Single room MVHR advice please

phil24_7

ComicGeek wrote: »

You can get decentralised MVHR units called dMVHR which will do supply and extract in a single room with heat recovery - no ducting, just a through the wall system. All the major manufacturers do them; Vent-Axia, Nuaire, Xpelair etc.

Intake and extraction is at the same point, so I wouldn't expect it to work well on deep rooms as no real throw on the supply air - designed primarily for bathrooms and utility rooms. 8.5m is fairly deep, so your idea of having this at one end and the dehumidifier at the other seems sensible to me.

Google Xpelair dMVHR - they have a good domestic ventilation system guide which shows all of the options and how they're installed.

Thanks. These are the ones I had seen but all of their marketing spiel says the same. I was wondering if anyone had any real world experience of them.

malc_b

phil24_7 wrote: »

malc_b I know all of this but thanks!

From my brief searching I can see that not ALL MVHR's are balanced.

I can't see how trickle vents are needed in most cases unless your house is air tight. I have effective extraction in my bathroom that just sucks air from the rest of the house, that is in turn sucked in through gaps in the fabric of the building.

An unbalanced MHRV unit is not recovering any heat, how can it? So anything calling itself MHRV that does not have both inlet and outlet is telling porkies. Even the dMHRV units have both ventilation and extraction.

Your room is 8.5m long, 3.3m wide. You don't say height so I'll assume 2.4m so total is 67.3m3. The formula for air change heat loss is Q = N x V x Sp.ht x dt where Q is Watts, N is changes per hour (say 1), V is m3, Sp.ht is specific heat capacity factor = 0.34 and dt is the temperature difference, say 20C inside 0C outside. For your room that is

Q = 1 x 67.3 x 0.34 x 20
Q = 458W

That's the power you need to heat the cold air coming or if you like the power you throw away letting the warm filter out. You can't have a HR if you aren't capturing that heat before it escapes. All you are doing is ventilation, positive or negative.

phil24_7 wrote: »

But this is all besides the point. I just want pointers as to which through wall, single room MVHR I should get!

Which is why I suggested BPC, they fit all sorts and can tell more than anybody who just the one type installed so can't compare that with some other make.

phil24_7

malc_b wrote: »

An unbalanced MHRV unit is not recovering any heat, how can it? So anything calling itself MHRV that does not have both inlet and outlet is telling porkies. Even the dMHRV units have both ventilation and extraction.

Unbalanced just means the in and out flows are not balanced not that one doesn't exist! If it sucks in less air than it kicks out it would such air from other parts of the house. It is still recovering most of the heat but it is unbalanced.

Your room is 8.5m long, 3.3m wide. You don't say height so I'll assume 2.4m so total is 67.3m3. The formula for air change heat loss is Q = N x V x Sp.ht x dt where Q is Watts, N is changes per hour (say 1), V is m3, Sp.ht is specific heat capacity factor = 0.34 and dt is the temperature difference, say 20C inside 0C outside. For your room that is

Q = 1 x 67.3 x 0.34 x 20
Q = 458W

That's the power you need to heat the cold air coming or if you like the power you throw away letting the warm filter out. You can't have a HR if you aren't capturing that heat before it escapes. All you are doing is ventilation, positive or negative.

Take your word for that!

Which is why I suggested BPC, they fit all sorts and can tell more than anybody who just the one type installed so can't compare that with some other make.

I have also said I wish to gain the knowledge of the wonderful folk on here before I go to a commercial company with a vested interest in seller their products.

coffeehound

malc_b wrote: »

An unbalanced MHRV unit is not recovering any heat, how can it? So anything calling itself MHRV that does not have both inlet and outlet is telling porkies. Even the dMHRV units have both ventilation and extraction.

Here's an example of a single room MHRV unit for you



HTH

OP I've worked in a building that had one of these larger MHRV units

in a large basement room perhaps 50 ft long and 20 ft wide with a high ceiling. It worked very well. The fact that the flow is sent out at a sideways angle probably forces a sort of circular flow so minimising dead spots in the room.

.

phil24_7

Thanks ch...hadn't looked at the larger units to be honest. I did bump in to the Viking House FreshR/FiWi as well as the bluMartin FreeAir 100 on my Google travels, which look interesting.

malc_b

I've seen pictures of dMHRV units before. IF that unit is MHRV unit then it has both air coming in (at side by the look of it) and extraction (on the face I would guess). The tube through the wall is probably 2 concentric tubes like a balanced flue. BUT, how effective is that at distributing the fresh air around the room? You don't want to be in a draught yet unless the air is pushed into the room with some force it is just going to trickle in and then get sucked back out again. Small MHRV are also less efficient too as rule.

Maybe it would help if you think of it as water. Take 1 l of water at 21C and 1 l of water at 1C. Put it through a heat exchanger that captures 90% of the heat to warm the new water, that is difference = 20C, 90% is 18C, so incoming water is 19C (18+1). Now consider what happens if you have a leak in the hot water side so it is 1 l in but only 0.5 l out. We still capture 90% but since we have have the water this is half the energy. Instead of an 18C gain we have half this, 9C. In effect the heat exchanger is 45% efficient not 90%. Water is warmed to 10C (9+1).

MHRV is just the same. You need balanced flows to get the performance figures. If they flows are unbalanced then the efficiency drops off.