Green Building Forum - MHRV

MHRV

jonny-b- — Fri, 29 Jun 2012 09:27:41 +0100

Hi everyone,
Just wondering if anyone can help us make a decision on a MHRV system for our self build.
At the minute we're caught between a nuaire MRXBOX95-LH2 and the Vent Axia Kinetic Plus unit.

Any info/practical experience of the above units would be great.

MHRV

Pugliese — Fri, 29 Jun 2012 11:17:05 +0100

Faced with a similar decision although I have yet to do a thorough analysis yet. I think the Kinetic has a built in humidstat whereas the Nuaire has to have room sensors - I could be wrong though so do check. If correct then the kinetic wins for me, as with 3 bathrooms we would have sensor overload!

MHRV

tony — Fri, 29 Jun 2012 11:53:24 +0100

Do these move enough air for you?

MHRV

RobL — Fri, 29 Jun 2012 12:12:58 +0100

I have the kinetic+, and I think it's great!

Love:
humidistat built in
low power
trusted make - be around a while for spares (none needed for me so far!)
automatic summer bypass & protects if <0C
Loads of programmability - based on humidity, analogue+digital inputs, time

Hate:
No CO2 sensor in it
humidistat+other inputs locked out when <0C (for safety I guess)

Practical:
Make sure it fits thru loft hatch/wherever. It is BIG
Use BIG pipes for main runs, 200mm?
Use 150mm vents for rooms (to be quiet)
They do 200mm spigots as an option

MHRV

jms452 — Fri, 29 Jun 2012 13:30:10 +0100

and I have the litle brother of the MRXBOX (same efficiency just smaller)

Cheap
Simple
high efficiency and low(ish) SFP

Sensors can be v cheap switches (few Â£) or nuaire's low voltage bespoke ones (about Â£70 - but you legally fit in bathroom etc.)

I think these are both good units but it depends if you want the bells and whistles/interfacing options.

MHRV

nbwilding — Sat, 30 Jun 2012 15:06:12 +0100

Also have the Vent-Axia Sentinel Kinetic Plus since January and been very pleased with it. Sorted our condensation problem after insulation/air tightness retrofit and the air quality in the house is much improved.

Ours runs at 20% normal and 50% boost which works fine for our house (180m^2) . On normal it consumes only a few tens of Watts. One of the secrets of keeping noise down is to get a unit that is oversized for the house so that the fan runs slowly. Agree with Rob about making sure you use generously sized ducting so that the air can move more slowly, thus keeping the noise down. We have 200mm trunks and 150 mm out to the rooms. There is nothing audible at the fresh air inlets in the rooms. Can hear it near the stale air outlet in the kitchen and bath room, but don't care about that.

Don't miss a CO2 meter there are only 2 adults and 2 kids of us and we don't have an open fire or anything that might generate a lot of CO2.

As Rob says, it locks you out of using the boost below 0 degrees and below -5 it turns down the intake of external air to keep frost off the heat exchanger. I seem to recall that ours shut down completely at -10 C or so, but that doesn't happen very often where I am based in the SW. Some of the continental models (eg Paul) have electrical heaters to protect from frost, but that probably isn't necessary in the UK climate.

It has foldable filters which can help with cleaning them if the unit is in a tight space.I just give them a gentle vacuum and reinsert.

Nigel

MHRV

PaulJ — Sat, 30 Jun 2012 18:22:03 +0100

Just fitted a Vent-Axia Sentinel Kinetic Plus in a V. small insulated loft in a new build. Very tight fit.

I notice that annual user maintenance includes unscrewing and removing the front of the unit and removing, cleaning and reinstalling the heat exchanger. Hardly realistic for an average house owner and the one I fitted will need to be dropped down from the loft to get the required access.

MHRV

nbwilding — Sat, 30 Jun 2012 19:06:27 +0100

That's a good point, though I assume other models advise you to clean the heat exchanger at intervals as well. Anyone any experience here?

On a different note, a friend of mine who lives 5 minutes away recently measured the Radon levels in his house and found levels above the recommended safe limit, particularly in the basement. It's quite common here in the SW for Radon gas to seep out of the ground. It hasn't occurred to me that MVHR is a good way of dealing with this problem, but it's another reason I'm pleased I got one.

MHRV

borpin — Mon, 02 Jul 2012 08:24:46 +0100

Was looking at the Vent-Axia but decided that it was too close in terms of performance for my build. Looking at a bigger model so it will run at max 50% capacity.

Key question for @jonny-b is what volume is the property?

On top of things mentioned above, is what filters are fitted. Most units have just G3 while some have F7 as well to reduce particulates further. This then has a knock on for the SFP (finer filter more power to draw air through).

MHRV

jonny-b- — Mon, 02 Jul 2012 08:55:32 +0100

Thanks for all the information so far.
Borpin the floor area of the house is 2380sqft

MHRV

borpin — Mon, 02 Jul 2012 09:56:10 +0100

Posted By: jonny-b-the floor area of the house is 2380sqft

What is the warm volume? I will have a warm roof construction plus vaulted ceilings in places so the volume of your house is the key parameter (mÂ³).

MHRV

jonny-b- — Mon, 02 Jul 2012 10:30:42 +0100

Sorry borpin I misunderstood.
Not too sure how to do the calculations really. I had sent my plans off to a few companies and they came back with the recommended units.

MHRV

Viking House — Mon, 02 Jul 2012 10:42:32 +0100

Posted By: jonny-b-Not too sure how to do the calculations really.

Hi Jonny, Your house is 220m2 floor area x 2.5m ceiling height = 550m3 volume.
550m3 x 0.3 airchanges/hr is 165m3/hr.

Also calculate the occupants and choose the largest figure, so 25m3 per person x 4 persons = 100m3.

So you would choose a unit that can handle 165m3/hr.

What efficiency are they quoting for these units and what price are they?

MHRV

jonny-b- — Mon, 02 Jul 2012 11:04:17 +0100

thanks for that viking house, the ceilings are 2.7m in the house, so following the calculations you done: it would be 596.7m3 volume, x 0.3 = 179m3/hr.
The efficiencies range from 87 to 92% according to the info online, and prices range from â‚¬3000 to â‚¬4500

MHRV

Viking House — Mon, 02 Jul 2012 13:05:55 +0100

The efficiency gained in winter by ducted HRV units is eroded by having to run them all the time, most ducted HRV manufacturers recommended to leave their units running 24/7/365, if theyâ€™re switched off in summer condensation in the ducts can cause fungus and mould growth, so they recommend cleaning out ducts prior to restarting in winter and at least every 2 years otherwise.

In 2010 18,000 HRV units were sold in the UK, in 2011 (a year later) only 2,000 filters were sold!

Counter current heat exchangers, duct pipes, filters and elbows all cause pressure drop in ducted HRV units, fan power consumption increases to overcome the pressure drop. Some HRV companies fit pollen filters but the units never have sufficient fan power to counteract summer overheating, so the windows still need to be opened to cool the building down!

These are just 3 of the reasons we moved away from Ducted HRV systems but my opinion is biased.

MHRV

djh — Mon, 02 Jul 2012 14:13:01 +0100

Posted By: Viking HouseHi Jonny, Your house is 220m2 floor area x 2.5m ceiling height = 550m3 volume.
550m3 x 0.3 airchanges/hr is 165m3/hr.

Also calculate the occupants and choose the largest figure, so 25m3 per person x 4 persons = 100m3.

So you would choose a unit that can handle 165m3/hr.

Building regs requires capability for 0.44 ACH, I think?

MHRV

SteamyTea — Mon, 02 Jul 2012 19:21:46 +0100

Posted By: Viking House8,000 HRV units were sold in the UK

That's encouraging

Posted By: Viking House2,000 filters were sold

That isn't

Posted By: Viking HouseThese are just 3 of the reasons we moved away from Ducted HRV systems but my opinion is biased

It is, but valid reasons all the same

I really must get up in the loft and retrieve the temperature loggers from mine. I do know that it used 4.25 kWh in 3 weeks of constant running.
I also know that one of the pipes fell off again, and am hoping the data will show me when.

MHRV

PaulJ — Mon, 02 Jul 2012 20:44:21 +0100

I agree that running MVHR all year seems a bit silly, as the HR is probably good for 5 months in England (SE). Wouldn't it be better to trickle vent intake with MVHR boost extract in the summer and use full MVHR the rest of the time? Lots of people turn heating and a/c on / off in summer winter, so I don't see it being too tricky for the average user. Maybe a few additional controls could make this simple.

MHRV

djh — Mon, 02 Jul 2012 21:48:22 +0100

Posted By: PaulJI agree that running MVHR all year seems a bit silly, as the HR is probably good for 5 months in England (SE). Wouldn't it be better to trickle vent intake with MVHR boost extract in the summer and use full MVHR the rest of the time?

The latest GBM has an article about the pros & cons of turning MVHR off and on.

There are no trickle vents in high performance houses - they're too much of a thermal bridge and too difficult to make airtight. Nothing to stop you opening the windows though.

MHRV

Viking House — Tue, 03 Jul 2012 17:33:27 +0100

Posted By: djh The latest GBM has an article about the pros & cons of turning MVHR off and on.

What are the main points Dave? I don't get the magazine.

MHRV

chriskemp — Tue, 03 Jul 2012 18:10:01 +0100

MVHR needs to run 24/7 to comply with building regulations - especially in buildings <3m3 airtightness.
you still generate moisture in the summer (showers/cooking) which requires extraction (especially with internal wetrooms).
MVHR can provide a boost (increased movement of air) which can be of benefit on still nights - or where you maybe reluctant to have windows open.
MVHR provides filltrered air which has been proven to help with asthma, plus by keeping moisture levels down (RH) this also helps reduce house dust mite levels and of course prevents mould growth.

dont forget filters can be washable. ;-)

dont forget strictly speaking (and its usually in the user guide) extractor fans should be cleaned annually too - but who does? Thats why some manufacturers have filters and some have developed "filterless" extract fans.

MHRV

jms452 — Tue, 03 Jul 2012 20:49:04 +0100

MHRV

jms452 — Tue, 03 Jul 2012 20:51:26 +0100

Posted By: Viking House
In 2010 18,000 HRV units were sold in the UK, in 2011 (a year later) only 2,000 filters were sold!

Our MVHR has a 5 year filter life with a suggested yearly vacuum clean, and then there are the washable filters and the fact that MVHR sales are probably ramping up fairly steeply...

Without filters efficiency will rapidly degrade - this must be true of the fiwi too?

And even if people are stretching out the filter life it shouldn't effect heat exchanger efficiency (negatively) - it just means they are then working slightly below building regs air flows which some people here seem to advocate anyway...

MHRV

JSHarris — Tue, 03 Jul 2012 21:29:57 +0100

Posted By: jms452
Without filters efficiency will rapidly degrade - this must be true of the fiwi too?

And even if people are stretching out the filter life it shouldn't effect heat exchanger efficiency (negatively) - it just means they are then working slightly below building regs air flows which some people here seem to advocate anyway...

When filters start to clog the fan efficiency will drop, quite dramatically. Fan efficiency is strongly dependent on total pressure ratio, so anything that restricts flow (and hence increases the fan pressure ratio) will hit power consumption.

A partially blocked filter will cause a flow restriction, which will increase the amount of power that the fan consumes for a given ACH.

MHRV

chriskemp — Tue, 03 Jul 2012 21:34:38 +0100

Agreed.
Some systems will ramp up the fan rate to over come resistance due to clogged filters to ensure "adequate means of ventilation"

MHRV

Viking House — Tue, 03 Jul 2012 23:16:16 +0100

Posted By: jms452 Our MVHR has a 5 year filter life with a suggested yearly vacuum clean, and then there are the washable filters and the fact that MVHR sales are probably ramping up fairly steeply...

Point taken and accepted!

Posted By: jms452Without filters efficiency will rapidly degrade - this must be true of the fiwi too?

Why do you think efficiency degrades without filters? I'd say efficiency degrades with filters!

Posted By: jms452And even if people are stretching out the filter life it shouldn't effect heat exchanger efficiency (negatively) - it just means they are then working slightly below building regs air flows which some people here seem to advocate anyway...

It affects the Watt/litre/sec figure.

MHRV

jms452 — Wed, 04 Jul 2012 07:40:49 +0100

Well there are two efficiencies here

1. Efficiency of moving the air -i.e. SFP
2. Efficiency of heat exchanger

Posted By: Viking House
Why do you think efficiency degrades without filters? I'd say efficiency degrades with filters!

Because the heat exchangers get covered with (thin) layers of greese and dust reducing the heat exchanger efficiency.

It affects the Watt/litre/sec figure.

Agreed - the efficiency of moving the air is reduced.

MHRV

joe90 — Wed, 04 Jul 2012 08:01:33 +0100

If you have your MHRV controlled by humistats and CO2 detectors wont the unit switch itself off when not required, i.e. windows and doors open in summer?

MHRV

DamonHD — Wed, 04 Jul 2012 08:50:44 +0100

My little MHRV has trickle or boost: no off unless I turn it of manually at the mains.

So that is what we have done for summer, and opened windows...

Rgds

Damon

MHRV

djh — Fri, 06 Jul 2012 13:55:47 +0100

Posted By: chriskempMVHR needs to run 24/7 to comply with building regulations - especially in buildings <3m3 airtightness.

Where in the building regs does it say that? I don't believe I've ever seen it.