MVHR Price Variables

What might affect the final project cost?

Estimates are never wrong, but they can be optimistic and there may be other factors ‘subject to design’ that affect the final price. Site specific challenges, client requests, or requirements from other stakeholders, including contractors, structural engineers, frame builders, acoustic or thermal modelling consultants, fire safety, environmental health, or Building Control officers to name a few. The truth is until you’ve a final, workable design and specification signed off by all parties, the final project cost is a moving target.

The question is, what factors might affect the cost and what can you do to mitigate the risks? Here are the usual suspects..


Late design of MVHR (after RIBA Stage 4)

Ideally the MVHR should be designed before the technical drawings are complete (RIBA stage 4). It is our objective to make the property ‘MVHR compatible’ from the outset. If it is designed early, you might for instance manage with one larger machine in a soundproof plantroom rather than two or three smaller units installed around the property (often adjacent to bedrooms through necessity).

One big unit means less noise, less energy, less maintenance, and fewer exterior penetrations through your airtight layer. With adequate prior planning you can avoid having to squash the MVHR in a ‘hot’ plantroom too, next to boilers or water cylinders that could inadvertently heat the air handling system during the summer, contributing to overheating risks. The MVHR unit really needs its own space at the ambient indoor temperature.

In terms of physical size, the MVHR and ducting is bigger than all your other M&E plant combined. When it’s delivered to site there will be roughly two cubic metres of material for every 100m2 of floor area. With prior planning 90% of that can be hidden inside the fabric of the building without boxing-in. Inside floors, internal walls, or even under the screed floor.

It’s a lot harder to hide the ducting if the building layout and structural elements are finalised before planning the MVHR. You’ll probably need more materials to work around impenetrable objects you could have gone through with prior planning. More ducting means more air resistance, more resistance means more effort is required to move the air, and more effort means more energy, normally more noise, and certainly more cost.

Designing the MVHR immediately before you need to fit the MVHR is like building a new car and then planning what goes in the engine compartment when it’s already on the production line.

If you are already on site and the construction drawings are complete, add 10% to the estimated cost.


Remote, missing, or insufficient plantroom

The optimal MVHR plant room/space would be central in the property, close to an exterior wall that preferably points North (so cooler air is available during summer), away from the road or neighbouring buildings (for noise and/or air quality reasons), ideally on the ground floor (so the air intake terminal is easy to reach for cleaning), ideally has a Posi-joist floor above (for distribution of ducting without needing suspended ceilings), and in a large property there should be some means of ‘riser’ access from the plantroom to the floors (to avoid needing separate systems on every floor).

If the MVHR plant is situated in a faraway corner of the building, the system will generally require more ducting. It could also be difficult to ‘balance’ the supply and extract side too, as air will always look for the path of least resistance. You can end up with too much air movement in the rooms closest to the machine, and not enough air movement in the rooms at the furthest extremity. The MVHR could potentially go in the loft and vent through the roof, assuming the loft is inside the ‘heated thermal envelope’, but you should avoid situating the MVHR in a garage for fire safety reasons.

It’s impossible to produce accurate costs without finalising the optimal plant location, so that is the first design priority. Design it early and you might only need one MVHR, but design it late and you may need one per floor. One machine will always be less expensive and more efficient than three.


Fire protection for properties with more than two floors

Since the Grenfell disaster and the subsequent introduction of the Building Safety Act (BSA), fire protection has never been under so much scrutiny. Whilst properties with just two floors will rarely need additional measures beyond smoke alarms and perhaps fire-rated air valves in timber frame homes, it is safe to assume any property with more than two floors, or a floor over 4.5m from ground level, or a basement, WILL need careful consideration. Failure to plan, execute, and evidence a robust fire protection strategy can be a very expensive mistake.

For every ‘notifiable’ new build or refurbishment project somebody must be responsible for Approved Document B compliance. This person is deemed the Duty Holder under the new safety act. With penalties for non-compliance that include unlimited fines or prison, even for your Approved Inspector or Building Control Officer, and liability that exists for ten years or more, signing off Part-B makes everyone nervous.

It’s important to establish who the Duty Holder will be from the outset, so they can review any proposals, including the MVHR design, before things happen on site. Don’t assume it’s the Principal Designer, the Principal Contractor, or the Building Control Officer, because if nobody is nominated that responsibility falls on the client. If you ask someone to sign off the fire protection strategy after the fact, they may ask you pull down ceilings to expose products that were never installed. Retrofitting those products is extremely disruptive and expensive, assuming it’s even possible.

With MVHR the general idea is to ensure the fire escape route from upper floors stays protected, i.e. if a fire starts in a room it can’t spread to the hall, stairs, and landings to impede your exit. Any ducting that passes through a floor or a fire protected boundary wall will need, as a minimum, intumescent fire collars on both sides of the barrier. Any air supply or extract terminal that penetrates a fire-protected ceiling will most likely need a fire-rated air valve. Potentially you may need to fit resettable fire and smoke dampers that are connected to the fire alarm, to seal off external air sources, or isolate certain areas.

Fire protection measures are excluded from our estimate as every job and every Duty Holder is different. If you have more than two floors add at 10% to the budget cost to cover parts and labour.  


Vaulted/cathedral ceilings

Calculated air flow rates normally assume you have 2.4m ceilings. But a vaulted ceiling adds to the building volume, so to achieve the minimum ventilation rate, which is roughly equivalent to one complete air change every 2.5 hours, or 0.4 Air Changes per Hour, the total internal volume needs to be calculated and then ventilation rates increased to suit. Achieving the elevated flow rates without adding undue noise or energy consumption could mean you need a larger MVHR unit, and/or additional ducting. If the ventilation rate is not adjusted to reflect the additional volume, you risk being under-ventilated.

Working at height also adds time for the installers and commissioner, as our operatives may need to work off safe access equipment (platform ladder or tower scaffold). You may also need additional duct support systems too.

Our estimated costs assume you have 2.4m ceilings throughout. If you’re planning vaulted ceilings, you should estimate how much they will add to the volume of the building, and then add that percentage to the estimated cost.


Solid (impenetrable) floor joists

While it’s sometimes possible to drill a small number of holes through TJI-joists, it’s unlikely solid timber joists or steels can be drilled once on site. If you do have impenetrable joists, it is likely you will need to introduce suspended ceilings in some if not all of the property. The service void depth needed to accommodate MVHR ducting is normally at least 150mm.

Posi-joists or Easi-joists are the way forward as all services can be hidden inside the floor, rather than below it. Be sure to specify these early in the project, as they are not always the builders first choice for cost reasons. But when you add the cost of a suspended ceiling, the open web posi-joists generally save money as well as space overall.

If you are planning to install solid timber, I-joists, beam and block, or reinforced concrete floors, if fact anything but Posi-joists (aka Easi-joists), the MVHR material and labour cost is not likely to change much. But you will need to budget for suspended ceilings, and they will require (typically) a 150mm clear void above. If you don’t want to compromise your finished ceiling height or window datum, you may need to build a taller property to accommodate those service voids.

Core drilling through reinforced concrete floors will also add cost.


Mechanical Purge for rooms without opening windows

Building regulation ADF(1) section 1.28 states that every habitable room in the property must have ‘purge’ ventilation, a means of achieving a minimum of 4 Air Changes per Hour which is ten times the normal ventilation rate. Purge ventilation is required when there are high levels of indoor pollution, for example excessive cooking odours, decorating odours, smoke, or excessive heat.

In most habitable rooms this requirement is met by installing openable windows, to be designed by a specialist with an aperture size based on the floor area of the room. But in those rooms where an openable window is not practical for noise, pollution, or security reasons, or if the room is below ground, you are required (by law) to install mechanical purge ventilation.

It is not mandatory to satisfy the mechanical purge requirement by upgrading the MVHR, but it is recommended for reasons including energy consumption, air quality, noise ingress, and physical space allocation. If you opt for a separate mechanical purge system, it not only needs to extract air at ten times the normal rate, it also needs to allow in ten times as much replacement air. In other words, you’ll need two more sizable holes in your airtight layer, potentially for every room affected.

If you need mechanical purge expect a substantial increase in cost, whether it’s provided through the MVHR or not. Purge ventilation is mandatory for every habitable room.


Active cooling through MVHR

The minimum background ventilation rate is equivalent to changing all of the air inside the property every 2.5 hours. Your MVHR must be able to deliver 0.4 Air Changes per Hour (ACH). However, to deliver meaningful cooling through the same MVHR, the system needs to comfortably deliver 1.0ACH. Effectively 2.5 times more air movement than a system without cooling.

Achieving this elevated air flow rate quietly and efficiently is what adds most of the cost. You may need a larger machine, more ducting, duct lagging etc, as well as a condensing coil. The amount of cooling you get is determined by the volume of air you can pass over the cooling coil. Fail to increase the ventilation rate and you will barely notice any difference. Traditional air conditioning recycles the air 4-6 times per hour!

AC doesn’t replace the air it just recycles it, so it’s not considered ‘ventilation’ under building regulations. That means if you want AC you’ll need a ventilation system too. The options are therefore:

  1. MVHR with active cooling (quieter, more efficient, lower cost)
  2. MVHR with separate AC (AC in isolated rooms or the whole property)
  3. Traditional extract-only ventilation with AC (expensive to run)

To upgrade your MVHR to support cooling can typically add anything from 25% to 100%, depending on the property and the amount of cooling required. You will need a reversible Heat Pump that can do heating and cooling, and possibly a rudimentary BMS to control/link the technologies if you want to use the same heat pump to warm your water and cool your air, as a single heat pump can’t do heating and cooling at the same time.

MVHR cooling done properly is quieter, more efficient, and around half the cost of installing AC throughout the entire property. But it will be dearer than fitting AC to one or two rooms only.


Passivhaus certification

When it comes to MVHR design and installation, the requirements of Passive House guidelines far exceed those of the building regulations. One is based more on occupancy and the other based on size. But even with a ‘certified Passivhaus’ you must still comply with normal building regulations, and if yours is a large property with low occupancy, meeting both the building regulations and the Passivhaus requirements can potentially result in over-ventilation and uncomfortably low humidity levels. It may be necessary to add a humidification device to keep above 40%RH.

Meeting the Passivhaus acoustic and energy targets means you’ll get a very quiet and very efficient system. Most of our systems meet Passivhaus guidelines (almost) by default. Ultra-low air resistance, below 1.5 Watts/litre/second, and normally around 25dB(A) system noise at point of delivery. However, if you’re going for the prestigious award additional attention is required. More duct insulation, more soundproofing etc. You/we can’t afford to jeopardise the entire certification process by gambling on the ‘as-built’ acoustic performance. The system must be over-engineered enough to comfortably meet all requirements.

If your ‘as-built’ MVHR is going to be scrutinised by a Passivhaus certifier, you will also need accurate, evidenced performance calcs before the job starts. You’ll then need pressure, energy, and acoustic measurements to verify the installed system performance matches the designed system performance.

The additional materials and labour required to guarantee Passivhaus compliance could add 20% or more to the overall cost. It requires more work and more materials, not just a ‘Passivhaus certified’ label on the MVHR.


Designer valves & colour-coding

By default, Solarcrest specify the standard Airflow supply and extract valves. The grilles or terminals you see in the ceiling. They’re more aesthetically pleasing than most and allow a wider range of adjustment so our technician can fine-tune the flow rates. They can also be painted with a quality emulsion too if they are rubbed down and primed, so they can better match the surrounding surface.

In the kitchen we specify a slightly different terminal with replaceable grease filter, as the kitchen extract duct is the most susceptible to contamination over time, not that that contamination will affect your air quality as the air from the kitchen ultimately leaves the building. The kitchen air valve filter reduces the need for cleaning inside the ductwork and inside the MVHR.

Designer air valves, linear slots, and climate beams are available on request, depending on the project, but they will add cost. Our designer will show you the options if you are not happy with the standard valves.

If you’re having exterior wall terminals rather than roof terminals and you provide a RAL colour  to match your facias, we can arrange for your exterior terminals powder coated to match. We can also provide colour-coded steel ducting if you want to create an ‘industrial feel’ by leaving some exposed. The additional cost depends on the additional work. For now, assume the standard terminals will be sufficient. It won’t be long before you don’t even notice them.


Working in occupied buildings

MVHR is very much a ‘first fix’ service. It goes in before plumbing, electrics, plaster, and décor. To retrofit MVHR to a finished property can be extremely disruptive, as to a certain extent the property has to return to first fix, i.e. you might need to pull your ceilings down or lift the floors. You will certainly need a local builder to expose void spaces, provide power, plumbing, and then to make good afterwards. Those costs are outside the scope of the ventilation installer.

If you are prepared to undergo the disruption associated with MVHR retrofit, and you’re willing to employ and manage a local builder to support the ventilation installer, then working in an occupied property adds another level of complexity (and cost). Moving fixtures, fittings, and personal belongings from room to room all adds time. The ducting needs to reach every room because MVHR is a ‘whole house’ system. If your property has traditional extract-only ventilation in one area, and balanced supply/extract ventilation (i.e. MVHR) in another, neither system will work correctly.

Note: If you’re switching ventilation strategy from traditional extract-only to a balanced supply/extract system (i.e. MVHR) then this is a notifiable measure, meaning you/we must notify Building Control when the job is complete. Fail to demonstrate building regulation compliance and you could struggle to sell the property one day, assuming you don’t fall foul of the Building Safety Act.

If yours is an existing, decorated, and furnished property and you are not planning other significant refurbishment works, add 50% to your MVHR budget cost and expect to (potentially) pay the same again for the local builder. MVHR retrofit is not easy.


Large property with low occupancy

The mandatory minimum ventilation rate is equivalent to replacing all of the air inside the building every 2.5 hours. It is set at this rate because that is what is deemed necessary to remove indoor pollutants for health reasons. Chemical, biological, or volatile organic compounds like formaldehyde from pressed wood products, or natural gas like argon. Removing odours, dust, and excessive moisture are secondary. In terms of moisture, 40%-60% Relative Humidity is the healthiest place to be.

For habitable comfort you must heat the house by heating the air within it, so ventilation with minimal heat loss will naturally reduce the energy cost. However, heating the air vastly reduces the relative humidity of the air. Old fashioned methods of heating like open gas fires release additional moisture into the air as a by-product of the combustion, but modern (sustainable) heating systems produce “dry” heat. Electric storage heaters, low wattage heaters, underfloor heating systems, ground or air source heat pumps, do not add any moisture to the internal atmosphere.

An MVHR system can decrease the moisture level by increasing the ventilation rate, but it cannot increase the moisture level by decreasing the ventilation rate, not beyond a certain point, as this has other negative effects.

According to the NHBC one person adds 3-6 pints of water per day through breathing, washing, and showering etc. A small high occupancy house will get damp without MVHR to deal with the excess, but in a very large, low occupancy house you may not be adding enough moisture to stay above 40% Relative Humidity. If you drop below 40% your contact lens’ will dry out and you could get a tickly throat, so you may need an automatic humidification device connected to your smart MVHR.

An automatic humidification device controlled by the MVHR would cost around £1500.


Remote location or city centre

Projects in remote locations or city centres incur additional travel and welfare costs. Transportation of men and materials, hotel accommodation, congestion or street parking all add to our overheads, so we kindly ask the client to meet these costs, normally ‘at cost’.

Solarcrest have successfully completed projects all over the UK, from Penzance to the Orkney Islands. We can help wherever you are provided you cover the additional costs. The same will likely apply for all specialist trades.


Other potential factors..

If yours is an HMO or a property over 18m tall you will need additional fire safety measures. Fire collars, resettable fire dampers, and fire-rated air valves are mandatory and will add cost. Any property over two floors will likely need some fire protection products.

If you want zonal temperature control from a heat pump connected MVHR, either heating or cooling, you’ll need additional controls possibly a BMS/ home automation system.

If your property is in close proximity to a road or other AQMA you may need a NOx gas filter to comply with local Environmental Health requirements.

If you’re very close to a neighbouring property you may need to reposition the plant or extend the ducting to avoid a causing noise nuisance. Standard air intake and exhaust terminals can be noisy when running on boost, although that noise can be reduced by enlarging the ducting close to the terminal.

If your property is in a coastal or exposed location, you may need to reposition the plant or extend the ducting to avoid strong prevailing winds. In this scenario the intake terminal could intake too much air, and the exhaust terminal could struggle to overcome the wind.

Whatever the scenario it is better to design the MVHR correctly rather than cheaply. The question 2a.1 on the ADF(1) completion certificate asks “Has the system been installed in accordance with the manufacturers requirements”. If you answer YES and it hasn’t, not only are you in breach of the regulations, but you will also void the warranty. Cheap MVHR is a false economy.

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Tell us about your project
If you prefer to avoid problems rather than having to fix them, work on the basis that every pound invested up-front saves two pounds worth of issues later
Eliot Warrington
MD and Founder
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