Does an HRV or MVHR system work?

Practical Analysis of a domestic HRV system in Scotland

This is our experience of using our Heat Recovery Ventilation System.

It covers ideas about choosing an HRV system and running costs.

I have also devised an anenometer and thermometer useful for adjusting the flows and monitoring the temperatures at the vents. They are not covered here but I will happily advise anyone trying to set up a system.

If you have any comments or questions, please feel free to contact me using the contact page.

Note: I measure temperature in °C

Roddy Young  updated January 2020

What is an HRV system?

One thing has to be clarified right at the start.

An HRV system is not a heating system; it is a ventilation system

An electrically powered Heat Recovery Ventilation system brings fresh air into a house and removes stale air all year round using as little energy as possible.

It does this using electrically powered fans and ducting together with a simple heat exchanger which transfers some of the heat warmth (heat) from the extracted air to the incoming fresh air.

Some rooms are supplied with fresh air through a ceiling "valve"; the other room valves are ducted for extraction.

The main purpose is to improve the quality of air in a controllable way without the need to open windows or vents.

In the UK, houses must by law have "trickle" ventilation at all times. This is usually achieved through little vents on the window frames. Using an HRV system allows these vents to be "deleted", ie windows may be fitted which have no such vents.

The HRV system reduces moisture build up in the house (eg baths, showers, cooking, drying clothes etc) and removes stale air, smells (eg toilet, cooking etc)  and air-borne pollutants(eg paint solvents).

HRV systems like this are sometimes called "Mechanical" Heat Recovery Ventilation systems because the air is moved by fans; they are suited to well sealed buildings and particularly to new builds.

HRV System Design

Our new build house is a timber framed and clad bungalow, approx. 132m² in south Scotland. The plans included underfloor heating and it was designed to be relatively airtight. The airtightness was to be achieved by lining the walls and ceiling with wind and waterproof membranes. These membranes were to be sealed where any wire, pipe etc. passed through, using an appropriate sticky tape or tube sealant.

We wanted an HRV system to provide the required ventilation without ugly window vents or other uncontrollable ventilation.

Looking at various HRV systems, it became clear that some did not totally separate the outgoing and incoming air flows. In particular, we felt that some rotating disc heat exchanger models might transfer contamination and smells to the incoming fresh air. We did not want to use any system like this and ruled out rotating disc HRVs.

Many HRV systems include an electric heater to warm incoming air. We were unhappy to have an invisible electric heater controlled by any device, making its own choice when to consume our expensive electricity.

We also did not want the main fan unit built into a kitchen cupboard. Despite manufacturers’ claims of near silence, we felt this might produce an irritating noise. The unit was, therefore, to be fitted in the (unheated) attic. Its condensate drain could not be ducted around the unheated attic (to avoid freezing up in winter) and so would have to go down immediately through the ceiling to the warmth. The logical location proved to be above the utility room.

We could not get reliable information from HRV suppliers and manufacturers concerning using a cooker hood in a house with an HRV system.

A separate extracting cooker hood was originally specified for the kitchen, however, we felt this might create negative pressure in the house which could overcome an HRV extract system, “forcing it into reverse”. It has to be said, however, that the one HRV manufacturer we consulted (Vent-Axia) did not consider this to be an issue.

It is also possible to connect a cooker hood into the extract side of an HRV system. In this case, we feared cooking grease might get through the filters in the hood, particularly if they were not replaced regularly and into the HRV ducts and heat exchanger, causing long term problems and possible fire hazard.

In the end, we opted for a standalone recirculating cooker hood with filters to remove cooking grease and rely on the HRV system to extract smells and moisture from the ceiling area.

The HRV detail design was entrusted to ADM of Skipton, Yorkshire. Fresh air would be supplied to living and bedrooms, stale air extracted from bathrooms, kitchen and utility room.

ADM’s evaluation assumes the air will migrate from supply rooms to extract rooms to provide the required air changes. Our installer followed their plans.

The total supply and extract flows were each calculated at 107m³/hr, giving air change rates (ACR) of 2 per hour in bathrooms, 1 in kitchen and 0.6 in other rooms. 

A Vent-Axia 443319 Kinetic BH unit was selected. It can run at various speeds and supply the required 107.5m³ /hr when running at less than 50% capacity. This would allow for a boost to 60% or more as required, for example, if a lot of steam were produced in the kitchen.

The precise meaning of these % capacity numbers is unclear. It is assumed that they refer to approx. airflow rate.

The unit has an automatic “summer bypass” for the heat exchanger. This operates if the temperature of the air being extracted is above a set level (24°), meaning that the house is too warm. Such a situation can occur in summer when there is a lot of solar gain (ie sun shining through the windows). When the summer bypass is operating, the incoming air is not preheated by the outgoing air.

It also has an anti-frost mode which reduces the supply fan speed in winter to avoid overcooling the extract air with resultant excess condensation and frost in the heat exchanger. It is not easy to fully understand how the extract fan can operate at normal flow rate while the inlet flow rate is reduced and that this should bring the required reduction of frosting. One might assume that air will be drawn into a sealed house at the same rate it is extracted ……………

Another feature of this unit is a humidistat to sense if dampness is being produced, e.g. in the kitchen when draining boiled vegetables, or in the shower room when showering. This automatically switches the unit on to a boost setting until the humidity falls again.

The consumption of this HRV unit according to Vent-Axia sales info:

 49-58W at 60%

25-30W at 40%

12-13W at 20%

 Vent Axia supply a small control panel which we opted to have fitted in our kitchen:

Installation

Our HRV installer was delighted to have easy access to the attic and made an extremely neat job. He used insulated rigid plastic ducting (150mm diameter), with short lengths (1200mm) of flexible ducting to isolate vibration before the connections to unit itself.

The ceiling valves were adjusted to give the correct flow rates.

The system was completed in winter and initial running produced a large quantity of condensate. We collected 2.5litres overnight for the first few days. This water came from the poured gypsum-based self-levelling floors.

Unfortunately, we later discovered that the condensate dripping down within the ducting had dissolved some of the duct sealing compound and the resulting sludge had run into the HRV unit. There was a thin layer of goo on the extract impeller. We hope this has not compromised the mechanical balance of the fan. (Until now, April 2018, it is not detectable)

The plumber did not understand the significance of running a condensate drain pipe through an unheated attic. He installed a horizontal 25mm pipe, connecting it into a vent pipe some 2 metres away. This had to be re-plumbed vertically down through the ceiling (as designed) to carry the condensate water quickly into a frost free area.

The house was designed to be well sealed – it has no letter box, window vents etc. and the builders tried to maintain the vapour barrier integrity using tape to seal access holes. It was not pressure tested and we can see that a few holes have been cut through the vapour barrier (particularly in the ceiling for downlighters).

 Commissioning and modifications

It was immediately obvious that the “air movement” sound from the ceiling valves was unacceptable with the unit running at 45% (the speed which supplied the required airflow). This was a hissing, fluttering noise.

When asked to correct this, our installer seemed happy to point a microphone about 300mm from the valves and take a relatively random reading as it wobbled around. The contrast between his precision equipment and his uncontrolled technique seemed quite amusing to us.

This “white noise” or “hiss” was traced to the smooth walled rigid plastic ducting which transmitted the fan noise to both supply and extract valves.

The addition of approx. 2m of flexible “acoustic ducting” on supply and exhaust sides of the unit brought the sound down to acceptable levels. The irregular interior surface of this new piping might have caused a minor flow reduction, however, the flow rates through the ceiling valves were not checked after this addition.

It was noted that the temperature of the supplied air varied by several degrees with distance from the HRV unit. On a day when the attic temperature was similar to the outside temperature of 4.1°, the vent in the lounge nearest the unit supplied air at 12.9°, while a second vent 5m away was at 10.2°, a loss of 1.7°. This was attributed to the ducting having only a very thin foam insulation.

Insulating mineral wool was laid around the ducting in the attic. The addition of a 150mm layer brought a noticeable improvement. Afterwards, typical readings were: outside, 3.8°, near valve 12.5°, further valve, 11.9°, loss 0.6°

Living with the HRV system

 Noise

The immediate impression is that the system is very quiet; however the fan unit itself does make a constant hum and air hiss noise which would have been annoying in the kitchen. Locating the unit in the attic was confirmed as a good choice.

Air entering the rooms through the ceiling valves is barely audible in daytime. Our house is in a rural location without background traffic or other noises. One has to stand immediately under the valve and listen carefully to hear the airflow when running at under 50%.

It can, however, just be heard at 60% throughout the living room and, in addition to the airflow, there is a low frequency hum in this room with a phasing effect. This may be caused by the use of 2 supply valves in the lounge ceiling.

To our surprise, there is a separate and completely unexpected noise problem. In order that the air can flow freely from the supplied rooms to the extract rooms, the tops of the doors are deliberately left with a hidden gap of some 12-15mm to their frames.

This means that noises also travel from room to room and through the house, particularly from the hall which has an echoey linoleum floor surface. The result is that our cuckoo clock in the lounge can be heard in the bedroom at night through 2 doors!

The exhaust vent on the outside wall also generates a continuous, low level, airflow hiss noise. That might have been distracting had we built a seating area in the garden nearby.

We have set the controller to run the system at 50% during the day and 33% at night. This ensures we cannot hear the system in the quiet of the night.

 Humidity

 The house is much drier than our previous one, where we controlled ventilation by opening windows as required.

The humidity is consistently less than 45% in all rooms (including the unused spare room), meaning that any condensation which appears on the glass shower enclosure or bathroom mirror evaporates quite quickly. Bath towels dry overnight on an ordinary unheated  towel rail, even with the windows shut.

The humidistat operates very effectively, sensing moisture in the air extracted from kitchen and bathrooms and automatically switching the unit to Boost.

On the negative side, we have found that, with the underfloor heating running in winter, the humidity falls further and this has led to our becoming dehydrated. We have to drink more water! In addition, there is shrinkage in some furniture. One 360mm wide softwood cupboard door has shrunk by almost 10mm across the grain.

 Smells

Cooking and bathroom smells generally do not linger and are soon extracted. They also tend to be drawn back into the room rather than seeping further into the house.

Having said that, smells such as burnt toast in the dining area of the lounge disperse slowly – similar to partly opening a window. The HRV system is not as effective as opening a window or outside door fully.

The HRV unit is intended to be left running 24 hours per day, so no “OFF” switch is readily available. (The main switch is located beside the unit in the attic). One reason for this is that our windows have no vents, so trickle ventilation (as required by planning regulations) must always be available by some other means.

As a result, one cannot prevent smells from outside being brought into the house.

Such smells include the farmer spreading slurry or a neighbour's  smoky wood burning stove. We have concluded that a software option to temporarily set the unit to a low or OFF position for a set period (say 30 mins) from its control panel would be very useful.

This inability to prevent smell ingress from the control panel is another unexpected issue and probably our most serious complaint.

To combat this, I built and fitted a wooden cabinet with a large active charcoal filter. I then rerouted the inlet duct. This is remarkably effective, though there is still a trace of smoke smell coming onto the house.

To compensate for the reduction of airflow, I had to raise the flow rates to 50% during daytime and 33% at night. There has been a marginal increase in noise.

The extract and supply vents are positioned on a vertical gable wall about 2 metres apart. This is supposedly  a "balanced system" where any increase of wind induced air pressure will be the same on each vent, with the exception of strong gusts which can momentarily overcome the fans.

 Comfort

Incoming air is seldom cooler than 13° at the ceiling valve in winter and is warmer in other seasons; this has proved very acceptable. Draughts from the HRV system are not noticed unless one stands directly under a supply valve. The ceiling is 2400 high.

Visually, we are pleased there are no ugly window vents but have to accept that each room has its ugly ceiling valve instead. These do have a fairly “industrial” appearance.

Extract on the left, Supply on the right, below:

Our bedroom window is usually open overnight so we can enjoy the early morning birdsong.

The system is left running 24 hours per day, 365 days per annum. We return from 3 week holidays to find generally fresh air in the house.

The draught free comfort is one major reason for recommending an airtight house build with HRV system.

Maintenance

 Maintenance comprises cleaning the filters and changing the clock for summer and winter time.

2 filters are fitted, preventing contamination reaching the heat exchanger with its narrow passages. The one in the fresh, incoming air (left in photo), collects an array of insects, vegetation and earthy dust, while the filter on the extract side (right in photo) gathers an astonishing amount of fine powdery household dust (usually reddy brown in colour - a result we assume of fibres from our red carpet being sucked away).

A “Change Filers” indication appears in the control panel at approximately 3 monthly intervals. The factory fitted filters can be renewed (in 2017, about £26) or washed. We have 2 sets; 1 in the unit, the other washed and ready to re-install. The job is very easy.

The first drum carbon filter (the one I added) lasted about 3 years before I thought it had lost effectiveness. Better quality (= more expensive) ones last up to 5 years. The inlet filter in the unit is much cleaner now with this extra filter.

Changing the clock for summer or winter time is relatively easy, though non-intuitive. The number of button presses involved on the control panel is difficult to remember from season to season, so one has to refer to the instruction manual each time. As a result we have decided not to change the settings at all. The clock now runs permanently on Winter Time!

The ceiling valves remain generally clean, though there were slight traces of grease on the kitchen extract valve after 4 years.

ENERGY ANALYSIS:

Temperature measurement techniques

The practical measurement of temperature in the air flowing through ducts and entering or leaving the ceiling valves proved much more challenging than anticipated.

A domestic digital thermometer was dismantled and the sensing element mounted at the end of a wooden stick about 1metre long. This proved almost useless, for example, jumping up and down within a range of about +/- 2° in free air outdoors according to the wind strength and direction and sensing nearby warm bodies including my own hand.

The sensing element (assumed to be a thermistor) was repositioned within a shroud which fitted into the ceiling valve openings. This brought some thermal mass and tended to average the readings. This element was positioned in the airflow at each ceiling valve and the readings allowed to settle for about 60 sec. for repeatability.

Thermistors were also fixed into the outdoors ends of the ducting after initial calibration over a range of temperatures (using a mercury lab thermometer). These also proved to be less consistent than anticipated, being influenced by sunlight which warmed the ends of the ducts, the warming effects of the current passed by the multimeter, by build-up of dust on the sensor, by humidity and (possibly) by other effects in the long unscreened cable I needed to use.

2 new IC temperature sensors (LC335) were sealed in heat-shrink tubing and fitted into the ducts further away from the sunlight and the results over a winter to summer period were considered reliable.

Comparison of the outside air temperature measured at ground level and the intake temperature measured at 3.5m above ground level also showed remarkable differences, often well over 1°.

Despite some reservations, relative values could be noted and trends became quite clear. Measurements were made over several weeks and repeatability was noted. The figures were used in calculations of possible air-borne heat loss - see later.

Energy consumption

We have found that manufacturers’ figures suggesting X% heat exchanger efficiency should be treated with some scepticism. Any measured value is likely to represent the most favourable working temperatures in laboratory conditions.

The efficiency of a heat exchanger will, by most calculations, be 100% when the extracted air is at the same temperature as the incoming air, for example, 20°.

An attention grabbing figure such as “efficiency” may not mean much in an installed situation where the heat loss across many metres of ducting can dramatically change any efficiency calculation.

In any case, heat exchanger efficiency is only a part of the total energy analysis. It ignores the fan motor power consumption.

Fan motor power consumption is, however, a relatively easy calculation.

 Fan power

Manufacturer’s figures suggest running power consumption of 25-30W. A domestic wattmeter was used to verify this, suggesting higher figures as below:

Using 1.09kWhr per day equates to an annual consumption of about 400kWhr per year, or a cost of £51.72 (at 13p per kWhr) .

Note that I have now fitted an accurate wattmeter to the HRV supply. This suggests the above figures are much too high - over 1 year the wattmeter recorded an average of 0.6kWhr per day (24hours).

 Energy lost within the exhaust air

Without installing many temerature and flow sensors, it is difficult to accurately calculate the energy lost within the exhaust air as there are so many variables:

•         The most significant is solar gain. In winter when the sun is at a low angle, we found our lounge can be heated to as much as 28°. Under such conditions, the central heating switches off so any energy expelled in the air exhausted through the HRV system would not be replaced; instead, the loss is welcome!

•         Any latent heat energy given up by dampness in the exhaust air condensing in the heat exchanger will vary, depending on activity within the house.

•         Similarly, energy absorbed by moisture in the incoming air will depend on its temperature and moisture content.

•         In summer (when the room temperature is above the set value) the bypass comes on and cooler air is introduced – another case where any energy loss is welcomed and not replaced by the central heating.

•        In cold conditions, typically below about 5° outside, the defrost mode is activated, so the airflow during that time is less than the designed 107.5m³ per hour.

Measuring and recording the difference between intake and exhaust air temperatures in the unit ducting immediately showed considerable inconsistency though, as expected, the trend was towards higher temperature difference at lower outside temperatures.

Because the approximate flow rates are known, it is possible to compare the energy stored in the extracted air with that in the incoming air

Ideally, one would employ 24 hour datalogging of all relevant temperatures and moisture content.

In the absence of such facilities, guesswork can be used to arrive at an estimated energy loss. The following is based on such values.

The calculation is for nominal energy losses during the following seasons:

• Spring and Autumn when the outside air temperature (and therefore air intake temperature) is between, say, +3° and +15°. 
• Winter when outside temperatures range from 0 to 3.5°
• Summer when inside and outside temperatures are similar.