Is it more energy efficient to ventilate a home with an HRV/ERV or “shock-ventilation”?

Question

In a reasonably cold climate like Hamburg, Germany where the standard practice is to open all your windows in the winter for up to 20 minutes once a day to ventilate your home (Stoßlüften or “shock-ventilation” as promoted by the German government), would a heat recovery ventilator or energy recovery ventilator be a more efficient solution than opening the windows once a day as is standard practice in a nation where many residents pride themselves on the energy efficiency of their homes?

I am aware of this similar question which asks for a non-specific location and answered with what appears to be mostly non-objective American sensibilities.

I just find it hard to believe that an advanced nation like Germany could continue promoting “shock-ventilation” as opposed to pushing for regulations to include an ERV/HRV in new construction projects unless the practice is at least as efficient as an ERV/HRV.

Certainly an ERV/HRV is more convenient for day-to-day ventilation (you don’t have to stop and open your windows once a day for 20 minutes) and you don’t have to wait for your home to warm up again, but if you can live with the inconvenience like the Germans do, is it more efficient under any circumstances to do “shock-ventilation”?

Answer 1

The aim of intense ventilation ("stoßlüften") is to exchange air in a room efficiently. Generating a substantial draft ensures, ideally, that all corners of the room are ventilated.

If you have a window ajar ("angekippt" -- because the Germans tilt their windows) there is a steady exchange near the window and very little in the rest of the room.

It is true that intense ventilation loses all the heat held in the air that goes out. For instance, if you have a room which is 4m x 4m x 2.5m, that is 40 cubic meters of air, which is about 50kg. If you have substantial 20K temperature difference, you are losing 20 * 50 * 700 J of energy. That might look like a lot, but there are 3.6 million Joules in a kWh. You are actually losing around 0.2 kWh. The important thing is to get the balance right: exchange the air but stop before the walls lose any significant amount of heat.

With continual ventilation you need to exchange much more air in order to ventilate all parts of the room, so it becomes worth while having a heat exchange. This appears to be the critical advantage of the "stoß" -- exploiting a strong draft to get fresh air in the whole space quickly.

Translated roughly from Richtig Lüften

As pointed out in a comment below, this implies that circulation within the room needs to be taken into account. Continual ventilation through a partially open window or poorly fitted windows and doors is clearly the worst option. To get full efficiency from Heat Recovery Ventilation you need not only the heat recovery system but some means of ensuring the level of good circulation achieved by intense ventilation. If you do this, the combined HRV + circulation system should give savings relative to intense ventilation.

Answer 2

The big questions of ventilation are:

• How much air is exchanged
• How much heat (or coldness) is recovered from the exhaust air

A perfect ventilation system for homes would have at least 0.5 air changes per hour and would recover 100% of the heat (or coldness) from the exhaust air.

This means the perfect ventilation system would not cause any additional heating or air conditioning need.

We're not there yet. We can achieve any air change rate, but the 100% heat recovery cannot be achieved.

The problems of opening the windows are:

• It's not a constant ventilation but rather too much ventilation during too little time, and then when you close the windows again, ventilation rate drops to practically zero
• Even during the time the windows are open, the ventilation rate is not constant, but rather dependent on wind for example
• Exactly zero percent of the heat in exhaust air is recovered
• Pollen is not filtered away from the incoming air unlike it is in centralized systems that have air filters
• It is manual, requires additional effort to ventilate unlike centralized systems that are fully automatic

I'd say opening the windows is the crudest possible ventilation system, to be avoided at all costs. It's not energy efficient unless you ventilate so little that it would be unhealthy due to accumulating volatile organic compounds inside the home air.

In real exhaust heat recovery ventilation systems, about 70-90% of the exhaust heat can be recovered. Far better than 0% of opening the windows. It also helps when the outside air is warmer than inside air. Then it recovers coldness, allowing to reduce air conditioning need.

An alternative for rapid ventilation would be to have less rapid ventilation combined with air purifiers. However, a good air purifier that actually works (so that it has a large particle pre-filter, a volatile organic compound filter, and a HEPA filter for small particles) has huge filter costs, surprisingly consumes lots of electricity, generally makes a louder sound than ventilation systems, and also steals non-negligible floor area away from your home. However, if you live in a rental home or an apartment building so that you can't affect the centralized ventilation system since you don't own it, operating an air purifier could be the only way to achieve better indoor air.

Answer 3

ERV/HRV AFAIK comes mainly in two shapes:

• as centralized system with a big heat exchanger and ventilation system for the rooms
• as decentral system with little ventilators in the rooms.

I'll tackle the decentral ones. They have a chance of being more energy efficient, but the difference isn't that large - all in all, I don't think the case is clear-cut. (Don't have numbers for central ones)

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The ones I've seen lately work with flow rates of, say, 10 - 60 m³/h. The lower the flow rate the better the heat/enthalpy recovery.

They either have

• a heat exchanger with heat recovery (these specs claim 94 % recovery at 10 m³/h, dropping to 60 - 65 % at 60 m³/h)
  However, their inflow and outflow are typically right besides each other (two sides within one circular tube in the wall). I'd expect the air flow to be similarly inefficient as tilted window (air exchange mostly in a small volume close to the ventilator).

• a ceramic body that gets heated up (and adsorbs moisture) when air goes out, and heats up and humidifies ingoing air. With them, the direction of the air flow is reversed about every minute.
  (these specs claim ca. 90 % heat recovery at 6 m³/h and ca 82 % at 22 m³/h [that's overall planned flow rate, they say max flow rate is twice as much])

  You can set them up so that one room has ingoing air while the other has it outgoing. Which makes for much better than having inlet and outlet withing a few cm of each other.

  In the minute they work in one direction, so, roughly 100 - 600 l of air pass. That's maybe 1 % of the air volume of @MJuckes' example room, or maybe even less if you install it across a flat or house.

So for both variants we in a situation where the outflowing air is close to the inflowing air (convection may help?).

The plan with shock/intense ventilation is to create a substantial draft (if possible across the house) that replaces pretty much the complete air within a short time (5 min - 20 min). That's like refilling a batch reactor, or idealized a plug flow reactor.
With the decentralized systems, it's more like a continuous tank reactor: a small amount of fresh air continually mixes with the content of the room(s). For an ideal CSR (which we don't have, but which I think we can use as 0th approximation).

With continuously 6 m³/h incoming and 40 m³ total volume and 90 % heat recovery,

• you end up with an average age of the air corresponding to the average air quality you have with shock ventilation every ca. 14 h,

• at energy losses that are only about half compared to a single shock ventilation cycle per day.
  I calculated this with temperature difference of 20 °C as in @MJuckes' example - that would be winter in Hamburg where freezing temperatures are rare. The 280 Wh thermal energy loss for a complete air exchange 20 °C -> 0 °C compare to about 60 Wh per day for running that ventilation at its lowest speed.

• A factor of 3 (2 for energy loss and 1.5 for same average air quality) is non-negligible, but OTOH, it can easily be eaten up by small additional inefficiencies.
  If the the air exchange is less efficient due to the changing flow direction, or wind makes the flow rates uneven in the direction, so heat recovery is less efficient, that difference is easily eaten up. Overall heat exchange efficiency of 7e5 % instead of the assumed 90 %, and temperature difference only 10 °C instead of 20 °C (ventilator power consumption stays constant), and the advantage is eaten up.

• Also, you could decide to do your daily shock ventilation in afternoon when ambient temp is highest (at an average +4 °C instead of -1 °C in Hamburg January) you can cut the shock ventilation losses by 25 %.

• If you're at home the full 24 h of the day, you'll need to ventilate more than if you're at home only for sleeping. It's easy to adapt shock ventilation to this since it works on a time scale of 5 - 20 min.
  The example ventilation system has a τ of 6:40 h, meaning that it needs 1 1/2 days to reach steady state (rule of thumb for ideal CSR: 5 τ).

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• There are norms for minimum ventilation. Wiki page in German cites for > 1/h for automated ventilation (and 0.5 - 3.6 for manual ventilation), and 25 m³/(h ⋅ person).
  8-o

  Neglecting infiltration air exchange (which would be valid only for new buildings) that would mean shock ventilation of the example room every 2 h when 1 person is there (which noone does...), but also the example ventilation system wouldn't be sufficient when working at highest speed (21 m³/h)

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• A human emits maybe 75 W heat, so having one more person present in the (admittedly not too large room) "allows" shock ventilation every 4 h (or half as often as prescribed per norm...) ;-)