Considering PIV

[MortarThePoint]

I know that MVHR can divide opinions, but the numbers don't work for me as my house has been conceived (having ASHP and not hyper insulated). I respect that it works well for some people's use cases.

 

I had planned to use window trickle vents, but take exception to how they look and how they would likely be lived with (I think most people just leave them closed). Trickle vents act as a point of ingress and egress for fresh, but cold, air from outside. They are often perceived as 'free' but will of course have a cost when included in a window quote. Though some have filters to keep out insects etc, I suspect they are not very effective. I think if I was starting from scratch I would consider airbricks with internal gratings, but that ship has all but sailed now.

 

PIV looks to offer a happy middle ground for my needs:

• It is relatively low capital cost and actually likely to be cheaper than or similar to window trickle vents.
• It has reasonable filters, so should provide good air quality
• It is centrally controllable so I won't find all the trickle vents closed
• It only has a single eyesore
• Benefits from solar gain in unheated loft space (dubious)

I don't perceive it as any more expensive to run than trickle vents except for the power consumption of the pump itself ~5W --> ~45kWhr/yr --> ~£9/yr. Both waste warm air in the winter.

 

Key concerns:

• Egress: My main concern is where does the warm air egress and does it cause condensation there. I'm not timber frame so am not bothered about OSB walls. If it is in the cavity that would likely be OK as it would drain. The graph below suggests that if the humidity in the house is at 50%RH and the temperature is 18C, then condensation would occur when that air is cooled down to about 8C. For the vast majority of the year it shouldn't be an issue then. [Please challenge this if you can think of why, I don't know much about dew point].
• Draught: It creates a centralised point of draught rather than a distributed one like trickle vents. The plan would be to put it in the 2 storey hallway that would make it >1.5m from people and have a direct 'drop' to ground floor level. I wonder about detecting peoples movements (plan to do it for lighting) and turning off the PIV when people are on the landing or stairs. You can have heated PIV, but the efficiency would be rubbish.
• Summer Heat: The PIV would be drawing hot air from the loft and so heat the house. We can counteract this by opening windows and I suspect the heat gain in the loft would be less due to the air being pulled through it.

 

We'll have conventional extractor fans in bathrooms that will likely turn off based on %RH.

 

Why not MEV:

• Where is the air coming in from and what is it bringing in with it
• No filter (similar to above)
• Multiple points of extraction so ducting required (and multiple eyesores)
• Draws in moist air so mould growth and mechanical consequences possible

 

I'd love to rant about how I'd design and build my own system but contrary to the playground quip, we don't live in a free country.

 

 

Edited May 26, 2021 by MortarThePoint

[joe90]

I too didn’t want trickle vents and as we live in an area of high winds at times any draught would be uncontrollable . I bought a cheap MVHR on Ebay (ex bankrupt stock) and DIY installed it myself. Frankly in summer I turn it off as windows and doors are left open but in winter the air is always fresh and it runs itself. I have never got round to properly balance it ?but it works fine. 

[MortarThePoint]

2 hours ago, joe90 said:

I too didn’t want trickle vents and as we live in an area of high winds at times any draught would be uncontrollable . I bought a cheap MVHR on Ebay (ex bankrupt stock) and DIY installed it myself. Frankly in summer I turn it off as windows and doors are left open but in winter the air is always fresh and it runs itself. I have never got round to properly balance it ?but it works fine. 

 

You're a maestro at finding a bargain.

[MortarThePoint]

They both look a bit Noddy (or should that be NooNoo) but the Drimaster and Pozidry both cost around £300

https://www.fastlec.co.uk/ventilation/nuaire-dri-eco-hc-drimaster-eco-positive-input-ventilation-unit

https://www.fastlec.co.uk/ventilation-extractor-fans/vent-axia-lo-carbon-pozidry-pro

https://www.fastlec.co.uk/vent-axia-lo-carbon-pozidry-pro-fd-with-heater-multi-storey

 

https://teletubbies.fandom.com/wiki/Noo-Noo

[Cpd]

I have the top one. I installed it in the same location you talk about but my situation was different, shite house no insulation at all and mould forming on the hallway walls as well as other rooms,  After instaling this it definitely  stopped the mould on the hallway walls but did little to help in other areas - can’t expect miracles. It’s virtually silent and the air is diffused  by the cover over the inlet pipe.  I really don’t see how these can be effective as a way of ventilation in a new house as they pump cold air into the house in the hall and in theory as the pressure rises that cold air escapes through gaps in the building therefore giving you an air change system....  in reality I think the air escaping just takes the easiest route and pretty much bypasses everywhere else. I am just in the process of insulating the house and will see how it performs then but it would NOT be my go to solution for ventilation on a new build. 

[MortarThePoint]

@Cpd I'm hopeful there would be enough mixing with the air already in the hallway. The volume of air in just the hallway is around 50m3 which is something like 10 minutes of the PIV's maximum flow rate. I could investigate the option of a trickle extract on the kitchen extractor fan which would help draw the air through more of the house.

Edited May 26, 2021 by MortarThePoint

[MortarThePoint]

>> The volume of air in just the hallway is around 50m3 which is something like 10 minutes of the PIV's maximum flow rate

 

I don't fancy delving in to the mathematics of diffusion, but I would expect to be able to smell a fart in the hallway within a number of seconds which is much less than 10 minutes. Consequently,  the air mixing due to diffusion should be plenty good enough for the PIV to work effectively.

Edited May 26, 2021 by MortarThePoint

[saveasteading]

I may be out of touch, and that MVHR has moved on since last I tested it.

I have never seen figures that justify the cost of the heat recovery element.

I did try to like it, and met technical reps, but they generally lost interest when asked for figures of power in against power out. If they had test figure then they would have been handing them out.

Some systems were clearly going to short circuit and treat a very small area, pointlessly. The sensible ones involved lots of ducting and a powerful pump.

 

Fresh air is essential I agree but to date,  window trickle vents and some air vents have worked, tho I think that chimneys and the fan vents in kitchens and wcs are probably doing a lot of the work, and losing heat at the same time. But then again, these vents will be there even if there is MVHR.

 

Am I wrong?

Edited May 26, 2021 by saveasteading

[MortarThePoint]

2 minutes ago, saveasteading said:

I may be out of touch, and that MVHR has moved on since last I tested it.

I have never seen figures that justify the cost of the heat recovery element.

I did try to like it, and met technical reps, but they generally lost interest when asked for figures of power in against power out. If they had test figure then they would have been handing them out.

Some systems were clearly going to short circuit and treat a very small area, pointlessly. The sensible ones involved lots of ducting and a powerful pump.

 

Fresh air is essential I agree but to date,  window trickle vents and some air vents have worked, tho I think that chimneys and the fan vents in kitchens and wcs are probably doing a lot of the work, and losing heat at the same time. But then again, these vents will be there even if there is MVHR.

 

Am I wrong?

 

Makes sense, though I'm looking at not having window trickle vents so that is the only difference to what you say

[saveasteading]

2 minutes ago, MortarThePoint said:

 

Makes sense, though I'm looking at not having window trickle vents so that is the only difference to what you say

 

Where do you consider the fresh air to come from? Multiple tiny gaps at doors etc? Could be enough I think.

 

Random thought: Do Passiv-haus designs have very special extract fans that close tight when not in use?

For an air test it is normal to blank off all these orifices, but they allow air passage on an everyday basis.

[MortarThePoint]

9 minutes ago, saveasteading said:

Where do you consider the fresh air to come from? Multiple tiny gaps at doors etc?

 The PIV system sucks air out of the ventilated loft space

[saveasteading]

As already mentioned above but forgotten already by me: sorry.

 

Cold air in, but not as cold as direct from outside. OK. 

 

I have long wanted to try stack effect ventilation, using the effects of wind across the 'chimneys' plus differential pressure around the building.

Wood burners, when not in use will provide the stacks, with handy vents for control.

 

It is shocking how much heat goes up the chimney, I have been up there and felt the warm draught of expensively heated air.

My inclination is that we don't need any more ventilation than that, or from new stacks, as long as some air gets in to replace it.

 

[ProDave]

1 hour ago, saveasteading said:

Fresh air is essential I agree but to date,  window trickle vents and some air vents have worked, tho I think that chimneys and the fan vents in kitchens and wcs are probably doing a lot of the work, and losing heat at the same time. But then again, these vents will be there even if there is MVHR.

 

Am I wrong?

Yes you are wrong.  With mvhr you have controlled ventilation to all rooms,  fresh air into living rooms and bedrooms and exhausted air from kitchens and bathrooms.  the heat from the exhausted air gets put back into the fresh air entering the building.

 

You don't have ucontrolled non heat recovered exhaust vents from kitchens and wc's.  Those all operate as part of the mvhr system.

 

I also don't believe a chimney in a properly installed room sealed stove is a big loss of heat (or an aid to ventilation of the house).  We ran the house for 1 year before the stove was fitted and the eating bill did not rise the next winter after fitting the stove suggesting any heat loss is "lost in the noise"

[Bitpipe]

1 hour ago, saveasteading said:

 

Where do you consider the fresh air to come from? Multiple tiny gaps at doors etc? Could be enough I think.

 

Random thought: Do Passiv-haus designs have very special extract fans that close tight when not in use?

For an air test it is normal to blank off all these orifices, but they allow air passage on an everyday basis.

 

No as MVHR is continually running as its the sole means of ventilation, there is summer bypass where you can avoid the heat exchanger.

 

The principal of airtightness is to control the egress of air that you've paid to heat vs having it blow uncontrolled out of the house.

 

For me, the heat recovery element of MVHR is secondary to its primary function of effectively ventilating the whole house.

 

As with others, I leave doors and windows open in summer but I can be more efficient in cooler weather and reduce my heating costs.

[MortarThePoint]

2 minutes ago, Bitpipe said:

For me, the heat recovery element of MVHR is secondary to its primary function of effectively ventilating the whole house.

 

That's why I like the look of PIV. It doesn't distribute the air as well as the multiple outlets of an MVHR system would, but it is fresh filtered air coming in. It's not heated by scavenging 90% or more of the outgoing air's energy, but those economics are discussed elsewhere.

[Nickfromwales]

1 hour ago, saveasteading said:

 

Where do you consider the fresh air to come from? Multiple tiny gaps at doors etc? Could be enough I think.

 

Random thought: Do Passiv-haus designs have very special extract fans that close tight when not in use?

For an air test it is normal to blank off all these orifices, but they allow air passage on an everyday basis.

PH MVHR systems are sealed systems delivering balanced fresh air to habitable spaces and extracting and extracting from wet / smelly areas. No connections ever from rooms to atmosphere as they ( MVHR ) rely on heat recovery to attain PH performance levels. If you dump air to atmosphere, instead of going via an MVHR unit, then you lose the heat energy plus you are also going to be pulling in the same volume of air direct from infiltration. That ‘replacement’ air will be freezing cold during winter and your heating bill will reflect it. 
Partially open strategic trickle vents, which would control infiltration far better in this instance, would be the least aggressive way of managing humidity / CO2 etc and have the least impact on losing heated air. 
Balanced MVHR is the only way to manage this properly, but if the house has an infiltration of ACH which is above the trickle rate of the unit then you’ll have just wasted a lot of money and effort. 
Serious amounts of draught-proofing and MVHR will serve you best.

[MortarThePoint]

10 minutes ago, Nickfromwales said:

Partially open strategic trickle vents, which would control infiltration far better in this instance, would be the least aggressive way of managing humidity / CO2 etc and have the least impact on losing heated air. 

 

I'm inclined to disagree*. Unless I am missing something, trickle vents are 100% inefficient just like PIV, so for the same amount of ventilation (m3/h) you'll have the same amount of heating cost. However, it's hard to know what flow rate your actually getting from trickle vents (e.g. effects of wind etc) so it is less controllable. If the trickle vents cost your heating less it's because they are exchanging less air with the outside.

 

* the caveat to this is if the air is so still that it is driven by diffusion in which case air will be coming and going in the same vent and so exchanging energy. This feels unlikely however.

Edited May 26, 2021 by MortarThePoint

[MortarThePoint]

This is interesting. 

 

Resting human breathing rate <8 l/min, so 4 people well less than 1l/s. Don't need >10l/s of ventilation for breathing then.

 

Wrong: C_eqm = equilibrium CO2 concentration
C_out = background CO2 concentration = 0.04%
q = air exchange rate = 60 l/s air
i = CO2 influx rate = 8l/min * 4people * 4% CO2 = 0.02 l/s CO2
At equilibrium:   i - q.(C_eqm - C_out) = 0
So,    C_eqm - C_out = i/q = 0.02l/s / 60l/s = 0.03%
C_eqm = 0.07% which is 700ppm or +75% over outside CO2 concentration

 

Thresholds vary by standards organisations, but 1000ppm is often seen as the upper CO2 limit for good air quality.

https://en.wikipedia.org/wiki/Indoor_air_quality

https://www.velux.com/what-we-do/research-and-knowledge/deic-basic-book/ventilation/indoor-air-quality?consent=none&ref-original=https%3A%2F%2Fwww.google.com%2F

 

[MortarThePoint]

Doing a similar calculation for humidity

 

C_eqm = 0.8% by mass based on 50%RH at 21C [1]

C_out = 0.64% by mass based on 80%RH at 10C [1]

Sources adding water to the air:

• Human breathing up to 1l/day, four people --> 4kg/day [2]
• Ignore sweat
• Evaporation about 3kg/m2day so guess as 3kg/day to cover water left in shower, basins and loos [3]
• Cooking and kettle, guess 2kg/day (ignores extractor or this amount gets round extractor)
• Bathing guess 250g/person/day --> 1kg/day (after extractor effects)

i = 10kg/day total based on above list which is hopefully an upper bound. I could imagine it being half this, but can't imagine it being much more unless clothes are drying into the air which is a bad idea.

 

q = i / (C_eqm - C_out) = 10kg/day / (0.8% - 0.64%) = 6250kg/day of air

Density of air is around 1.2kg/m3 so that flow rate equates to 5200m3/day which is about 220m3/hr or 60l/s. Coincidence?

 

Both calculations assume that the air leaving the house is representative of the house as a whole, i.e. good mixing.

[saveasteading]

Thanks for the input. Have done some reading . several thoughts arise.

 

Came across several statements, including from manufacturers, that cooker vents should not go to heat exchanger because of the fats, but be recycled to the room through a carbon filter. I don't think that the recycled method is going to satisfy 'management', as the filters restrict the airflow, which is the sole purpose of the fan.

There may well be manufacturers who take the greasy air but I didn't find them.

 

I was also looking for a simple explanation of how you can get 90% of the heat back.  When I have asked specialists to recover the waste heat from chiller plants ( alot of quite hot air) they have told me that they can only salvage 10% of the energy, and that it was not cost-effective

Domestically, if air at 20C passes around air at 10C, in identical quantities, then both apparently don't become 15C.

Examples show that you get 19C but I am not following the logic.

With plain English and pictures if poss. please.

 

We will have very high spaces, which probably alters the logic considerably too. The air becomes stale less quickly for one, and so air change rate could be less.

 

I haven't considered what air temperature is needed with underfloor heating. the logic has always been that with warm feet , the air temperature can be lower. (I might alter the thermostat dials so that 18 shows 20!)

[Nickfromwales]

1 hour ago, saveasteading said:

I don't think that the recycled method is going to satisfy 'management', as the filters restrict the airflow, which is the sole purpose of the fan.

Incorrect assumption. That’s EXACTLY the job they are designed to do. The more cooking you do, the higher the frequency of the filter changes.

Your thinking is flawed. 

[Mr Punter]

1 hour ago, saveasteading said:

Came across several statements, including from manufacturers, that cooker vents should not go to heat exchanger because of the fats, but be recycled to the room through a carbon filter. I don't think that the recycled method is going to satisfy 'management', as the filters restrict the airflow, which is the sole purpose of the fan.

There may well be manufacturers who take the greasy air but I didn't find them.

 

I like cooker hoods to have stainless steel grease filters that go in the dishwasher and are externally vented via min 150 dia ducting.

[saveasteading]

Nickfromwales        Incorrect assumption. That’s EXACTLY the job they are designed to do.

 

Is it your recommendation that the kitchen fan doesn't go to the heat exchanger, but recycles through its own filter to the kitchen?

 

My logic was that when I have been involved in commercial kitchens, the extraction rate is huge (there must be  a better word for a large amount of air flow) and it is to outside. there is not attempt to capture this waste heat.  The working kitchen that you would expect to be hot is near to cold.

Meanwhile every domestic extractor I have known has been underwhelming, even when venting outdoors, and disappointing through filters back to the room. 

I guess then that the solution is a very much more powerful fan than the normal range, at 4 x the price.

 

Advice on carbon filters seems to be to use them in winter, and vent back to the room, then remove and vent outdoors in summer.

This is getting complicated.

 

Mr Punter: Stainless steel filters are indeed easy to clean and don' t need replacing, but you can see through them so must be catching  just a proportion of the oil, to reduce the muck in the duct.

 

This is not becoming clearer, especially the cost/benefit.

 

[MortarThePoint]

3 hours ago, saveasteading said:

Domestically, if air at 20C passes around air at 10C, in identical quantities, then both apparently don't become 15C.

Examples show that you get 19C but I am not following the logic.

I don't know how they work but you have presumed a single thermalization chamber that averages the temperature of the incoming and outgoing air. If instead you had three in series, the middle one would be at 15C, the inner at 17.5C and the outer at 12.5C. I'd imagine MVHR works on a succession of thermalization points. I presume the airs don't actually mix however. Another method would be to have a heat pump extracting heat from the outgoing air and putting it into the incoming air.

 

Do you ever get condensation issues with MVHR?

 

38 minutes ago, saveasteading said:

there must be  a better word for a large amount of air flow

There is, wind ? couldn't resist it. Doesn't fit the context as well as a gale though.

[MortarThePoint]

5 hours ago, saveasteading said:

When I have asked specialists to recover the waste heat from chiller plants ( alot of quite hot air) they have told me that they can only salvage 10% of the energy, and that it was not cost-effective

 

This sounds intriguing. Is that like food chillers? You need someone that wants to keep something warm or does a lot of endothermic reactions. Converting hot air to electricity is pretty inefficient (thermoelectric generators, thermopiles or even Peltier).

 

Heating offices etc another good option.