Green Building Forum - Using 'Specific Fan Powers' to Compare HR Units?

Using 'Specific Fan Powers' to Compare HR Units?

Gotanewlife — Fri, 05 Jul 2013 18:45:58 +0100

I am trying to compare numerous HR and Extractor Units. They don't all give full info on power usage...I have found Specific Fan Power quoted from 0.5 to 3.4 W/l/s but taking one example Xpelair Xcell 150: it gives maximum SFP of 3.2, maximum supply/extract (FID) of 161 M^3/h and Max power usage of 146W.

Just being logical, I assumed that if I convert the max extract to l/s (161 m^3/h = 44.7 l/s) and then multiplied it by 3.2 I would get 146W but it is 134W - OK 10% out is not much but it worries me.

Then of course can I also extrapolate using the SFP in any way perhaps by assuming some sort of extra % for system losses/duct runs etc? In this case I find on the performance dia 100m^3 at 50Pa is exactly speed 2 of 6 - can we guesstimate how many Watts that is?

Another HR unit gives 5 different Watts values with corresponding supply and extract rates but at 110 Watts supply is 253 and Extract is 371 -(FID) how does the SFP work in this case?

Using 'Specific Fan Powers' to Compare HR Units?

barney — Fri, 05 Jul 2013 18:56:27 +0100

You can't extrapolate - the fan power curve isn't linear with airflow - there's a sweet spot - also electrical power in includes losses, power factor etc so again there is no correlatio directly between electrical power and airside power

SFP is a unit of measure across the operating range of the fan so you can't get exact agrement on the numbers

For the volumes you need I wouldn't contemplate anything much above SFP = 1

Regards

Barney

Using 'Specific Fan Powers' to Compare HR Units?

Ed Davies — Fri, 05 Jul 2013 20:07:00 +0100

Posted By: GotanewlifeJust being logical, I assumed that if I convert the max extract to l/s (161 m^3/h = 44.7 l/s) and then multiplied it by 3.2 I would get 146W but it is 134W - OK 10% out is not much but it worries me.

161'000/3600 * 3.2 = 143.111â€¦, not 134. Much closer to 146. Did you swap the digits?

Using 'Specific Fan Powers' to Compare HR Units?

Gotanewlife — Fri, 05 Jul 2013 20:16:35 +0100

Posted By: Ed DaviesDid you swap the digits?

Thanks Ed. can't reproduce error, it is late and I have spend 12 hours today on trying to solve my ventilation problems. Still it is nice to know I can use a fans max FID figure with the Wattage to get SFP and hence compare units.

Using 'Specific Fan Powers' to Compare HR Units?

Gotanewlife — Sun, 07 Jul 2013 11:40:43 +0100

Posted By: barneySFP is a unit of measure across the operating range of the fan

Actually it is not Barney - it is always quoted as ''Max" (edit - not true, just sometimes) and corresponds with the max extract rate (FID) and max power usage (Edit: if it is Max SFP); it is a worst case figure and hence I can work out a W/l/s for units that don't quote a SFP if they give max extract rate and wattage consumed, I feel sure this is the max SFP used in specs, after all it is in the same units. The SFP of HR units is better when working as less than Max, which is what you are implying Barney.

What is really good though and even better than comparing quoted max SFDs (Edit SFP!) is that if the Units give Volume/Static Pressure graphs, usually with several lines on them representing power levels, you can work out the SFP for YOUR specific likely scenario - ie the power level you will be using for your required extract level given your likely Pa pressure. It wouldn't surprise me if there was a variety in the shape SFDs (Edit SFP) across a Units extract performance range (Edit - yup turned out to be quite some difference), especially as few (I guess) people would check this out and rely wholly on the Max SFD as an indicator - hence manfs might optimize their units' SFD for max power. I hope you guys follow this - can explain more if you want....

Using 'Specific Fan Powers' to Compare HR Units?

barney — Mon, 08 Jul 2013 10:00:41 +0100

LoL - OK then, have it your way - but perhaps read your post again, to see the contradiction in your first and second paragraph.

As an observation, it is not constant for a given fan, but changes with both air flow rate and fan pressure rise - trust me on that.

Now that you know where to look on the fan performance curve, and I've indicated to you how to produce a system resistance curve you have the operating point - all you now need is the fan power curve and from the OP you have Specific Fan Power.

Barney

Using 'Specific Fan Powers' to Compare HR Units?

Ed Davies — Mon, 08 Jul 2013 10:40:23 +0100

Gotanewlife: your second paragraph above uses â€œSFDâ€ throughout; just checking but is that a rather consistent typo?

Barney: I don't see a contradiction between his first and second paragraphs, could you explain? Gotanewlife could be a bit clearer, though. I think his first paragraph is talking about the quoted SPF which he says is the one at maximum power (or the worst case one, which (I imagine) is the same thing) whereas the second paragraph is talking about the actual SFPs across the operating range. Is that what you're referring to?

Using 'Specific Fan Powers' to Compare HR Units?

barney — Mon, 08 Jul 2013 12:56:36 +0100

Specific fan power cannot be used without knowedge of a system - if it's quoted at FID then fine - but perhaps check power curves for say backward curved and foward curved centrifugal fans - the former will have a totally different power curve than the latter at FID (in fact the latter will have an almost runaway power curve at FID as it needs system resistance to stabilise the fan).

My comment was based on the "it's always quoted as max" - for compliance or any basic energy assesment it's based on actual operating conditions. Comparing max SFP from unit to unit is pointless.

Regards

Barney

Using 'Specific Fan Powers' to Compare HR Units?

djh — Mon, 08 Jul 2013 13:50:29 +0100

Posted By: barneyComparing max SFP from unit to unit is pointless.

That'll be why it's used as a figure of merit in both SAP Appendix Q and Passivhaus assessment systems then?

http://www.sap-appendixq.org.uk/search.jsp?parentID=1&categoryID=3
http://www.passiv.de/komponentendatenbank/kleine_lueftung/

Using 'Specific Fan Powers' to Compare HR Units?

barney — Mon, 08 Jul 2013 14:48:39 +0100

What's the definition of SFP - does that SAP data base actually say "down to" rather than "less than"

Building regulations requires it to be no worse than eg 1.5w/l/s for whole house mech vent.

I don't disagree that it's a measure of efficiency - but one of the system as a whole rather than the fan in isolation - ie my comment about SFP at FID without knowledge of the fan type.

So, if I put a relatively poor performing fan in a system with very minimal resistance the SFP might be "good"

but if I put a super duper fan in a system with abnormally high resistance then the SFP may well be "bad"

Try looking as I suggested for the power curves on say foward and backward curved centrifugal fans - and see where the power curve takes you on abnormally low and abnormally high system resistances.

Regards

Barney

Using 'Specific Fan Powers' to Compare HR Units?

Gotanewlife — Mon, 08 Jul 2013 14:52:56 +0100

Specific Fan Power is defined by the Energy Savings Trust as:The power consumption, in Watts, of the fan (plus any other electrical system components) divided by the air flow through the system, in Watts per litre per second (W/l/s). Specific Fan Power Calculation: Heat Recovery Ventilation

For a small house with a Kitchen + 1 Wet Room the minimum extraction airflow rate is 21 l/s (Part F: Eng & Wales 2010)If the unit consumes 15 watts to achieve this rate of air flow then: Specific Fan Power (SFP): 15 / 21 = 0.71 w/l/s.

Posted By: Ed DaviesGotanewlife: your second paragraph above uses â€œSFDâ€ throughout; just checking but is that a rather consistent typo?

yup what a mess - I'll fix it with an edit so as not to confused future readers.

I have now compared LOTs of HR units back to back, cut and paste performance curves to compare, looked at SAP numbers, etc etc. I am of the firm opinion that the best way to compare is to estimate the SFP at the point on the graph that, as far as you can estimate, corresponds with your likely installation. Not always easy as some don't list the watts against each curve but you can always find the worst case SFD as all but one unit quoted max power usage. Most high quality units have flat SFP 'curves' ie best 0.41 worst 0.6 but some, like the one I bought a few moments ago, has best 0.53 worst well over 1, which is why the headline SFP should just be a first level weeder, ie see SFP = 3.2, look no further! Manufacturers seem to vary how they 'headline quote' their SFPs not always 'Max' as I had found before. It is always correct to say that the highest SFP is at the highest fan power so when the SFP is not quoted this is a good place to start.

Using 'Specific Fan Powers' to Compare HR Units?

barney — Mon, 08 Jul 2013 15:59:46 +0100

So, as I said then, its a measure of efficiency of the fan in a system and not the fan in isolation.

As for best, I keep suggesting to you that the type of fan has a hige impact - it can still be a horrible cheap and nasty unit, but if it uses centrif fans instead of axial fans, it's performance will appear to be much better.

As for the highest SFP being at the highest fan power, that's just one of the terms - you also need to know flow rate (or pressure rise to determine flow rate).

ie Specific fan power is measured in watts per litre per second - if you assume seconds to be constant then it's watts per litre - you need both quantities to make tha comparison.

Which is back to my initial response

Regards

Barney

Using 'Specific Fan Powers' to Compare HR Units?

Gotanewlife — Mon, 08 Jul 2013 17:42:25 +0100

No doubt you're right as usual Barney, at least as much as I can be bothered to thrash it out - but since I have now assessed half the units on the web against each other and settled for 2 and bought them - I couldn't give a monkeys now because next pressing matter is the ducting followed by how to give in-laws Sky, need a multi LNB, blah. Give me strength...

Using 'Specific Fan Powers' to Compare HR Units?

Ed Davies — Mon, 08 Jul 2013 17:43:19 +0100

Posted By: barneyie Specific fan power is measured in watts per litre per second - if you assume seconds to be constant then it's watts per litre

More like joules per litre.

Using 'Specific Fan Powers' to Compare HR Units?

SteamyTea — Tue, 09 Jul 2013 06:57:59 +0100

Posted By: Ed Davies
Posted By: barneyie Specific fan power is measured in watts per litre per second - if you assume seconds to be constant then it's watts per litre
More like joules per litre.

An easy mistake to make

Using 'Specific Fan Powers' to Compare HR Units?

barney — Tue, 09 Jul 2013 20:30:02 +0100

LoL - ok then

Regards

Barney