Sizing of MHRV Units

creedp

I'm looking for some advice on the sizing of a MHRV Unit fir a 2,800 sq ft (260 sqm) house. I've got conflicting views from 2 suppliers - one says its large unit which has a max ventilation capacity of 400m3/hr is more than sufficient for this size while the second says that this is too small and would result in the unit running on med/high setting as standard thereby significantly reducing its efficiency. The second guy is recommending his largest unit which has a max ventilation capacity of 500m3/hr and saysthat this extra capacity will enable the unit to run on a low setting as std.

I appreciate its difficult to be definitive on something like this without having all the facts but I would appreciate if anyone could comment on the above views concerning appropriate size of these units.

Thanks

tred

creedp wrote: »

I'm looking for some advice on the sizing of a MHRV Unit fir a 2,800 sq ft (260 sqm) house. I've got conflicting views from 2 suppliers - one says its large unit which has a max ventilation capacity of 400m3/hr is more than sufficient for this size while the second says that this is too small and would result in the unit running on med/high setting as standard thereby significantly reducing its efficiency. The second guy is recommending his largest unit which has a max ventilation capacity of 500m3/hr and saysthat this extra capacity will enable the unit to run on a low setting as std.

I appreciate its difficult to be definitive on something like this without having all the facts but I would appreciate if anyone could comment on the above views concerning appropriate size of these units.

Thanks

I heard the same from a few suppliers. Advice I was given have more power then under powered for cost in the long run. Is there a huge difference in price?

cuculainn

creedp wrote: »

I'm looking for some advice on the sizing of a MHRV Unit fir a 2,800 sq ft (260 sqm) house. I've got conflicting views from 2 suppliers - one says its large unit which has a max ventilation capacity of 400m3/hr is more than sufficient for this size while the second says that this is too small and would result in the unit running on med/high setting as standard thereby significantly reducing its efficiency. The second guy is recommending his largest unit which has a max ventilation capacity of 500m3/hr and saysthat this extra capacity will enable the unit to run on a low setting as std.

I appreciate its difficult to be definitive on something like this without having all the facts but I would appreciate if anyone could comment on the above views concerning appropriate size of these units.

Thanks

Currently in the same situation myself.......as previous poster stated though, better to have it oversized than under sized.......

Btw could you PM me what two models you are looking at?

Thanks

creedp

Actually the bigger unit is a little bit cheaper but it is less sophisticated, e.g. it doesn't have an summer bypass option. I am more familiar with people who have installed this unit but I'm a little concerned about the lack of a summer bypass. Anyone any views on whether the lack of a summer bypass option would be a dealbreaker?

cuculainn

I know of one person who installed MHRV and didn't include the hte summer bypass. it was going to cost about 100 quid more. They regret not getting it and advised me to get it.

maybe last summer was abit hotter and it was needed, but i cant imagine it making a huge difference. I'm gonna get it anyway.

CJhaughey

I would definately get the Bypass, we use it every year and not just the hot ones like this one.
It would be unbearable if the HRV was pumping warm air around when you are trying to sleep.
Do the units you are looking at have easily replaceable/cleanable filters?
I change mine every 4-5 months and they are full of stuff. My HRV is on the ground floor and you can just open the door and switch it off and remove the filters.

creedp

@ Cuculainn

Yes the filters can be easily removed and cleaned with a vacumn cleaner 3 - 4 times a year and replaced annually. Mine will be in the attic but I will have a spacious attic with a proper stairs access so won't be difficult to access the Unit.

uptowngirly

Hi Creedp

I'm looking into the price of MHRV systems at the moment as well - any chance you could PM me the name of the two you are looking at - my house is 255 sqm and about to apply for PP but want to get a proper idea of costs first as there's no point in us getting PP for a house we can't afford to build!

By the way - did you look at DCV at all - I've seen some chat about it bandied about and I know it's cheaper than MHRV but don't know if it's a good plan or not....

creedp

By the way - did you look at DCV at all - I've seen some chat about it bandied about and I know it's cheaper than MHRV but don't know if it's a good plan or not....[/QUOTE]


Hi I looked at DCV having seen articles about it in Construct Ireland. I spoke to a few people and no one seemed to know anything about. I got a provisional quote and although cheaper than the MHRV I came to the conclusion that although it did provide a source of controlled ventilation I was prepared to pay more for the heat recovery aspect of MHRV. Of course the advantage of the latter is of limited value if build quality isnt up to scratch.

uptowngirly

I think you're right - the heat recovery part of the ventilation is a definite big plus in my book!

You must be reading the same articles as me

Now, once the planning permission comes through, all I'm going to have to do is stand over the builders to make sure that their idea of airtightness and mine (thank you clever people in Boards!) are the same so as the MHRV system will work the way it should!

cuculainn

uptowngirly wrote: »

I think you're right - the heat recovery part of the ventilation is a definite big plus in my book!

You must be reading the same articles as me

Now, once the planning permission comes through, all I'm going to have to do is stand over the builders to make sure that their idea of airtightness and mine (thank you clever people in Boards!) are the same so as the MHRV system will work the way it should!

I would advise getting an airtightness spec put on the plans...say less than 3 (whatever the units are)

Then if you are getting a contractor make it clear that this spec has to be met and get and airtightness test done after it airtightness has been completed but before any ceiling slabbing etc is complete.

If you are doing direct labour it will prob be down to you to keep an eye that everything is done right

L driver

Hello,
Did any of youse look up FIWI hrv? Looks very impressive from both performance and dollars point of view

uptowngirly

L driver wrote: »

Hello,
Did any of youse look up FIWI hrv? Looks very impressive from both performance and dollars point of view

No - but am currently reading about it now that you mentioned it! I'd be interested in hearing from anyone who has it re any hum that you can hear in bedrooms at night.

The dollar point of view is also key - mmmm - more things to think about!

runawaybishop

You need to exchange a 1/3rd of the air in your house per hour. Work out the volume of your house by multiplying the square footage* ceiling height *.85 (this is to account for internal walls, furniture etc) - you need a unit that can shift this amount on its lowest settings comfortably - you definetly do not want the unit running on medium or high settings constantly - its efficieny drops a lot and it is a lot louder.

You also need to account for the resistance in the ducting, but that depends on what you use. Rigid ducting has better airflow, but carries sound more. Flexible ducting has better sound reduction but more resistance. The sound from a unit running on minimum shouldn't be an issue though, just make sure you site it well away from your bedrooms if at all possible.

The summer bypass isn't really that important, but it doesn't cost a hell of a lot to get it usually. Temps typically will not rise that much in the whole of your house that it needs to kick in (some individual south facing rooms might exceed it but that soon gets balanced out).

I installed our system myself with the help of an electrician mate, we picked up the unit from a company in the UK that stocked a model from an italian company - it was about half the price that the irish companies were charging with more features (humidity detector, summer bypass, co2 detector, automatic modes + remote etc). I found that a lot of the irish companies were just chancers, they were pricing me for expensive units that couldn't actually shift the air I required. I assume a lot of them hopped on the bandwagon.

The only item I didn't get with my system, that i will shortly be picking up, is a heater for when its extra cold. To stop the unit frosting over it periodically bypasses the heat exchange (this occurs only when it reaches a certain minimum temp) for a few seconds and blows the warm air around the unit to keep the temps up. This involves running at full speed and activating the bypass which is noisy. At night this wakes me up and is a bloody nuisance. I'm going to pick up a heater that sits in the pipe and will warm the unit itself for about 60 quid (reduced from about 100 quid during the "boom") - hopefully this will ensure the unit is silent all of the time.