MVHR with adiabatic cooling

[Garald]

We are carrying out a renovation of (most of) a (small) house from 1930 in the Paris area. We (meaning really: the architect and I) will install MVHR by insufflation (i.e., the reverse of the usual direction). One possibility is to install adiabatic cooling (either now or later) - apparently it goes well with this sort of ventilation system.

 

It would be nice to hear about other people's experiences with adiabatic cooling. Is it appropriate to the local climate?

 

Paris is within the same broad climate zone as all of England (Cfb); compared to (say) London, its summers are hotter (by about 2C) and slightly more humid.

 

hideClimate data for Paris (Parc Montsouris), elevation: 75 m (246 ft), 1991–2020 normals, extremes 1872–present
Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Record high °C (°F)	16.1 (61.0)	21.4 (70.5)	26.0 (78.8)	30.2 (86.4)	34.8 (94.6)	37.6 (99.7)	42.6 (108.7)	39.5 (103.1)	36.2 (97.2)	28.9 (84.0)	21.6 (70.9)	17.1 (62.8)	42.6 (108.7)
Average high °C (°F)	7.6 (45.7)	8.8 (47.8)	12.8 (55.0)	16.6 (61.9)	20.2 (68.4)	23.4 (74.1)	25.7 (78.3)	25.6 (78.1)	21.5 (70.7)	16.5 (61.7)	11.1 (52.0)	8.0 (46.4)	16.5 (61.7)
Daily mean °C (°F)	5.4 (41.7)	6.0 (42.8)	9.2 (48.6)	12.2 (54.0)	15.6 (60.1)	18.8 (65.8)	20.9 (69.6)	20.8 (69.4)	17.2 (63.0)	13.2 (55.8)	8.7 (47.7)	5.9 (42.6)	12.8 (55.0)
Average low °C (°F)	3.2 (37.8)	3.3 (37.9)	5.6 (42.1)	7.9 (46.2)	11.1 (52.0)	14.2 (57.6)	16.2 (61.2)	16.0 (60.8)	13.0 (55.4)	9.9 (49.8)	6.2 (43.2)	3.8 (38.8)	9.2 (48.6)
Record low °C (°F)	−14.6 (5.7)	−14.7 (5.5)	−9.1 (15.6)	−3.5 (25.7)	−0.1 (31.8)	3.1 (37.6)	6.0 (42.8)	6.3 (43.3)	1.8 (35.2)	−3.8 (25.2)	−14.0 (6.8)	−23.9 (−11.0)	−23.9 (−11.0)
Average precipitation mm (inches)	47.6 (1.87)	41.8 (1.65)	45.2 (1.78)	45.8 (1.80)	69.0 (2.72)	51.3 (2.02)	59.4 (2.34)	58.0 (2.28)	44.7 (1.76)	55.2 (2.17)	54.3 (2.14)	62.0 (2.44)	634.3 (24.97)
Average precipitation days (≥ 1.0 mm)	9.9	9.1	9.5	8.6	9.2	8.3	7.4	8.1	7.5	9.5	10.4	11.4	108.9
Average snowy days	3.0	3.9	1.6	0.6	0.0	0.0	0.0	0.0	0.0	0.0	0.7	2.1	11.9
Average relative humidity (%)	83	78	73	69	70	69	68	71	76	82	84	84	76
Mean monthly sunshine hours	59.0	83.7	134.9	177.3	201.0	203.5	222.4	215.3	174.7	118.6	69.8	56.9	1,717
Average ultraviolet index	1	2	3	4	6	7	7	6	4	3	1	1	4
Source 1: Meteo France (snow days 1981–2010),[110] Infoclimat.fr (relative humidity 1961–1990)[111]
Source 2: Weather Atlas (percent sunshine and UV Index)[112]

 

These are 1990-2020 averages; of course things are getting warmer everywhere. Enjoy the coming heat wave!

[jack]

1 hour ago, Garald said:

We (meaning really: the architect and I) will install MVHR by insufflation (i.e., the reverse of the usual direction). One possibility is to install adiabatic cooling (either now or later) - apparently it goes well with this sort of ventilation system.

 

I've never heard the term "insufflation" used in the context of building ventilation. Is this some sort of positive pressure ventilation system?

[Garald]

5 minutes ago, jack said:

 

I've never heard the term "insufflation" used in the context of building ventilation. Is this some sort of positive pressure ventilation system?

I'm using the French term - there's most likely a different technical term in English. Fans are located so that air is sucked into the building, filtered, and, depending on the season, possibly heated or (as we are discussing now) cooled.

Edited July 18, 2022 by Garald

[JohnMo]

Not sure of the MVHR in reverse is.

 

Just been looking at the price of the coolers, they are not cheap, you could buy Aircon for the same money.  Not sure how effective the adiabatic cooling is at the air flow rates of MVHR as you are only doing a 0.3 to 0.5 ACH. 

[jack]

Ah, that sounds like positive pressure ventilation. 

Try putting this search term in google: positive pressure ventilation site:buildhub.org.uk

 

Incidentally, positive pressure ventilation isn't considered MVHR, because it doesn't include heat recovery (= the HR in MVHR). 

[Garald]

1 minute ago, jack said:

Incidentally, positive pressure ventilation isn't considered MVHR, because it doesn't include heat recovery (= the HR in MVHR). 

 

Right - I was translating the French term PIV. My architect has some ad hoc solution for heat recovery (if I understood correctly).

[Garald]

3 minutes ago, JohnMo said:

Just been looking at the price of the coolers, they are not cheap, you could buy Aircon for the same money.  Not sure how effective the adiabatic cooling is at the air flow rates of MVHR as you are only doing a 0.3 to 0.5 ACH. 

 

I think part of the idea is (a) much higher efficiency (b) ease of installation/space needed. I do have the option of getting a reversible heat pump, but it would be a very bulky Stiebel Eltron that would take up most of my storage space; it would make what is now a garage into a small and inconvenient bike shed.

 

But yes, my concern is whether adiabatic cooling will be enough, with heat waves such as the coming one becoming more frequent. Of course I'm also installing high-grade outside shades and the like.

[SteamyTea]

A number of things going on in your home at once, hard to keep track sometimes.

 

I am a great believer in doing 1 thing, but doing it really well.

So separating heating, cooling and ventilation may be better.

A traditional MVHR system is not designed and installed as a heating or cooling system. MVHR is really just to replace the air in an airtight building, in a controlled manner. The HR side of it is really just a bonus and a simple modification that scavenges some energy from the expelled air. 

With any heat recovery system, efficiency changes with the temperature difference.

 

It is possible to make a material difference to internal temperature and humidity levels with an air based heating and cooling system by oversizing the pipework and increasing the flow rates. This takes up more space and costs more to run. So probably only worthwhile in exceptional circumstances. Having said that, it may be possible to have just one or two rooms that have a greater flow rate, and fit FCU (fan coil units) in just those ducts. FCUs can be run from an ASHP in both heating and cooling mode.

You can also get MVHR units with build in heat pumps that can make a small difference.

 

As usual, there are many ways to sort out the same problem.

But basically all you need to do is control internal air for temperature and humidity, without causing condensation problems.

Edited July 18, 2022 by SteamyTea

[MikeSharp01]

This is a really hot (pardon the pun) topic today.

21 hours ago, SteamyTea said:

It is possible to make a material difference to internal temperature and humidity levels with an air based heating and cooling system by oversizing the pipework and increasing the flow rates. This takes up more space and costs more to run. So probably only worthwhile in exceptional circumstances. Having said that, it may be possible to have just one or two rooms that have a greater flow rate, and fit FCU (fan coil units) in just those ducts. FCUs can be run from an ASHP in both heating and cooling mode.

 

OK so we are at the install stage of the MVHR suppose I double up the pipe runs to the bedrooms and cool the air at the MVHR I wonder if could get close to replicating a fan coil unit in the same place. Appreciate that this will cost but its a toss up given the infrastructure fan coil units need. At this stage much easier to run ducts than highly insulated water pipes and we are squeezed for space in the ceilings. I dimmly recall you @SteamyTea doing some calcs on airs ability to carry cooling but I cannot find them on BH - probably looking in the wrong place. 

 

Did find this link which seems to say that if I can get to 2 ACH then it might be effective: https://www.greenbuildingstore.co.uk/can-an-mvhr-system-be-used-for-cooling/

 

Edited July 19, 2022 by MikeSharp01
Added link found

[SteamyTea]

8 minutes ago, MikeSharp01 said:

I double up the pipe runs to the bedrooms

Or double up the diameter and get 4 times the airflow.

[MikeSharp01]

2 minutes ago, SteamyTea said:

Or double up the diameter and get 4 times the airflow.

Yes maybe for those three rooms. Good thought!

[SteamyTea]

5 minutes ago, MikeSharp01 said:

Yes maybe for those three rooms. Good thought!

Easy to do if you can pipe directly from a unit in the loft.

[MikeSharp01]

32 minutes ago, SteamyTea said:

Easy to do if you can pipe directly from a unit in the loft.

No Loft! Unit is in GF Utility room but I think I can find space to run 4 x 90 rather than 2 x 70 flexi pipes.

[SteamyTea]

4 minutes ago, MikeSharp01 said:

4 x 90 rather than 2 x 70 flexi pipes

Other than the faff, and a bit of extra cost, I can not see a down side of running more/larger MVHR pies.

 

We could do the numbers and work out what the optimal size pipe area is.

 

Building Volume x ACH x Delta T x SHC x hours = Energy.

Then, from one of my temperature distribution charts, proportion the hours.

May find that you can have a low airspeed in the pipes and cover a lot of your heating with just FCUs.

[MikeSharp01]

3 minutes ago, SteamyTea said:

May find that you can have a low airspeed in the pipes and cover a lot of your heating with just FCUs.

Yes, we have passive standard insulation and the PHPP gives us only a few days (2% = 7 days) of overheating but that is obviously based on past temp data (2018 was our last update to the PHPP package) so we are not far from comfortable. Our only heating upstairs was to be a towel rail in the family bathroom and let the slab do the but if we have an FCU post the MVHR unit then we have that option as well.

[MikeSharp01]

51 minutes ago, SteamyTea said:

Building Volume x ACH x Delta T x SHC x hours = Energy.

Is that equation right, do I have the units correct - I guess it must be, its definitely me, but I get an odd outcome:

 

So 451m³ x 4  x 15 x 1.012 x 8 = 219077J = 7.6Js^-1 = 7.6W of cooling which sounds all wrong - need to go back the Thermodynamics 101, which I hated I seem to recall, - or maybe that 451 should be in l rather than M³ and/or the SHC should be the capacity of the whole volume - what have I missed?

 

4 ACH is the boost rate for the whole house which I guess you would not object to if you needed cooling.

[SteamyTea]

24 minutes ago, MikeSharp01 said:

that equation right

No idea, but you need the air volume converted to kg I think.

ACH seems high. And the 8 hour run time. 16 tonnes of air.

564 [kg] x 4[ACH] x 15 [°C] x 8[h] x 1 [SHC] = 270,600 kJ.

 

I get a similar figure.

Does seem high.

Edited July 19, 2022 by SteamyTea

[JohnMo]

If you can get 4 ach from MVHR your unit must be massively oversized.  Normal flow is 0.3 to 0.5 ACH and boost 25% or so above that.

[MikeSharp01]

22 minutes ago, JohnMo said:

If you can get 4 ach from MVHR your unit must be massively oversized.  Normal flow is 0.3 to 0.5 ACH and boost 25% or so above that.

Have not ordered the unit yet but thought I would go for a good amount of headroom to keep it quiet. 

[JohnMo]

I would check the min flow rates, as you don't want too many air changes in winter, as the air can get too dry.

[MikeSharp01]

25 minutes ago, JohnMo said:

I would check the min flow rates, as you don't want too many air changes in winter, as the air can get too dry.

Good thought - I had expected to be able to control the fan speed continuously (so right down to very slow) but if I only have a few fixed settings it may be more difficult I guess.

 

[SteamyTea]

55 minutes ago, MikeSharp01 said:

Good thought - I had expected to be able to control the fan speed continuously (so right down to very slow) but if I only have a few fixed settings it may be more difficult I guess.

 

Could you put and extra couple of smaller fans on the inlet and outlet pipes, then cobble up your own speed/pressure/volume control.

May need a bypass, or not.

[MikeSharp01]

1 hour ago, SteamyTea said:

No idea, but you need the air volume converted to kg I think.

ACH seems high. And the 8 hour run time. 16 tonnes of air.

564 [kg] x 4[ACH] x 15 [°C] x 8[h] x 1 [SHC] = 270,600 kJ.

 

I get a similar figure.

Does seem high.

These threads are getting mixed up. I redid the sums and I think that I would need about 3.7Kw of cooling to achieve that but if I drop the delta and the ACH it obviously comes down dramatically (proportionally). Looking around this unit (https://www.paulheatrecovery.co.uk/wp-content/uploads/2020/07/ComfoPost-CW6-PAUL.pdf) will almost get to the above, assuming the ASHP can cope.

[Dan F]

10 hours ago, MikeSharp01 said:

These threads are getting mixed up. I redid the sums and I think that I would need about 3.7Kw of cooling to achieve that but if I drop the delta and the ACH it obviously comes down dramatically (proportionally). Looking around this unit (https://www.paulheatrecovery.co.uk/wp-content/uploads/2020/07/ComfoPost-CW6-PAUL.pdf) will almost get to the above, assuming the ASHP can cope.

Couple of things I releaised (late) with the data in this spec sheet:

- The cooling power is based on 28C supply temperature from MVHR  (in practice it's never going to get to that due to the "heat (cool) recovery".. ours got up to around 26C at most today but typically it's only around 24C on a hot day. (a degree or two above internal temperature).

- The cooling power is relative to the MVHR supply temperature, not the room temperature.  So if room temperature is 20C, MVHR supply temperature is 24C and Comfopost output temperature is 12C, the effevtive cooling power is only 2/3 of that stated.  Depends how you look at it though, and if you MVHR would be on at same speed without a comfopost.

 

I'm seeing very minimal cooling of first-floor with our Comfopost (that's with automated blinds down).  It does mean though, that on days like today we can run MVHR on medium speed and not heat the house up in the progress.  The last few days we've been using UFH to keep ground floor at 22-23C and first floor has stayed under 24.5C throughput.  Not sure what it would have got to if we hadn't been running the Comfopost, but we'd most likely have need to put MVHR on "away" mode today to stop the first floor getting uncomfortably hot.

 

While the Comfopost outputs air at 12C, by the time it gets to the plenums in the first floor rooms (even though all ducting is insulated) it's at 16C.

 

 

Edited July 19, 2022 by Dan F

[jack]

9 hours ago, Dan F said:

The last few days we've been using UFH to keep ground floor at 22-23C and first floor has stayed under 24.5C throughput.  Not sure what it would have got to if we hadn't been running the Comfopost, but we'd most likely have need to put MVHR on "away" mode today to stop the first floor getting uncomfortably hot.

 

Following on from my earlier comment about being surprised that it wasn't sweltering upstairs the night before last, I had the same experience last night. Back-to-back hottest days on record after a week of high and increasing temperatues, and it was still only a bit warm upstairs. Perfectly comfortable to sleep with a pedestal fan on low.

 

I can only assume that by getting the underfloor cooling on early and leaving it running 24 hours a day for the last handful of days, the temperature upstairs hasn't had a chance to get out of control. It's a puzzlement.