Mvhr newbie

[Painy]

I'm a general builder that had to look up what mvhr was ?.

im going to build a 3 bed detached house (3rd bedroom in the loft ) and was just going to have underfloor heating downstairs and rads upstairs , I contacted the plumbing merchants about getting a price for air source heating rather than a boiler and the manager said to me have you thought about mvhr !!!! 

I have no idea about it so would like advice from you guys regarding cost, effiency etc as I have no idea about it or am I better with an air source heat pump ??

thanks in advance 

[Crofter]

Hi and welcome

MVHR is a ventilation system, and does not replace a heating system.

There are essentially two ways to ventilate a house: the traditional 'accidental' way, using trickle vents and various random leaks around windows and within the fabric of the build, and the 'controlled' way, where you seal up the house as much as possible and then use a fan and ducting to blow fresh air into the house, and to extract stale air.

It's fairly obvious that accidental ventilation leads to a lot of random variation. Some corners of the house are too drafty, others are stuffy. The amount of ventilation will depend entirely on wind speed and direction.

If you choose to control the ventilation, you can have a constant supply of fresh air directed exactly where it's needed. And as a bonus, if you control ventilation using a MVHR system, there is a heat exchanger which recovers the warmth from the stale, moist air that you are getting rid of. A good system will recover 90% of this heat. Compare that to simply opening a trickle vent where you recover 0% of the heat. There is a running cost for the fan, but it is likely to be less than £50 a year.

 

In a new build, it can actually be easier to aim for an airtight house and then use MVHR, rather than try to build a leaky but not too leaky house that gets by without. If it's too leaky, it fails building control altogether.

 

A MVHR system should cost around 2% of your build budget, give or take. Yes it is an expense, but the rewards are better indoor air quality and lower heat loss. It also helps your SAP score, which means you can pass building regs more easily and might not need such expensive windows or insulation.

[Jeremy Harris]

As above, MVHR always cools a house when the outside temperature is lower than the desired temperature, but it cools it a LOT less than if normal ventilation is used.  As a general rule, in addition to the much better overall ventilation and improved air quality that MVHR gives, it will recover at least 80% of the heat that would otherwise be lost from conventional ventilation, like trickle vents, leaving windows open a bit, extractor fans, etc.  That's a big saving.  In our new build around 70% of the total heat loss in winter would come from ventilation heat loss, if it were not for the MVHR.

[MikeSharp01]

The essential point probably is that installing an MVHR system with enough insulation, great air tightness in the fabric, windows & doors alongside a UFH system driven through a reversible Air Source Heat Pump (ASHP) - so you can cool the slab as well as heat it, you probably won't need those rads upstairs just a couple of towel rails in the bath rooms and your energy bills will be tiny.

[Painy]

Thank you so much for the in depth advice, is there warm air heating systems that are efficient ?? Or am I still better off using underfloor heating ??

[Crofter]

Water carries much more energy than air can. This is why central heating uses pipes filled with water, not air.

To heat a house using air, it really works best if the house has low heating requirements. However the big advantage is the easier and cheaper installation of a warm air system, compared to a full wet central heating system.

 

In other countries, e.g. the US, warm air heating is very common. It's still quite unusual over here, probably because ot's only relatively recently that houses have become efficient enough to have sufficiently low heating requirements in our climate.

[Jeremy Harris]

As an example of the capacity of water to move more heat energy than air, as @Crofter says, here are some numbers that illustrate this (with a bit of playing fast and loose with the terminology - but the comparison is still valid):

 

100 litres of water that is 20 deg C warmer than the room needs will be able to shift 2,300 W of heat into it.

 

100 litres of air that is 20 deg C warmer than the room needs will be able to shift 0.67 W of heat into it.

 

To move the same amount of heat with air, rather than water, for a given temperature difference, means moving a volume that is about 3,400 times larger.  This also helps explain why an MVHR system can't shift very much heat around.