MVHR on all time or not

[Jane & Peter Harris]

Any ideas if our MVHR should be on all the time or not as it seems to me that there is an inherent heat loss as it is 90/95% efficient   The house runs at between 17.5〫and 22〫depending on when the wood burning stove is on

Peter & Jane Harris

[Mark Siddall]

If the MVHR is left running all the time then you will still consume less energy than nat vent alone (or wind cowl with HR for that matter). The point at which leaving it switched on consumes more than it saves (in any give instant) is dependent upon the efficiency of the HR and the specific fan power. A PH certified unit, 75% efficient spf 0.5 W/m3, has a balance point of about 14C (turn it off above this temp to save the most energy/carbon). The best PH units have a balance point of about 17C (90% efficient, spf 0.3 W/m3). For the best SAPQ rated systems expect a balance point of about 14C. (This is due to differing test methods).

Hopethis helps.

Mark

P.S. The balance point temps stated are external.

[Kate de S]

Mark, I understand what you are saying about when its not very cold, there starts to come a point when there's not so much heat loss to save, so fan energy starts to approach heat saving, etc.

Pardon me if I'm being a bit dim, but even when its really cold, is there any harm in switching the MVHR off for a bit, so long as you are dry enough and well enough ventilated (and of course, not planning to open a window instead)? On the grounds that just because you can leave it on, doesn't mean you have to?

And might it even save a (tiny) bit more energy, as Jane suggests, plus add to comfort if you are not a fan of v low RH and out of the condensation danger zone?

Your wisdom is awaited!

Kate

[Mark Siddall]

Hi Kate,
No doubt you have an very capable engineer within feet of you. No doubt he can explain it better than I :-) But I'll give it a go.

Good MVHR is more energy efficient than gas heating and the colder it gets the more energy it saves - unlike a heat pump (ask Alan if you want details). If I owned a superinsulated home such as the one built by Peter & Jane then I would first turn on the MVHR  when the external temp falls below the balance temps noted above, then once that no longer surfices I'd turn on the heating system. What this means is that not only do you avoid uncomfortable drafts but you also ensure that the indoor air quality is maintained at the highest possible level.

The fresh air provided by a well sized MVHR system is based upon "just right" design - not to much, not to little. There are settings to allow higher and lower vent rates if desired, say in the event of abscence (lower) bathing (higher), <0C externally (lower; to improve relative humidity) etc. During the heating season switching off the MVHR is not something that I would consider, especially in a home with a wood buring stove. The reason for this is that particulates from the stove get into the room and have a negative impact upon the indoor air quality. (PHI found that many stoves are not adequately sealed.)

Not sure if this is what you're looking for - hope it's wise enough. Perhaps Alan can tidy up the loose ends.

Mark

P.S. To maintain excellent IAQ always change the filter if the MVHR system has been switched off for a prolonged period. (At the start of the heating system).

[Kate de S]

Thanks Mark, that makes sense.

[Nathan Williams]

As Mark says, I think the MHVR should operate all the time to maintain air quality - the unit in "trickle" mode should be doing just that, with boost to higher speeds to get rid of steam etc (bathroom, cooking etc). If the system is designed correctly then there should not be a problem with low humidiy - should balance out with the latent loads in a house. MHVR is only really effective (in energy terms) in a very air tight house where you are replacing the conventional window trickle vents with a MHVR system.

[Nick Grant]

Agree with Mark and Nathan, leave running in heating season, ie when windows not opened.

[Andrew Farr]

Hi, I've only skim read the other entries and I appologise if I've missed that someone else has already said this. The reason for ventilation is not just de-humidification and oxigenation but also removal of other polutants such as off gasing from modern materials.

I really don't get why peopole are obsest with turn it off. Depending on how you work it out and the design of the system the neutral point (energy recovered = energy used) is approximately 2 to 5 degrees C temp difference between inside and out. At 0 degrees difference it is still doing something useful, namely giving you reliable ventilation and clean air.  If you want to spend your life thinking about whether to open or close a window, that's fine. However, with a highly air tight house if you turn the MVHR off, please remember to turn it back on again!

[Jane & Peter Harris]

Thank you all for the input  I get the drift of the ideas  But one further point  If the unit is say 95% efficient then surely 5% of the heat is been wasted from the house so that over a number of hours the temp will go steadily down  So what about putting the unit on a timer to go on and off 2/3 time per 24hours  I accept that air-quality is the thing to keep an eye on  once again thanks for the imput

Peter & Jane Harris

[Mark Siddall]

Peter & Jane,
There is nothing that can be done about the efficency of the MVHR. This is fixed. What is not fixed (if the system is flexible enough) is the volume of air supplied by the system, and that the energy lost through it. If you reduce the volume you reduce the ventilation heat losses. Turning the system off is an extreme measure. To me it this excersise is one that places carbon/energy savings above health and well being. As  consequence this is not something that I could recomend.

One way to reduce ventilation heat losses even further would be to control the supply air volume through the use of a humidistat or CO2 detector. If set up correctly it would customise the required ventilation to your needs without compromising IAQ. This would make the system is more responsive to your demands. I understand that CO2 detectors are not very robust and drift away from there setting. Humidistats are more reliable as they are less complex - I've heard of some consist of a nylon thread attached to a micro-switch (the theory being that the thread reacts to the humidity, this then adjusts to volume flow rate of the MVHR via the micro-switch). When you are away from the house, and you are not contributing the humidity, the RH falls, as the RH falls the ventilation rate is reduced. Conversely when at home, as the RH rises, the ventilation rate increases to match that required by your activities.

Hopefully this strikes the best balance between ventilation IAQ and heat loss/carbon emissions. Any thoughts/criticisms anyone?

Mark

[Jane & Peter Harris]

Mark

That seems close but we will wait to see if there are any other thoughts  Thanks

regards
Peter & Jane

[Andy Simmonds]

We should also quantify this issue: I aim to (one day!) set up a sub meter to see what our MVHR consumes. When I turn it on/off my Electrisave power meter registers a diffrence of c. 20 - 25 watts. This is what it seems to consume when on 'normal' (ie 12hrs) and will consume less I assume when on 'min' (at night = 12 hrs).

Although I also spend time thinking about when I could/should/might turn the MVHR off (a problem I don't have with our fridge, even though the fridge is noisier) my biggest concern is electricity consumption via appliances and lighting etc, which I think is currently more worthy of my attention. Also my home office at bottom of garden, which is not a low energy space.

So currently I don't intend to turn it off, except maybe in high summer and then I will drive my wife mad by opening and adjusting windows for that perfect natural ventilation feel! Unless it gets noisy outside with people enjoying themselves late at night, in which case...I will turn it on again and shut the windows. Again, annoying my wife.

By the way, Peter and Jane - many thanks for putting your house on the database! We are designing a nice new front for the database, in conjunction with the technology strategy board. This development will also make adding several years energy performance data (utility bills, fuel used etc) much easier.

[Rob Rickey]

Wolfgang Feist said at the AGM that human health must be the first priority, followed by energy efficiency. MVHR provides predictable indoor air quality, and windows do not. The attached slide shows clearly that you do not save anything by turning off the MVHR in cold weather since you are losing 250W/person with an open window. The ventilation heat loss plus fan power is much lower than that! I assume that you don't want to compromise on health.

[David OLIVIER]

Because of the fan laws, it uses less energy to run it at half speed all the time than full speed half the time. This assumes modern DC fans/motors which should save roughly 85% at say 30% of full airflow.

A few decades ago, all electric motors were controlled by methods which were analogous to driving a car at full throtte and controlling the speed by applying the brakes. So they didn't save anything in part-load operation. However, that's in the distant past (hopefully).

So on-off operation is probably a bad idea, rather as it now is with boilers (see thread elsewhere on this forum) .

D.

[Alan Clarke]

The designers of MVHRs intend them to be run continuously. One looks at the total need for ventilation over 24hrs, based on moisture generation and other internal pollutants, which indicates a specific volume of air needing changing. Then the speeds of the MVHR ought to be set so that this total is met by a mix of say 8 hours slow speed, the rest on normal with maybe a couple of hours of boost (when bathing & cooking). However if the system is set for more people than actually occupy the house, based on area or number of bathrooms, then you could be getting too much ventilation. The solution though is to reduce the fan speeds rather than turn the system off for part of the time.

As David says, if you turn it of for a while, but need the same volume of air, then when the system is running it needs to run faster, with higher fan power and probably less efficient heat recovery (this is a bit complicated since it does rely on the system being commissioned properly with equal intake and exhaust airflow rates).

It would be nice if a set ventilation rate suited us all the time, but it isn't quite that simple - one of the main factors in indoor air quality is humidity, and for a given ventilation rate this depends on the outside air moisture content as well as internal moisture generation. Outside moisture content varies of course, in particular there is a noticeable seasonal shift from the damper autumn to dryer late winter, so you may find you want less ventilation at some times than others. This is where internal humidity control, switching ventilation to a higher speed as required to control humidity, makes sense.

Alan