Mechanical Heat Recovery and Ventilation systems (MVHRs)

[Mysticbreeze]

Hi. Has anyone here got an MVHR in their house? My question is a little strange but hope someone here can help
We are building a new house which will be super-insulated and air-tight. This means that unless we open the windows, we need to mechanically ventilate the house. We also intend to recover the heat from the stale air being sucked out to warm the fresh air being drawn in.
I recently spent a couple of hours in a house with MVHR and found afterwards that my eyes (I wear contact lenses) were dry and very uncomfortable. Is this an effect of the MVHR? Anyone got any experience? It may have been that I was sitting under vent in the ceiling and the air flow was blowing down on me. Anyone got any experience? Don't want to invest in this system if it makes the atmosphere in the house uncomfortable for me.

[Svalberg]

The air supplied should be warmed fresh air, so the relative humidity will be less than that of the outside air (warm air holds more moisture than cold air) so as the system isn't dehumidifying, there will be the same absolute moisture content as the outside air but at a lower relative humidity (see en.wikipedia.org/wiki/Relative_humidity and en.wikipedia.org/wiki/Psychrometrics ).

As to its effects on your contact lenses, it will depend upon what the outside conditions were when you were in the house with a MVHR system - sorry I'm not of more use!

[shovetheholly]

I don't have anything to add technically - Svalberg's post is amazing, but if you are building to Passivhaus standards, you don't have many other options do you?

I find that my eyes hurt in hotel rooms because of the air con. However, it's quite a temporary thing and they sort of adjust after a couple of days. A bowl of water in the room helps!

[Mysticbreeze]

Thank you Svalberg. That's useful information

[johnd2]

Yes with a normal "counter current" heat exchanger you'll find the air coming in is quite dry in winter, and especially with the movement of air it can dry your eyes. A lot of people notice this in offices too.
You can use a wheel heat exchanger which is less common but it also moves some of the humidity across as well. I've not seen them for sale in many places though.
I think you'll get used to it soon, but make sure it's designed by an experienced company to make sure the drafts are minimised.
Part of the point is to remove humidity and circulate fresh air, so it's a balance really. You should get some level of adjustment though, we are planning to fit two units in two zones for upstairs and downstairs, to give better control over ventilation rates.
Good luck with your house though, sounds like you'll never be cold again or shocked by a bill!

[johnd2]

I forgot to mention that there's nothing to stop you running a humidifier, although that would use energy it wouldn't damage anything and the centre will still bring in plenty of fresh air.

[Mysticbreeze]

Thanks johnD2. My DH and I have had several 'lively' discussions about this. He swears the air is only warmed, not dried. We are going with an experienced company and we are going to use the recovered warmth to heat our domestic hot water so I am keen to incorporate the system into the house. The humidifier sounds like it will be the solution if I get dry eyes - thanks!

[PigletJohn]

If you bring in cool air from outside, and warm it, its humidity will drop.

This is because humidity is measured as RH (relative humidity) and is the amount of water vapour that it holds, as a percentage of the amount that it could hold at that temperature. Warm air can hold more water vapour than cold. Hence condensation, when warm air is cooled and dumps the water it can no longer hold.

A cubic metre of hot, dry air in the Sahara hold more water that a meter of cold, damp air in Manchester.

[johnd2]

I'd be surprised if you'll be using the recovered warmth to heat your water - the air leaving the house will be about 20c and you'll want the water at well over 40c if you're storing it. And it would be ideal to warm the incoming air.
Are you sure you don't have a separate air source heat pump to warm the water?

[johnd2]

Ps if you don't have mvhr you'll need either constant extraction or supply, and/or trickle vents. So you'll be bringing in the outside air somehow and heating it which will reduce its humidity whichever way you do it.
I think the draughts are your main worry and you should discuss with the company designing it to see what they say.

[nueday8]

We have had it in a new house for past 8 years, doesn't make my eyes dry, I love the fact the house doesn't feel stuffy, I do intermittently switch it off in summer but then open the windows. Really don't think you will heat much hot water using that, what is evective is solar panels, heats our water tank up really well. I would recommend that to anyone as know it works so well.

[nueday8]

As to draughts you can adjust what comes out of the vents in all rooms, I have not experienced a draught from ours.

[nueday8]

If you have wood burner stove the fire air intake needs to be ducted outside as it will cause the fire to reverse pull if you got heat recovery. Also if you putting a wood burner in put back boiler in it for hot water.

[PigletJohn]

the cost of hot water is trivial. If you have gas, your usage will probably be in the region of 15p-30p per day in summer (plus standing charge).

Have a look at your bills and see.

If you could halve that, how much would you pay for the equipment to do it?

[shanghaismog]

We have an mvhr and the ecocent hot water. Love both and with a 4kwh set of solar panels, love the lack of bills 😀.

We don't notice dry eyes but I don't wear lenses anymore. I would imagine if you're sitting under a vent that it might be draughty, but the company should place the vents away from where you'll be sitting. Love the lack of dog smell in the house too.

The hot water works a treat, we simply never touch it and have never run out of hot water. BUT coming up to the first service we found out that it will be approx £400!

Are you putting any kind of heating in at all? We are built to passive standard but for the bifold doors and the wood burner. Wood burner is doable but needs calculation and correct placement of vents.

It all works fine but mega bucks to install and service but the FIT payment from the solar panels pretty much pays for our electricity used (we tumble dry everything and have induction range, as well as the hot water) over the year.

[Mysticbreeze]

Shanghaismog. Thank you for your post. We are also looking at the ESP Ecocent for hot water and Air-source Heat Pump for the underfloor heating - and a woodburner. We are not building to Passiv house standards but the house will be well insulated & airtight. The FIT for solar panels is now not worth the effort :-( although the RHI for the ASP will pay back installation costs (eventually)
£400 for an annual service sounds steep - they didn't mention that. Thanks for the warning.
On a culinary note, are you pleased with your Induction range?

[Mysticbreeze]

PigletJohn - we are off mains gas, otherwise it would be a no-brainer
JohnD2: Ecocent units are combined DHW cylinders with fully integrated heat pumps. They all generate hot water by using heat ducted to them from wherever there is excess heat in the house. The ASP will drive the UFH and the Ecocent will make use of this to heat the water. More technical info here: esavep.com/
Thanks to everyone for the comments. From what I've read, I think I will be fine with my contacts
I know the system will work well, it's just the potential for causing 'dry' eyes that I was concerned about but I think that is a draught issue rather than humidity of the air

[shanghaismog]

Love my induction range 😍

It's a shiny Stoves one that actually gets terrible reviews but touch wood all good so far.