Paul MVHR Summer bypass

[D Walter]

Hi, just wanted to check I have the Summer Bypass function right.  The attached extract from the manual shows a "Maximum" and a "Minimum" extract air temperature setting.  My reading of this is, if I want to maintain the highest level of cooling (minimum level of heat re-cycling) in order to keep the house as cool as possible in summer I would set the "Maximum" its lowest settings (20C ).  Then, whenever the extract air temperature exceeds 20C the bypass will be activated provided the outside air temperature is lower than the extract air temperature.  Have I got this right?

 

As for the "Minimum" setting, I assume this is less significant as the indoor temperature will never drop below 19.5C in summer but setting it at 19.5C would give the best opportunity for cooling.....

 

Thanks.

 

David

 

 

[TerryE]

Broadly the situation on non-heating days is that the indirect heat sources (solar gain, warm bodies, waste heat from misc electricals) generally exceeds the fabric heat losses, so the house will steadily heat above your desired setpoint.  Having your MVHR bypass its heat exchanger so the outgoing house air is swapped for unheated external air is one mechanism for dumping excess heat.  However, you don't want to do this if the external air is warmer than the internal setpoint as you would want the heat exchanger running to keep heat out -- hence the min / max setpoints on the MVHR.  You can do the sums on flow volumes + temp delta for your system to work out the kWh equivalent effects.  Or you can set them by experimentation, but there is no standard "right" values -- these will depend on the characteristics of your house.

 

My Ventaxia has a set of zero-volt contacts which can be used by a home automation system to fine tune all of this strategy, and I did initially plan to do this, but in the end we adopted my wife's strategy: we just open the odd window when needed. (We live in the country so the air quality is good, so long as the farmers aren't muck spreading. 🤣)  However, we do exploit the fact that our 3 storey house is a sun trap on the principle SE aspect and the rear NW stays cool until mid-afternoon, so we crack a rear window in the morning and (if needed) a front one in late afternoon; this plus the roof light at the top of the hallway gives a very effective heat pipe to dump excess heat. I know that some members would regard opening windows in a house with MVHR as heresy, but as we don't have special needs (e.g. asthma suffers), we don't see why not when we want to dump heat.

 

 

[ProDave]

I experimented with "summer bypass" when I first installed mine (different make to the one being discussed) and I found the window when you could actually usefully bypass the heat exchanger was very limited.  As pointed out, when it is warmer outside than in, you want the heat exchanger back in circuit.

 

I found the simplest way to cool an overheated house was keep it shut in the day to keep the warm air out, and keep the heat exchanger in circuit to stop drawing in warm air.  then in the evening when the temperature drops open up all the windows for a night purge of cool air to cool the house down for the next day.  Once cooled down a well insulated house will stay cool for the next day when closed up again to keep the heat out.

 

Then you only need to consider active cooling if the night time temperature does not drop below what you consider a comfortable temperature. 

[jack]

On 07/05/2022 at 10:07, D Walter said:

Hi, just wanted to check I have the Summer Bypass function right. 

 

Yes, you do.

It certainly doesn't hurt, but I can't say I've found ours to be hugely effective. The main issue is that unless you run the MVHR on boost overnight (which is when it will generally be triggered), you don't actually exchange all that much of the cooler outside air for the warm air inside. You also use a lot more power on boost compared to the standard setting.

 

As long as you don't have issues with noise or insects (or you have insect screens), opening windows is much more effective. If I were doing this again, I'd include concealed roll-down insect screens on all bedroom windows, and on at least a couple of downstairs windows that can be locked open a crack at the rear of the house, to take advantage of stack ventilation without being eaten alive by insects.

[AliG]

I agree, summer bypass works like this on my system but makes little difference.

 

Opening windows works much better.

[saveasteading]

2 hours ago, ProDave said:

I found the simplest way to cool an overheated house was keep it shut in the day to keep the warm air out, and keep the heat exchanger in circuit to stop drawing in warm air.  then in the evening when the temperature drops open up all the windows for a night purge of cool air to cool the house down for the next day.  Once cooled down a well insulated house will stay cool for the next day when closed up again to keep the heat out.

Immaculate logic. The Mediterranean way, applicable for only a few days per year here.

Closing blinds and curtains (shutters better) towards the sun to be added to this list.

If thermal mass was a real thing, you could imagine them giving off their heat during this process, ready to absorb more heat the next day.

[D Walter]

I agree with all the above.  We have installed external roller blinds on the more W facing windows to cut out the solar gain from the low sun in the summer.  In addition, the blinds act as mosquito screens so we can safely open the windows at night to allow for cooling.  Unfortunately, I gave in to time/cost pressure and engineering complexity and did not insist on an opening window at the top of the atrium to vent warm air at night.  That being said, the external blinds make the temperature manageable.