Am I correct in summarising exhaust air heat pump for hot water?

Figure are always interesting, but no one has convinced me yet :)

- The best MVHR units are independently tested as being able to recover >90% of the heat (see https://database.passivehouse.com/en/components/list/ventilation_small) and have done so for years

- MVHR is also about good, controlled air quality all year round, not just heat recovery

- MVHR also means no trickle vents, so no unnecessary external noise

- High airflow rates tend to be noisy, unless you increase the duct size to compensate

- Running costs may be low, but the capital costs I've seen have been very high

- etc.

BTW, Drexel & Weiss, Genvex, Viessmann, Effiziento (and no doubt others) also make similar combination units.

Posted By: minisaurusCan you explain more when you say "independently tested"?

Independently tested by a laboratory approved by the PassivHaus Institute, following PHI protocols; scroll down to 'Building Services':
https://passiv.de/en/03_certification/01_certification_components/02_certification_criteria/02_certification_criteria.htm

If a company hasn't submitted their products for testing, I'd also doubt their figures (and wouldn't buy from them either).

Posted By: minisaurusYou don't have to have trickle vents with exhaust air, there are other ways to build

True, but they come with various challenges and aren't as energy efficient. There's one interesting thread here:
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16772

Posted By: minisaurus we have around 800 properties - schools, elderly care and other care homes

Then this list of larger capacity units will also be of interest - up to 15,000 m³/hr, with the best again over 90% efficient and using cross-flow / counterflow heat exchangers:
https://database.passivehouse.com/en/components/list/ventilation_big

Posted By: minisaurusflow rates for e.g. the Nibe can be as low as 20 l/s

It would be interesting to calculate how much hot water can be obtained / hour at that flow rate.

CTC EcoVent i360F exhaust air unit £750

That's cheap. Just looked up the Nibe F370 to check it wasn't my memory, and that seems to be retailing at over £3K.

Thanks for those links and comment Mike1 (sorry I don't know how to do that blue quoting thing you do)

I've had a quick look at the calculation and if I've understood correctly I can see how you'd be able to get over 90 % in a passive house. Although probably not in the buildings I work with :)

The calculation used most where I work is (Temp after exchanger - outdoor temp) / (Exhaust air temp - outdoor temp) * 100

Of the units in the list, we have Swegon Gold and various Fläkt units in our properties.

djh - I'm in Sweden, and appear to have mistranslated, but yes, as you say, counter-flow. No passive house buildings where I work.

djh - all incoming air has to be heated - with MVHR you recover let's say 85 % depending on the calculation you use. With a heat pump, you boost that 85 % say 2-2,5 times. So for the same electrical input as MVHR, you get more heat. Plus you get that heat all year round; you don't vent it out on the warmer days, as you do with MVHR.

djh - the EcoVent is one example of exactly what I'm talking about! see my previous post :)

The attached image demonstrates my point better. You have circa 20 degrees exhaust air (GT41). And 0.6 degrees after the heat exchanger (GT43) The pump is using 9kW (Tillförd effekt) and "generating" 23 kW (Avgiven effekt). The bottom of the image shows the hot water being sent to the acc tank - 50.1 degrees (GT21)

The unit in this example is an IV Eco Heater

This is why connecting heat pumps to your exhaust air is great!

Yes, that other formula is also used here, the best is of course to measure both; I think my organisation may be working towards that. However, I really don't think, with a unit in a good or even reasonably condition, you can get so much air sucked in from your room that it significantly affects the result. Sorry, I'm not buying that argument :)

The CTC EcoVent i360F brochure states that "with the compressor located outside the building, you get a considerably quieter indoor climate". Yet the diagram shows the CTC unit inside the house - is that incorrect then?

Posted By: minisaurusThe attached image demonstrates my point better.

Sorry I'm not going to work through a diagram in Swedish to try to understand whatever point you're trying to make, especially since I know it is a specious point.

Heat pumps work. They are a good idea. They (ASHP) work by taking an arbitrarily large volume of air at some temperature and expelling it at some lower temperature in order to produce a measured increase in temperature of a measured quantity of some fluid (either air or water).

MVHR also works. They are a good idea. They work by taking a measured volume of air from habitable space and cooling it to approximately ambient external temperature, whilst heating the same volume of air from ambient to approximately internal temperature. The good ones do this very efficiently at very low power.

Trying to combine the two is a step too far IMHO.

Posted By: Mike1 And MVHR 'performance outperforms natural ventilation by between 20 and 30% in terms of carbon emissions for all levels of airtightness'. See https://www.passivhaustrust.org.uk/guidance_detail.php?gId=46

That's a highly questionable conclusion. The paper by the Passivhaus Trust relies on one paper from 2000 that comes to this conclusion. In addition, it's early proposition about the failings of natural ventilation in the paper are almost entirely incorrect. There's been plenty of research and real world studies since then that demonstrate different results. At the end of the day it depends on both the design of the building together with the design of the ventilation system. In my view this report represents a really poor and lazy standard of research by an organisation that really should know better!

Posted By: djh

Trying to combine the two is a step too far IMHO.

Posted By: Mike1Agreed - I've not yet seen a convincing argument for combining them...

Well, in John Cantor's book, Heat Pumps for the Home, he makes specific mention of heat pumps used to supplement heat recovery ventilation. This is suggested to be ideal in mopping up waste heat missed by the passive heat exchanger (especially in passivehaus) and these systems do provide useful net heat input of a COP around 3.

In commercial settings, I'd imagine the system being an ideal solution to utilise waste heat, from both ventilation and water drainage, for instance.

Posted By: djh

Posted By: SimonDThat's a highly questionable conclusion. The paper by the Passivhaus Trust relies on one paper from 2000 that comes to this conclusion.

I think you must have misread Palmer's report. He explains just above that quote how HE calculated those results last year, not in 2000. The 2000 work is one of those criticised for reaching incorrect conclusions and he has now updated to use modern carbon factors as well.

No, I don't believe I have. Whilst it is acknowledged in the report that things have changed, in particular grid carbon intensity and MVHR efficiency, there is no wider referenced support for his conclusions. In addition, the conclusions appear to be based entirely on desktop analysis, not of real world performance data. Nor does it seem to expand upon what a natural ventilation strategy consists of.

For example:

'An MVHR system will ensure a reliable air change rate throughout the year, whereas this cannot be guaranteed by a natural ventilation strategy.'

There is plenty of research to demonstrate that natural ventilation can and does provide reliable air change. And over the long term can present a more reliable and less costly, both in terms of capital outlay and running costs, ventilation option.

'The results, shown in Figure 4, indicate that this change alone results in MVHR ventilation out-performing natural ventilation by a significantly larger margin than previously, at any infiltration level.'

References to wider research to support this theoretical calculation?? None..

Anyone who has read wider research on MVHR will be aware of a fairly wide array of issues experienced in real world testing of MVHR systems, all the way from poor indoor air quality, to unanticipated inefficiencies. E.g. that despite laboratory and manufacturer testing, the systems often fail to live up to the promises, including and in particular, efficiency figures. It's also fairly common knowledge that occupant behaviour and environmental preferences play a significant if not overriding role in the efficiency of the ventilation system.

Unfortunately, the report also fails to consider the contexts within which MVHR is not an ideal option for occupants.

'Natural ventilation' is also such a terribly poor term as it can mean anything from someone opening a window to drilling a large hole through a wall, to a properly designed ventilation system that works with the building within its context, utilising natural pressure differences across the house and which gives the occupant full local control of the atmosphere. So really, in the context of the report, it's reference to natural ventilation is totally meaningless! Hence why I suggest it's just poor and lazy

Posted By: SimonDWell, in John Cantor's book, Heat Pumps for the Home, he makes specific mention of heat pumps used to supplement heat recovery ventilation. This is suggested to be ideal in mopping up waste heat missed by the passive heat exchanger (especially in passivehaus) and these systems do provide useful net heat input of a COP around 3.

Exhaust air heat pumps do exist, but I've not yet found one that's compact and affordable. See also http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11668

Posted By: SimonDIn commercial settings, I'd imagine the system being an ideal solution to utilise waste heat, from both ventilation and water drainage, for instance.

I'd imagine so too; It's often cooling that's the bigger problem in commercial settings.

Posted By: SimonDThere is plenty of research to demonstrate that natural ventilation can and does provide reliable air change

It can but, unless there are mechanical controls, over or under-ventilation is likely, which MVHR overcomes - hence the 'reliable' bit of that sentence. But yes, it does require that the system is correctly designed and installed, and that the occupants understand what it does, why it does it, and how to use & maintain it. Of course occupant behaviour is a key reason why 'natural' ventilation systems can have problems - blocking vents leading to poor indoor air quality, over-ventilation leading to wasted energy (a significant problem in Germany, where they apparently like to open the windows for an extended period every day), and the like.

Posted By: djh Incidentally, you missed also his critique of the previous papers' methodologies.

No, I didn't miss that. I just didn't comment on it. I think I probably illustrated quite clearly that I think the report is methodologically flawed which call into question its conclusions.

Posted By: djhI think you're considering the report to be something more than it is. And a report that included a lot of the information you're looking for would undoubtedly be a better report, but that doesn't make the present version completely useless IMHO.

I don't think I am. The report is essentially a statement being made by a trusted organisation upon which people are likely to make significant and expensive decisions.

I didn't say it was completely useless. I said it was lazy and poor and that the conclusions are highly questionable.

Frankly, I think it comes across as MVHR propaganda.

Posted By: WillInAberdeen
It should be possible to design MHRV that ventilates each room by the correct amount depending on how it is being used and on the outside air humidity, but examples are proving hard to find.

Lindivent have great systems for this - we have them in quite a few schools, but I don't think they do residential (?) I imagine you could put one of these https://www.lindinvent.com/products/sensors/htdt2500/ in your incoming air duct before your MVHR

Posted By: RobLOur mvhr came with humidity control built in, I think that's pretty standard now. I added an analogue output 0-10V CO2 sensor, which needed a longer 3 wire cable adding to it, but is completely compatible with the vent axia sentinel kinetic (0v, signal, +24V power from the mvhr):

The mvhr controls are expecting CO2 on a 0-10v line, and you can program in what level of CO2 you'd like to give 50% power or somesuch from memory.

This is a LOT cheaper than the manufacturer supplied CO2 sensor for my MVHR!

The controller for my MVHR has a similar option for external sensors. I am working on driving this input port from an already existing monitoring system which combines humidity and CO2 data from distributed sensors. The only item missing is an outdoor temperature & humidity sensor to avoid boosting the MVHR when it can't bring humidity down due to outdoors conditions.

@Mike not quite the same as a proper sensor....
For monitoring I got the Sensirion SCD-30. Probably the cheapest proper CO2 sensor. It does not have a 0-10V interface though, only I2C, UART and PWM. Nice for Arduinos and Raspberry Pis, but not for direct connection to an MVHR controller.

You can't "gain" more heat from a heatpump than there is available "in" the air. If the heatpump drops the air temperature by the same amount as the MVHR (ie. DT = internal temp - external temp), then it's the same heat - COP doesn't come into it (ignoring latent heat). Heatpumps are great, but they're not magic.
You could drop the temp to lower than external, but then what's the point, may as well use an A2W heatpump - it will probably be more efficient.

So 25A should say P=qxpxcPxDTxeff. 25B will give about the same answer as 18B.