Considering PIV

[tonyshouse]

Low capital cost, high running cost when you add inthe heat losses.

 

I think it should be illegal, it does work yes but continuously pumps warm out at the same rate as it fans (pumps) cold air in. 

[MortarThePoint]

20 minutes ago, tonyshouse said:

Low capital cost, high running cost when you add inthe heat losses.

 

I think it should be illegal, it does work yes but continuously pumps warm out at the same rate as it fans (pumps) cold air in. 

 

In theory it should be no worse than trickle vents, except for the power consumption of the fan itself.

[tonyshouse]

Trickle vents do almost nothing on calm days, little bit of convection 

 

PIV blows air into a home 24/7 and whatever volume of new air is blown in an equal amount of warm air must leave the home. There is a cost in blowing warm out as well as the cost of the fan power though most likely that at least is heating the building 

[MortarThePoint]

44 minutes ago, tonyshouse said:

There is a cost in blowing warm out as well as the cost of the fan power

 

Completely agree, the cost of powering the fan (20W?) is small compared to the cost of heating the cold air up (400W of electricity at 60l/s and 20C temperature difference). 200W at a more normal dT=10C.

 

But if trickle vents are/were doing the job they're supposed to then they'd be exchanging the same volume of air and costing 400W too.

Edited May 27, 2021 by MortarThePoint

[Bitpipe]

15 hours ago, MortarThePoint said:

Do you ever get condensation issues with MVHR?

 

Condensate forms inside the unit and flows to an output pipe which is connected to waste water system.

[saveasteading]

Is that like food chillers?

 

Yes. Have designed and built several factories/stores for food storage, chilled or frozen. The heat coming out as waste from the chillers is in vast quantities (at about 30C I would guess,) and no attempt to harness any of it.

Even with an office or ambient warehouse attached they don't use any of it.

I reckon most chiller contractors don't know or care about any such possibility, and the clients just choose the cheapest. But I did meet an energy consultant who quoted the 10% recovery possibility, but a very long payback on the capital.

If I ever told a client of a good idea to save the planet, they wanted to know payback on the capital. 10 years is the normal guide, and that can be a guide to the carbon efficiency too,  but in real life people want better than that.

 

I am not doubting the efficiency of MVHR in a philistine way: It is based on trying for 30 years to design buildings more efficiently, and seeing gimmicks and crazes come and go.

Wind turbines, GSHP, diesel from crops and cooking oil, wood burning boilers in central London, were all trendy and wasted clients' money to please designers and planners.

Also, early entry can be inefficient, eg the first solar panels, and early heat recovery.

 

Still haven't seen any proofs, not that it is for Build-hub members to justify it, but for the manufacturers and testing labs.

 

[Bitpipe]

12 minutes ago, saveasteading said:

Is that like food chillers?

 

Yes. Have designed and built several factories/stores for food storage, chilled or frozen. The heat coming out as waste from the chillers is in vast quantities (at about 30C I would guess,) and no attempt to harness any of it.

Even with an office or ambient warehouse attached they don't use any of it.

I reckon most chiller contractors don't know or care about any such possibility, and the clients just choose the cheapest. But I did meet an energy consultant who quoted the 10% recovery possibility, but a very long payback on the capital.

If I ever told a client of a good idea to save the planet, they wanted to know payback on the capital. 10 years is the normal guide, and that can be a guide to the carbon efficiency too,  but in real life people want better than that.

 

I am not doubting the efficiency of MVHR in a philistine way: It is based on trying for 30 years to design buildings more efficiently, and seeing gimmicks and crazes come and go.

Wind turbines, GSHP, diesel from crops and cooking oil, wood burning boilers in central London, were all trendy and wasted clients' money to please designers and planners.

Also, early entry can be inefficient, eg the first solar panels, and early heat recovery.

 

Still haven't seen any proofs, not that it is for Build-hub members to justify it, but for the manufacturers and testing labs.

 

 

The COP of MVHR did not bother me that much as, given my house was designed and built with very high airtightness, I *needed* an efficient all room ventilation system so that was my primary concern and the efficiency of the heat recovery was important but secondary.

 

That said, I found this https://www.paulheatrecovery.co.uk/products/methods-of-calculating-the-heat-recovery-efficiency/ so maybe somewhere to start.

 

 

[SteamyTea]

On 27/05/2021 at 14:05, 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

Is it really a lot of hot air, it may be air at 50°C, but probably not that much in volume (or mass really).

As said above, commercial kitchens are a different problem, they need tonnes of air moved though them, this is why it is so cold waiting to pick up a Chinese takeaway, all the air is sucked out the building (actually bows our roof) by the extractor.

21 hours ago, MortarThePoint said:

Do you ever get condensation issues with MVHR

Some energy will be recovered by the MVHR as water vapour turns to liquid.

https://www.engineeringtoolbox.com/water-properties-d_1573.html

[saveasteading]

Eureka. The point I have been missing is that these 80% efficiencies are for intake of very cold air, so that there is more capacity to absorb the extracted heat.

I found a simple calculator that says that for 20C inside, the efficiency varies from 0% with external air at 15C, to 80% for external of -5C.

That makes more sense to me now.

With the latter example, the intake air has been heated from -5C  to 15C.

 

My previous thought process was with a differential of 10C, and that seems to be 50% efficient, as might be expected, not 80/90 as the headline figures.

Energy, not degrees.

 

 

 

[saveasteading]

5 minutes ago, SteamyTea said:

Is it really a lot of hot air, it may be air at 50°C, but probably not that much in volume (or mass really).

Yes it is a great deal. It has to be directed upwards so as not to be a nuisance. 

The air flow is similar to that of a big air heat pump, as you might expect when I think about it, as it is the same in reverse.

If the food is brought in frozen, and stacked tightly, there isn't so much waste , but if actually freezing it first then there is a lot of heat to get rid of.

 

For a biggish building, say 700m2 chilled and 1000m2 frozen (and so a very big volume) there can be an array of extractors say 8-10m2 of outlet. That would be a lot of heat if pumped into a useful space such as a retail warehouse.....and so we have invented the easy answer, build a b&q right next to a freezer unit and pump it straight in (in winter).....instead of the oil they burn.

Or to a garden nursery to warm the plants.

[SteamyTea]

2 minutes ago, saveasteading said:

It has to be directed upwards so as not to be a nuisance. 

This may be to preserve the chillers, though I would have thought the hot part was outside, I find it hard believe they you be using larger standard chillers and freezers, which need a decent airflow around them.

Down here, when we get a hot week (Cornish maximum temperatures are very really above 26°C), the local fridge engineer company is overwhelmed with fridges and freezers stopping working.

A mate of mine had the same problem in his large pub kitchen, I suggested that instead of having the units against the walls, they full they out a few more inches, and get some fans blowing air behind them.

And to ditch the stand alone A/C unit as this just warms a room up overall, unless it can be fitted in a doorway blowing in.

I also suggest that they put tin foil on the roof light.

 

Not had a hot week in years to try out my ideas.

 

[MortarThePoint]

41 minutes ago, saveasteading said:

Eureka. The point I have been missing is that these 80% efficiencies are for intake of very cold air, so that there is more capacity to absorb the extracted heat.

I found a simple calculator that says that for 20C inside, the efficiency varies from 0% with external air at 15C, to 80% for external of -5C.

That makes more sense to me now.

With the latter example, the intake air has been heated from -5C  to 15C.

 

My previous thought process was with a differential of 10C, and that seems to be 50% efficient, as might be expected, not 80/90 as the headline figures.

Energy, not degrees.

 

Well that's vitally important as it's very rarely -5C around here. It's efficiency at an outside temperature of 6C is probably more representative.

 

Can you share a link to the calculator please

[MortarThePoint]

1 hour ago, saveasteading said:

With the latter example, the intake air has been heated from -5C  to 15C.

 

I'm not seeing the relevance of 15C and having a quick Google suspect you may have used the EngineeringToolbox calculator but the air after heat exchanger temperature won't be constant at 15C.

 

I can imagine the recovery (i.e. heat exchanger) efficiency being lower at smaller temperature differentials, but I can't find data readily available.

 

[MortarThePoint]

Thus far I have only found the graph below which is in someone's academic thesis and based on Stockholm where it is much colder. The temperature transfer efficiency is looking pretty linear and taking it as 87% at -4C and 95% at -16C it would be about 80% at 6C. These curves are suspiciously absent from the sales literature of MVHR systems though to verify.

 

 

Blauberg Komfort Ultr 64-72%

I can see heat recovery efficiency vs flow rate:

Edited May 28, 2021 by MortarThePoint

[saveasteading]

You are right that it was that calculator: probably not very accurate but was enough to show me the logic.

That attachment of yours shows the same, that the efficiency is 90% (more like 80% really?)  when it is 0C outside. None of the sales literature I had seen mentioned this.

Do you know what the downward curve is? I would like to see the graph extended into positive temperatures.

 

I am now almost decided that to spend £8k on a heat exchanger is not for me, whether it ticks boxes or not. That buys an awful lot of insulation, for example, with no running or maintenance costs.

If it is stuffy I will open some of the trickle vents. If it is cold I will close them.

Fortunately, the air is pure and filtering is not an issue. 

Also, for a conversion we only have to take 'reasonable' steps to efficiency, and that includes cost.

 

In my opinion the next best thing for energy saving, after airtightness and insulation, comes entrance porches for an air-lock when doors are opened in a howling winter gale.

 

 

[MortarThePoint]

16 minutes ago, MortarThePoint said:

Blauberg Komfort Ultr

That was a pretty weedy and cheap system. Looking at a more expensive Blauberg unit the numbers are better (below). The blue shaded text suggests the efficiency is quite flat with outside temperature, varying much more with flow rate which makes sense as that is what impacts the 'thermalisation' time.

 

Blauberg EC S5B 270 £1250

 

[Bitpipe]

1 minute ago, saveasteading said:

I am now almost decided that to spend £8k on a heat exchanger is not for me, whether it ticks boxes or not. That buys an awful lot of insulation, for example, with no running or maintenance costs.

 

Your choice obviously, although £8k sounds very expensive for MVHR. The entire system for my 400m2 cost £4k.

 

In my opinion, MVHR's primary function is to deliver continuous controlled ventilation to every room, the heat recovery capability means it does so efficiently.

 

In an airtight house it is essential but as yours is a conversion, you may not reach the airtightness levels to really take advantage of the efficiency anyway.

 

5 minutes ago, saveasteading said:

In my opinion the next best thing for energy saving, after airtightness and insulation, comes entrance porches for an air-lock when doors are opened in a howling winter gale.

 

In a sufficiently airtight house you can leave the front door open and there will be no draught in or out as there is nowhere for the air to go.

[MortarThePoint]

Sorry for so many posts, but the picture is forming slower than I'd like

 

Looking again at that unit (Blauberg EC S5B 270 £1250) it consumes a lot of power and it all looks to be fan related (i.e. no resistive heater or heat pump). Comparing it to a PIV system:

• PIV at 60l/s, dT=13C P_fan=20W, Q_heat=780W --> P_heat=260W (based on COP=300%) TOTAL=280W
• MVHR at 60l/s, P_fan=150W, Q_heat=780W*(100%-87%)=101W --> P_heat=34W TOTAL=184W

So at an outside temperature of 6C that MVHR is saving you about 100W of electricity over PIV.

 

Edited May 28, 2021 by MortarThePoint

[PeterW]

But you can’t compare MVHR to PIV..? 
 

At 60 litres/sec, the PIV will need to be moving air at 3.5m/s through the air input. That’s going to be a pretty fierce flow. Also, if it is in the attic it will be dragging a lot of air into the attic space, something like replacing the entire airspace of a standard house in around 30 minutes. That has to come from outside, so you will rapidly increase the moisture levels in the attic on a damp day. 
 

Also, I can’t find the pressure values for those PIV fans - how do they create enough positive pressure to force air out through trickle vents etc ..?

[MortarThePoint]

There is a lot of focus on PIV not being as good as MVHR on the heating bill side, which is only one dimension of this and I alluded to accepting that MVHR suits many so I'm more interested in people's thoughts on the other concerns.

 

PIV Pros:

• Clean (filtered) fresh air blown in contributing to good air quality (AQ)
• No need for window trickle vents
• Controllable flow rate so can potentially be put into an AQ feedback loop
• No duct work and only one vent
• Low cost and low maintenance

PIV Cons:

• Not as energy efficient as MVHR
• Many not distributed the fresh air as well as a ducted system
• Fabric condensation risk below 8C outside temperature ?more of a Timber Frame concern?
• Draught concern (if I was making a homebrew one I would include an ASHP powered radiator or the like)

I am considering adding dMEV into the mix which would then greatly reduce the amount of air that is exiting via the fabric as well as improve bathroom dryness

[MortarThePoint]

8 minutes ago, PeterW said:

But you can’t compare MVHR to PIV..? 
 

At 60 litres/sec, the PIV will need to be moving air at 3.5m/s through the air input. That’s going to be a pretty fierce flow. Also, if it is in the attic it will be dragging a lot of air into the attic space, something like replacing the entire airspace of a standard house in around 30 minutes. That has to come from outside, so you will rapidly increase the moisture levels in the attic on a damp day. 
 

Also, I can’t find the pressure values for those PIV fans - how do they create enough positive pressure to force air out through trickle vents etc ..?

 

Sorry Peter missed your post.

 

 

Ventilating an attic is normally a good thing isn't it

[MortarThePoint]

12 minutes ago, PeterW said:

But you can’t compare MVHR to PIV..? 

 

Why not? Both draw fresh air in from the outside. MVHR distributes it better and does a better job of reducing energy waste but they are performing a similar function.

[PeterW]

16 minutes ago, MortarThePoint said:

No need for window trickle vents

That isn’t correct - BC may still require them and you need some method for the air to exit. 
 

Other issue will be your airtightness test and you will get a poor SAP score due to ventilation loss as if you don’t make the house “leaky” your PIV won’t push airflow through the property. 
 

I also think you still need bathroom extract and other wet room extract to meet building regs. 

[MortarThePoint]

1 hour ago, PeterW said:

That isn’t correct - BC may still require them and you need some method for the air to exit. 
 

Other issue will be your airtightness test and you will get a poor SAP score due to ventilation loss as if you don’t make the house “leaky” your PIV won’t push airflow through the property. 
 

I also think you still need bathroom extract and other wet room extract to meet building regs. 

 

I'll need extractor fans in the usual places and they will provide good 'leaks' I'm considering having them as dMEV too.

 

I spoke to my SAP assessor and he thought it would be OK from the SAP perspective. My BCO has PIV in his own home.

[PeterW]

So given you need to do all this, why not use MVHR and get all the benefits ..??