MHRV - does it mean that whole house has to be at 20deg

fclauson

I am in the final stages of planning and am in two minds on MHRV systems

Does it mean the whole house has to be at the same or close to the same temp.

The air leaving the vents will be all the same temperature, I find that sometimes the heating will occasionally not come on in one of the rooms as the vented air is sufficiently warm enough to trip the room thermostat off.

Not a problem, rather a "feature". It could be a problem if you want to have some rooms at a different (several deg C) temperature.

fclauson

Dolan

thanks for your reply - but if one does want different temps across the house the MHRV does not make sense. e.g. we sleep with our windows open every night - the bedroom is "cool" in this weather - but out kitchen is warm - in a MHRV house this would not really work would it ?

fclauson

fclauson wrote: »

Dolan

thanks for your reply - but if one does want different temps across the house the MHRV does not make sense. e.g. we sleep with our windows open every night - the bedroom is "cool" in this weather - but out kitchen is warm - in a MHRV house this would not really work would it ?

Having open windows defeats the whole reason for having a MHRV system, you can get a partial system for just the kitchen if you want.

beyondpassive

@fclauson

For houses over 250m2 it can sometimes make sense to have 2 seperate ventilation circuits corresponding with heating zones, i.e Living zone and bedroom block. The set point of the bedrooms can be 18 degrees and for living rooms 21 degrees.

Where you use a single system, the air supply recovering 20 degrees would be 18 degrees in winter, add to this the cooling effect of blown air and it would feel like 17 degrees.

In ultra low energy houses the MHRV wouldn't perfectly equalise the heat from the warmer living spaces to cooler bedrooms. However, there is a scenario where bedrooms can become hotter than living rooms. In two story passive houses, the high degree of comfort throughout the house means that doors are mostly open, combined with double height spaces, this results in warm air rising and temperatures stratifying. This can result in higher temperatures upstairs. While ventilation from opening windows can reduce this, a better solution might be a higher forced air ventilation rate of say 0.4 or 0.5 air changes.

Its not such a big issue as the house will maintain about 16-17 degrees in winter without any heating system. Once you set a temperature for the bedrooms, the heat will not be demanded until the ambient temp drops below say 18.

The precise solution depends on the house in question, solar gain being the major factor to consider. Being familiar with your plans, I don't think you should be overly concerned that bedroom overheating is going to be particularly tricky for your building form.

HurlyBurly

Ha Id love if mine would keep the air that warm. I think it's missing the HR bit. Does a great job of keeping fresh air moving around the house but not remotely close to the temperature I set, usually 21. Don't see how it could anyway, the bathrooms don't get that much use that they'd heat cold air.

HurlyBurly

HurlyBurly wrote: »

Ha Id love if mine would keep the air that warm. I think it's missing the HR bit. Does a great job of keeping fresh air moving around the house but not remotely close to the temperature I set, usually 21. Don't see how it could anyway, the bathrooms don't get that much use that they'd heat cold air.

It depends on whether the pipes are in a cold attic and properly insulated, also the exhaust vents need to have warm air going into them as well.

A small heater in the ductwork would bring the temperature up to the desired level, you could fit a small radiator off the CH to do this.