 , , , , ,

Instead it was the renewable energy sector that took them up. From the outset they were established as a renewable energy which begged the question – what was the payback?

The answer, although this point will be strongly argued, is that there is no payback. Ventilation is NOT a renewable energy. It is a mechanical device which uses energy to ventilate a building. It is, however, an essential component of any building and will indirectly help save energy overall by allowing you build with high levels of insulation and airtightness.

MVHR systems should be a simple process but unfortunately there are many issues that can arise and cause problems. However, if each step of the process is controlled, these problems can be eliminated. The key elements to having an effective MVHR system are as follows:

Specification                                                                                 A detailed specification should be prepared, not just for compliance with Part F of the Building Regulations but to ensure that the system will be fit-for-purpose and provide “adequate” ventilation for the occupants and the dwelling. The specification should cover the unit and the air; what airflows above the minimum standard are required (if applicable); the ventilation capacity of the unit; and the ducting type and its dimensions.

If sections of the air distribution or the unit are in an un-insulated section of the dwelling and require insulation, this should be specified and not just relied on as being a regulation. The required efficiency across the exchanger and the noise levels should also be specified. It is equally important that the specification is achievable from an installation perspective, particularly in regard to the air distribution.

In regard to the noise level, there is no point specifying that the system must be less than 25dB(A) unless a significant budget is in place to facilitate this. Specifying that noise levels from valves must not exceed 28dB(A) on the low setting and 30dB(A) on the high setting is very realistically achievable.

Design                                                                                                                                                                   A detailed specification will enable a detailed design. In many cases the specifier may well be the designer of the system. The design should clearly show the location of the unit, primary supply, primary exhaust and the air distribution system. It should state the exhaust and supply airflow rates in the relevant rooms and these should be shown with the unit operating at low speed and high speed.

The duct runs with their dimensions and the location of each air termination valve should also be shown on the drawing to scale. The design also needs to show how the condensate connection to the waste water pipe will be made. The design should match the specification.

Installation                                                                                                                                               Installation of MVHR systems is simple. If the specification and design are detailed  and easy to follow, it greatly reduces the potential for problems. I will not address installation problems due to poor workmanship, which is a separate issue.

The point is that installation is not complex but the installer needs to have a design that specifically suits the application.

Commissioning                                                                                                                                                 The purpose of commissioning a system is to (a) ensure that the installation is in compliance with Part F; (b) that the system was installed in accordance with the specification and design of the system; and (c) that the end-user receives the handover of all operation and maintenance manuals and is shown how to use the system.

Commissioning should be carried out by a competent person using a calibrated (INAB or equivalent) anemometer. It is important to ensure that the anemometer is suitable for low-volume, low-velocity, measuring.

Maintenance                                                                                                                                                   MVHR systems are generally perceived to be far more complex than traditional ventilation systems and therefore require more expertise to service them. Most of the servicing contractors will have plug and- play style diagnostics from the manufacturer to facilitate quick and easy analysis. Filter cleaning and filter changing are the only frequent items to be considered and these are usually done by the occupier of the house. Annual servicing should be carried out by an MVHR service specialist.

Most common problems                                                                                                                                   Last minute changes – Where the home owner has decided to change the location of the unit or the use of a room, installers should not try to oblige the end user. They should immediately contact the system designer to see if changes can be made on site or if a re-design is required.

Water leaking – This can be caused by the condensate drain attached to the MVHR either being incorrectly fitted or not fitted at all. Exterts at Air Duct Cleaning Garland TX say, water leaking through termination valves and collecting in ducts happens because un-insulated ductwork has been used where it should have been insulated.

Noise – Undersized units, undersized ducting, excessively long duct runs and too many bends can all cause noise, as can using “roof tile vents” as the primary supply without sufficient free area.

Unbalanced systems – An incorrectly-balanced system can lead to draughts in some rooms with inadequate ventilation in other rooms.

Excessive condensation in some wet rooms – This is usually a design issue, particularly with north-facing walls which are tiled floor to ceiling and have high-volume power showers. Wet rooms with these conditions require higher levels of ventilation.

Conclusion                                                                                                                                                           In conclusion, mechanical ventilation units – with or without heat recovery – are an important aspect for providing more control over the ventilation rate required. With an MVS system running 24/7-365 you require a lower m3/h air change rate. This means that less infiltrated fresh air needs to be heated by the dwelling’s heating system. This alone is saving energy.

If you add heat recovery then your MVHR is further reducing your primary heating energy load by transferring a percentage of the waste heat energy to the incoming air. Also, MVS and MVHR are generally not affected by wind and external pressure differences.

Finally, MVHR systems in general provide a significantly higher rate of indoor air quality than traditional systems – this is primarily due to the filtration systems incorporated into the units.

I have seen many problems with MVHR systems and in most of these cases it is not a single-fault issue. It is always that at least one of the above five steps has been compromised or missed out completely. The primary fault is that ventilation is pretty close to the bottom of most peoples’ priority list and, as a result, it is usually the “cheapest” system that goes in.

Economy of scale applies to ventilation systems also. The most expensive system is not always the best and in some cases the least expensive may be the best option. However, if two companies are quoting against a specification there should be very little difference between quotes.

The bottom line is that energy efficient ventilation is here to stay. It is functional, practical and it does what it says on the tin. However, if any of the five steps are left out or compromised it will not work. My advice is that if the budget does not allow for an energy efficient ventilation system (MVHR or DCV), then don’t build a low-energy home.

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SDAR Journal 2020

About the Author:

Ciaron King