MHRV

[EccentricAnomaly]

How do the economics of these things work out?

If its only recovering around 1kWh of energy and its on 24/7 at 20 watts drain, is only saved around 0.5kWh energy per day?

Did somebody say it was only recovering 1 kWh/day?

I read somewhere that it's about 1KW of heat that's saved/recirculated.

That matches what I calculate (loss for a small house maybe matching recovery for a slightly larger house).

1 kW of heat recovery would be 24 kWh/day.

[EccentricAnomaly]

No, high moisture levels actually make the air feel hotter.

Indeed, but do high moisture levels also make cool air feel cooler?  Consider how people can be comfortable in "but it's a dry sort of cold" snowy conditions when they'd be uncomfortable at higher temperatures where it's damp.

[pcmadman]

This is unfounded, but I recon it doesn't matter at low temperatures.

My argument for that:
http://www.engineeringtoolbox.com/water-vapor-saturation-pressure-air-d_689.html

At low temperatures there simply can't be a lot of water in the air, so it doesn't matter whether it's near saturation or very dry.

What does matter is that dry air is uncomfortable, because you get a dry mouth and such, but very moist brings the danger of condensation.

Moreover the saturation density is about 9gr/m³ at 10C, so that's in the order of magnitude of 1% of the heat capacity of air.

[martin W]

Lots of MHRV systems explicitly don't let you connect a cooker hood. Something to do with it greasing up.

Some do, but I suspect the filters will need regular attention.

Richard, its as I suspected, but I thought I'd ask... grease get evcerywhere in these systems and we use very little oil in our normal cooking.

[martin W]

PCMADMAN,

I was reffering to cold air temps say in winter. I thought that it took more heat energy to heat say 80% humity air to heat to 19°C than say 50% humity air.

Mind I am probably wrong.....

[Justme]

How will these do in a dusty environment?

We have a wood stove & soon will have a wood fired range so dust is abundant will it block the system to much?

[Hendo]

This post is particularly aimed at PCMadMan who's mentioned Central Vacuum Cleaners & HRV filters before but if anyone else has experience of these beasts, your comments would be appreciated.

In 2009 PCMadMan posted about one of the benefits of an HRV being less dust and you mentioned that you have a Central Vacuum Cleaner and good filters on the HRV. I'm about to install an HRV into my home and have a couple of questions:

1. Do you think its worthwhile upgrading on filters or are the standard ones OK?
(Most HRV's come with a G4 filter but you can upgrade to a MERV9 or 12 or F7. It seems that the filters cost the same but I suppose might need more regular replacement if they collect more dust. Also they create more resistance which reduces the number of air changes per hour or if you want the same rate of air change you can switch the HRV to a higher setting and spend a few extra pennies on electricity.)

2. I'm also considering installing a Central Vacuum Cleaner (CVC) but I'm not so sure about the benefit of this. How much of your clean undusty atmosphere do you attribute to your HRV and how much to the CVC? I noticed in your 2009 comments that your brother's room is more dusty and that it doesn't have HRV. Does his room have CVC?

[wookey]

HRVs have improved (in power consumption) since this thread started (and got cheaper).

A couple of good ones are the Ventaxia Sentinel Kinetic (9W on min): http://www.vent-axia.com/range/sentinel-kinetic.html (min 0.6 W/l/s) £1000-odd
Envirovent energivent flow (0.7 W/l/s)
S&P IDEO 325 (0.3 l/m/s), min power 21W
Nuaire MRXBOX95. 0.49 W/l/s for the big version (£650) available in july, 0.7 W/l/s for smaller model (£550)
Greenwood have a nice unit too HRV2. Also went down to 9W. £650.
Itho HRU ECO4 (0.5 W/l/s) £1100
All prices ex-VAT.

If you are interested in very high efficiency, low power and low cost, then Viking Houses's FIWI units (http://www.viking-house.ie/fine-wire-hrv.html ) are very good, and can be DIYed into your own box+control setup if you like: http://www.navitron.org.uk/forum/index.php/topic,15839.msg177729.html#msg177729

[bath_ed]

Is MHRV only suitable for very air-tight houses or would a house of more average air-tightness benefit?

[A.L.]

hello,

Is MHRV only suitable for very air-tight houses or would a house of more average air-tightness benefit?

MVHR systems generally add 0.5 to 1 air change per hour (ach) to total ventilation. 'Average' houses will have a natural air leakage  rate (infiltration) in excess of 1ach and sometimes considerably more. Minimum required ventilation for occupants health is around 0.5ach so all MVHR would do is increase fuel bills. MVHR only shows benefits with infiltration rates less than about 0.2ach.

[dhaslam]

Is MHRV only suitable for very air-tight houses or would a house of more average air-tightness benefit?

It should work in much the same way.  It is quite a common  to  overstate the difference between airtight houses and normal ones in terms of natural air changes.  Tests are done under pressure  that doesn't  represent normal ventilation.  
Many houses only have air leak ventilation  because  windows are kept closed and in that situation there is a big benefit in having  some kind of  planned ventilation.   If there is a slight positive  pressure in living rooms it would help prevent draughts but at the expense of losing some heat.  Also constant slow extraction from kitchens and bathrooms is better than  open windows that with some wind directions blow air into other rooms.

[wookey]

Yes they are only suitable for reasonably airtight houses. Where the normally quoted threshold is 3ach. Current building control sets a maximum of 10ach, which is frankly pathetic.

[SimonHobson]

If one is into BIY, then there was a thread in this very forum ...

http://www.navitron.org.uk/forum/index.php?topic=15839.0