MHRV Ventilation Requirements

An Eccentric Anomaly: Ed Davies's Blog

Having got the design of my heat exchanger significantly wrong I'll, for now, just post the requirements bit which I'm fairly happy with.

General Plan

The usual scheme is to extract air from “wet” rooms - kitchens and bathrooms and supply air to bedrooms and living rooms. For my simple house design that's pretty easy: extract in the bathroom and kitchen and supply to the two bedrooms and the study with the main airflow percolating from the main bedroom and study through the living room to the kitchen extract.

All the ducting will run back and forwards through the loft areas with the supply to the main bedroom across the cathedral ceiling of the living room and study on a shelf suspended below the apex of the room (which will also carry other services like power and network cable and have lights on the bottom).

The heat exchanger will be mounted on the west gable end of the main house within the porch/greenhouse with ducts taking air into and out of the house going through the gable wall at floor level in the loft, over the small bedroom, to high up in the greenhouse.

Ventilation Requirements

Scottish Building Regulations

Obviously these are definitive on what's actually required from the building control point of view. Frustratingly, however, the Domestic Handbook 2013 is somewhat less than explicit on what's required for mechanical ventilation despite going into some detail for trickle vents.

For houses with infiltration rates better than 5 m³/(h·m²) at 50 Pa they say (in section 3.14.10) that mechanical ventilation in accordance with CIBSE Guide B2:2001 and BRE Digest 398 should be installed. The CIBSE Guide has now been superseded and the new one costs about £100. Hmmm…

The BRE document is available online but is not terribly informative when it comes to required ventilation rates. All it says is 0.5 to 0.7 ac/h (air changes per hour) with the ability to boost by 50% in individual rooms or 25% for the whole system.

The Domestic Handbook also gives some rates for when natural infiltration is in the range 5 to 10 m³/(h·m²) which, I suppose, need to be at least equalled in a more airtight house:

Kitchen 30 l/s (108 m³/h) over the hob or 60 l/s elsewhere
Shower room 15 l/s (54 m³/h)
Toilet 3 ac/h

I'm not overly happy about that kitchen requirement. What I'm planning is a cooker hood over the hob area (looked at ones with 170 m³/h (47.2 l/s) and 260 m³/h (72.2 l/s) capacity the other day) operating in re-circulation mode (acting to clean the air without extraction) with a separate extractor away from the hob (to avoid getting grease deposited in the MHRV).

In practice I don't do a lot of greasy frying or anything so I'd only expect to use the cooker hood occasionally but it'll be there when it's needed. With the kitchen open-plan to the living room and study and also the hallway, water vapour should be fairly well diluted and able to be extracted at sensible rates.

I propose to argue that a 47 l/s recirculating hood is providing the effect of at least 40 l/s of the kitchen extract requirements so the MHRV extract away from the hob only needs to provide 20 l/s.

My “bathroom” (shower, WC and basin) will be about 2.4 × 2.4 × 2.4 m³ so 3 ac/h comes to about 42 m³/h (11.5 l/s) so the shower room requirement of 15 l/s sets the rate needed.

Passivhaus

Another good source of guidance on ventilation requirements in this sort of house is the Passivhaus standard. The Green Building Forum has a very good post on their criteria (though with a quibble on calculation for higher than typical rooms - see further down the thread).

Per Occupant

Assuming two occupants for each of the two bedrooms requires 4 × 30 m³/h = 120 m³/h.

Per Extract Room

Kitchen (60 m³/h) and bathroom (40 m³/h) = 100 m³/h.

Per Floor Area

Well, volume, actually. Excluding the loft areas the internal volume would be 254 m³. Actually it could be a bit less if the thermal store volume was excluded as well. For 0.3 ac/h plus 30% boost that comes to 99 m³/h.

PH Extraction Requirements

Taking the limiting case of four occupants we need to be able to extract 120 m³/h (33.3 l/s). Apportioning that in proportion to their requirements among the extract rooms gives: kitchen 72 m³/h (20 l/s) and bathroom 48 m³/h (13.3 l/s).

Combined Extraction Requirements

In summary:

Kitchen Shower/toilet Total
Building Regs 20 l/s 15 l/s 45 l/s (162 m³/h)
Passivhaus 20 l/s 13.3 l/s 33.3 l/s (120 m³/h)

Remember that these are worst case rates when the system is on boost. I'm going to guess that normal operation will be around 15 l/s with typical boost (kitchen or bathroom but not both) at 30 l/s and design to be able to reach the building regs required 45 l/s but operate efficient heat recovery up to 30 l/s.

With air at typical sorts of temperatures and pressures having a density a little below 1.3 kg/m³ these rates correspond to 58.5 g/s and 39 g/s. Let's call them 60 g/s and 40 g/s.

Exchanger Energy Transfer

There are two considerations which might set the size required for the heat exchanger: the amount of energy to be transferred, which sets the exchange surface area required, and the need to keep the flow rates moderate to avoid noise, which sets the cross-section of the flow channels. While thinking about the construction of the exchanger I'll just talk about the energy transfer considerations for the moment.

Two forms of heat need to be transferred from the outgoing air to the incoming: sensible heat (actual temperature) and latent heat (stored in water vapour which is condensed). Sensible heat scales pretty much with the temperature difference (the specific heat capacity will vary slightly as the temperature changes, but not much) so we could calculate simply in terms of the ΔT. However, things are more complicated with condensation of water vapour as that's close to an exponential function of temperature. Therefore, it's probably best to calculate for a plausible, but slightly extreme, combination of indoor and outdoor air conditions:

Indoor Outdoor
Temperature 20 °C 0 °C
Relative Humidity 50% 100%

and allow a little margin for variations in the conditions.

Note that the actual outdoor relative humidity is irrelevant, what's of interest is the relative humidity of the outgoing air once it's been cooled by the incoming air. If we assume the outgoing air is cooled to the temperature of the incoming air (it won't be - it'll be slightly warmer as the exchanger is not 100% efficient but some heat is recovered from condensation) it'll reach a high relative humidity.

In principle the RH should be 100% or a little higher but in effect it will be higher still in that some of the condensate will be carried out with the outgoing air as suspended droplets. Only a portion of the latent heat in those droplets will be recovered. But calculating the area of the heat exchanger on the assumption that the RH is reduced to 100% will make the exchanger a little larger than actually needed which is probably helpful.

Sensible Heat Transfer

The specific heat capacity of air varies a bit with pressure and temperature but is about 1 kJ/(kg·K) so at the highest rate we want optimum operation (40 g/s) and 20 K temperature difference the sensible heat transfer will be:

0.04 kg/s × 1000 J/(kg·K) × 20 K = 800 J/s = 800 W.

Latent Heat Transfer

This table says that the equilibrium water vapour content of the air under our indoor and outdoor conditions would be 15 and 3.8 g/kg (grams of water vapour per kilogram of air) respectively. At 50% RH the indoor vapour content would be 15/2 = 7.5 g/kg.

At the flow rate we want to optimize (40 g/s) the condensation rate would therefore be:

0.04 kg/s × (7.5 - 3.8) g/kg = 0.148 g/s.

The latent heat of vaporization of water is commonly given as 2260 kJ/kg but that's only valid at 100 °C. For MHRV-like temperatures this formula seems more appropriate, giving latent heats over the relevant range of:

Temperature / (°C) Latent Heat / (kJ/kg)
02500.8
52489.0
102477.3
152465.6
202453.8

The spread's not so wide that it's unreasonable to pick a round number near the middle so let's call it 2470 kJ/kg. At 0.148 g/s that's about 365 W.

Net Heat Transfer

The total amount of heat power we need to transfer from the outgoing air to the incoming air is the sum of the sensible and latent heats: 800 W + 365 W = 1165 W. Let's call it 1200 W for round numbers.

Note that a kilowatt or so of heating is not insignificant in a well insulated house. Even if the usual operation is at lower airflows this is an indication that heat-recovery is well worthwhile.