mvhr MVHR Efficiency

[sonicboom]

I am planning on upgrading my 10 year old PIV system to an MVHR. What kinds of efficiencies should I expect at current temperatures (-2.5c). My PIV churns out a draughty 7c and that is with the 400w heater enabled. i would ideally like something above 19c (internal temp is 22c 24/7). I  have my eyes on the Zehnder Q350 which goes up to 96% but they do not produce charts on how that performance degrades as the temperature drops.

[MikeGrahamT21]

Where will you be running your ducting? Mine is in cold loft, but heavily insulated so it loses a bit more temp on its way to the terminals.

 

When it was like -3C the other week, internal temp was showing as 21.2C (extract) and the supply temp was at 19.4C, with a Vortice HR200BP unit, not sure what its rated at efficiency wise but it'll be less than the Zehnder. By time it gets to terminals its down to around 15C on a very cold day like above, i'm hoping to improve on that and have got a thermal camera to check everything, just not had time to do it yet

[FarmerN]

My Q350 now shows

Extract Air         20 degC

Exhaust Air       11 deg C

Ouside Air        6.5 deg C

Supply Air        18 deg C

Running at 65l/sec (setting 2) at the WARM supply setting  in a 215 sqm Bungalow

There is a Temperature Profile setting COOL , NORMAL, WARM.

 

Reducing flow to 34l/sec (setting 1 ) will probably increase Supply Temp by 0.5 Deg. Currently useing 17W at 34 l/sec

May put an edit on when it has run for a bit at 1.

I'm always surprised that ruducing flow rate does not have a bigger impact on supply temperature

[sonicboom]

22 minutes ago, MikeGrahamT21 said:

Where will you be running your ducting? Mine is in cold loft, but heavily insulated so it loses a bit more temp on its way to the terminals.

 

When it was like -3C the other week, internal temp was showing as 21.2C (extract) and the supply temp was at 19.4C, with a Vortice HR200BP unit, not sure what its rated at efficiency wise but it'll be less than the Zehnder. By time it gets to terminals its down to around 15C on a very cold day like above, i'm hoping to improve on that and have got a thermal camera to check everything, just not had time to do it yet

15c? that's a lot of heat loss, haven't you insulated your runs ? or maybe not enough ? what kind of duct work are you using ?

[JohnMo]

41 minutes ago, sonicboom said:

planning on upgrading my 10 year old PIV system to an MVHR

Are you also making improvements in your airtightness. You will be introducing warmer air as you have heat recovery. How close the supply air temperature is to extract, depends more on how well the system is balanced, as much as anything.

 

However if you have a leaky house all you are doing is adding to the ventilation loads, so be careful. 

 

PIV as you say dumps a lot of cold air into the house. If your house is leaky, I would 

 

Remove PIV system. Then either install dMEV or MEV system but ones that run in an on-demand basis. So they monitor humidity levels and ramp down to minimum throughput when not needed and ramp up only when needed. For these systems to be effective you need trickle vents in dry room, none in wet rooms. The trickle vents should be self modulating (react to humidity). A MEV system will have a central unit and extract points in wet rooms, dMEV system a fan in each wet room. 

[sonicboom]

23 minutes ago, FarmerN said:

My Q350 now shows

Extract Air         20 degC

Exhaust Air       11 deg C

Ouside Air        6.5 deg C

Supply Air        18 deg C

Running at 65l/sec (setting 2) at the WARM supply setting  in a 215 sqm Bungalow

There is a Temperature Profile setting COOL , NORMAL, WARM.

 

Reducing flow to 34l/sec (setting 1 ) will probably increase Supply Temp by 0.5 Deg. Currently useing 17W at 34 l/sec

May put an edit on when it has run for a bit at 1.

I'm always surprised that ruducing flow rate does not have a bigger impact on supply temperature

Not bad 18.5c would be 92% if I maths is correct - Although, for 6.5c, I was expecting the efficiency to be a little higher. What does the WARM setting do ?

[Beau]

Sounds like you are after a proper MVHR but I ran a little test on our decentralised MVHR to see how good it was and was pleasantly surprised. It's one of those that runs out for 70seconds charging a cartridge with the exiting heat and then runs back in supposedly returning the heat in the cartridge. I waited for a cold day to do the test and expected to see larger fluctuations in temp from it running in and out. We run the house coolish as you can see but it was 0.5C outside at the time. Had a temp probe set up just by the in/out vent

[sonicboom]

2 minutes ago, JohnMo said:

Are you also making improvements in your airtightness. You will be introducing warmer air as you have heat recovery. How close the supply air temperature is to extract, depends more on how well the system is balanced, as much as anything.

 

However if you have a leaky house all you are doing is adding to the ventilation loads, so be careful. 

 

PIV as you say dumps a lot of cold air into the house. If your house is leaky, I would 

 

Remove PIV system. Then either install dMEV or MEV system but ones that run in an on-demand basis. So they monitor humidity levels and ramp down to minimum throughput when not needed and ramp up only when needed. For these systems to be effective you need trickle vents in dry room, none in wet rooms. The trickle vents should be self modulating (react to humidity). A MEV system will have a central unit and extract points in wet rooms, dMEV system a fan in each wet room. 

The house is a little too tight for extractors and PIV hence MVHR - The PIV is not as effective as it used to be and the improvements in heat retention just make it very costly to run.

[FarmerN]

16 minutes ago, sonicboom said:

 What does the WARM setting do ?

No idea!

There is just A "Temperature Profile" setting , WARM , NORMAL, COOL

We leave it on WARM most of the time, set it lower in very warm weather, but had no problem with over heating this summer.

Increasing flow rate from 34 l/sec to 65l/sec increased energy use from 17W to 51 W.

Edited November 29 by FarmerN
more info

[MikeGrahamT21]

27 minutes ago, sonicboom said:

15c? that's a lot of heat loss, haven't you insulated your runs ? or maybe not enough ? what kind of duct work are you using ?

its 75mm semi rigid stuff, currently buried under around 300mm of wool insulation, but the loft is cold, very cold, almost no heat from the bungalow gets up there as under the 300mm wool, there is 100mm PUR insulation board. The manifold boxes (made from metal) are also up in loft, again buried under a lot of insulation. Theres definitely improvements to be had, and those figures were at -3C outside, so loft will also have been near this. I agree though, it is a lot of heat loss, and i hope to improve on this.

[Iceverge]

37 minutes ago, sonicboom said:

The PIV is not as effective as it used to be and the improvements in heat retention just make it very costly

 

Are you over ventilating? Maybe you could turn down the PIV flow rate a little. 

[SteamyTea]

The formula you need is here:

 

https://www.engineeringtoolbox.com/heat-recovery-efficiency-d_201.html

[G and J]

35 minutes ago, MikeGrahamT21 said:

almost no heat from the bungalow gets up there as under the 300mm wool, there is 100mm PUR insulation board

Good grief there’s a warning for me.  I’ll be running ducts in the loft and had intended to run on top of the first level of fluffy stuff, underneath a couple more.  Ok your 100mm of PIR is more insulative than our 100mm of fluff (technical term!) but still, methinks I need to rethinks….

[MikeGrahamT21]

18 minutes ago, G and J said:

Good grief there’s a warning for me.  I’ll be running ducts in the loft and had intended to run on top of the first level of fluffy stuff, underneath a couple more.  Ok your 100mm of PIR is more insulative than our 100mm of fluff (technical term!) but still, methinks I need to rethinks….

I'll do some poking about first, only got the thermal camera other day, so didn't even know this was an issue. Could be that i've just made a glaring error that needs fixing lol. Personally didn't have much choice but to run in loft, without making a serious mess of everything i'd already done, not for me thanks!

[MikeGrahamT21]

2 hours ago, sonicboom said:

15c? that's a lot of heat loss, haven't you insulated your runs ? or maybe not enough ? what kind of duct work are you using ?

 

You were right, just been up with thermal camera and i must have moved the insulation about when doing something else, and not done a great job of putting it back.

 

Some of the pipes are also crossing awkwardly, as i put another 2 in after the first ones. Think its going to be a weekend sorting it out! lol

[sonicboom]

8 hours ago, Iceverge said:

 

Are you over ventilating? Maybe you could turn down the PIV flow rate a little. 

Its currently running on its lowest speed. The house has a stale air smell in some rooms due to them being too tight. Speed 2 helps (there are 6 settings) but it gets very cold very quickly.

[sonicboom]

8 hours ago, MikeGrahamT21 said:

its 75mm semi rigid stuff, currently buried under around 300mm of wool insulation, but the loft is cold, very cold, almost no heat from the bungalow gets up there as under the 300mm wool, there is 100mm PUR insulation board. The manifold boxes (made from metal) are also up in loft, again buried under a lot of insulation. Theres definitely improvements to be had, and those figures were at -3C outside, so loft will also have been near this. I agree though, it is a lot of heat loss, and i hope to improve on this.

In my experience I have found wool insulation insufficient for pipework, primarily because its not dense and air can flow through it. I had some metal pipework that was losing huge amounts of heat under wool - 25mm pipe insulation fixed it.

[sonicboom]

8 hours ago, Beau said:

Sounds like you are after a proper MVHR but I ran a little test on our decentralised MVHR to see how good it was and was pleasantly surprised. It's one of those that runs out for 70seconds charging a cartridge with the exiting heat and then runs back in supposedly returning the heat in the cartridge. I waited for a cold day to do the test and expected to see larger fluctuations in temp from it running in and out. We run the house coolish as you can see but it was 0.5C outside at the time. Had a temp probe set up just by the in/out vent

Thanks - what is your internal temp ?

[Beau]

46 minutes ago, sonicboom said:

Thanks - what is your internal temp ?

According to a different thermometer in that room at the time 16.6C. 

 

Generally got the rest of the house around 18C but no rad in that room yet. 

[sonicboom]

50 minutes ago, Beau said:

According to a different thermometer in that room at the time 16.6C. 

 

Generally got the rest of the house around 18C but no rad in that room yet. 

Interesting, so the mvhr is passively heating that room. It is impressive whatever the case - 0.75c between exhaust/inlet which would make it 97% efficient ? Seems too good to be true. What make/model is your unit ? and what is the flow rate ? 18c is freezing btw, don't know how you survive.

Edited November 29 by sonicboom

[Beau]

13 minutes ago, sonicboom said:

Interesting, so the mvhr is passively heating that room. It is impressive whatever the case - 0.75c between exhaust/inlet which would make it 97% efficient ? Seems too good to be true. What make/model is your unit ? and what is the flow rate ? 18c is freezing btw, don't know how you survive.

Yes I'm sure its not that efficient but clearly not shocking either. Claimed efficiency is up to 90%. Suspect the 70 second interval didn't allow the temp probe enough time to fully register the changing temps. I'm sure it's not heating the room as it only uses 6watts so most likely just variation between the two thermometers. It's a BSK Zepher and until today I would be singing its praises but just this afternoon it died! Only a few months old so will be under warrantee but a bit disappointing none the less. Until a month ago the house had no central heating so 18C is luxury 🙂 

[sonicboom]

11 hours ago, JohnMo said:

How close the supply air temperature is to extract, depends more on how well the system is balanced, as much as anything.

The supply will be from a 150mm roof vent installed for the bathroom extractor. The exhaust will be in the bedroom right next to the unit.

[JohnMo]

6 minutes ago, sonicboom said:

The supply will be from a 150mm roof vent installed for the bathroom extractor. The exhaust will be in the bedroom right next to the unit.

Not sure if what I wrote wasn't clear or misread.

 

The distance isn't a concern. By balance I mean how well matched the flow rate between in flow and extract.

[MikeGrahamT21]

3 hours ago, sonicboom said:

In my experience I have found wool insulation insufficient for pipework, primarily because its not dense and air can flow through it. I had some metal pipework that was losing huge amounts of heat under wool - 25mm pipe insulation fixed it.

went up after work and decided to tackle it whilst it’s still cold out, doing work under the eaves is no fun at the best of times!

 

found quite a few bits which I’ve now corrected, where the ceiling joists end was warmer than I’d expected it being, so the wool is clearly doing it’s job, was just too patchy in places, fingers crossed it’ll do the trick anyway. Managed to cut out some excess pipe too from the supply runs, around 3m in total which should help also

[sonicboom]

3 hours ago, JohnMo said:

Not sure if what I wrote wasn't clear or misread.

 

The distance isn't a concern. By balance I mean how well matched the flow rate between in flow and extract.

Thanks for the clarification. I will be extracting from 3 rooms, and supplying to 4. I've not gone as far as to think about pipe sizing, but current line of thinking is 90mm for extract, 75mm for supply.