MVHR 'Reverse' Operation

[MJNewton]

As is mentioned from time to time here the heat transfer in an MVHR is bidirectional insofar that heat transfer occurs in whatever direction the temperature delta is. Thus, if the outside air is warmer than inside then heat will be transferred from the intake air to the air extracted from the house and in doing so cool the supply. I normally only see a degree or two tops in practice but the recent heatwave gave some more extreme figures producing just shy of 5°C of cooling on occasion:

 

 

Of course, with the low air volumes and heat capacity it likely didn't make all that much difference but every little helps - even if only psychologically! I think I've hit refresh on my monitoring charts and Met Office forecast app more times in the last two days than the last two years!

 

Also evident from the chart though is that even with the summer bypass activating when appropriate (extract < intake and all relevant thresholds being met) there was still some heat transfer taking place and raising the incoming air by a couple of degrees. In my unit the bypass diverts the extracted air around the side of the heat cell and so I suspect there's still a chance some heat transfer will take place from the close proximity, and not to mention the 40°C loft space that's going to eventually heat the unit up somewhat despite its insulation.

Edited July 20, 2022 by MJNewton

[Russdl]

  On 20/07/2022 at 22:32, MJNewton said:

I normally only see a degree or two tops in practice but the recent heatwave gave some more extreme figures producing just shy of 5°C of cooling on occasion:

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Really interesting that you mention that.
 

Over the last hot spell we saw a difference of 9 degrees for exactly the reasons you’ve explained, but why is our temperature difference so much larger than yours? We have an enthalpy heat exchanger, does that make the difference or are we just lucky?!?

[Dan F]

I saw temperature difference of up to around 12C, with a max supply temperature of around 26C when it reached 24C exhaust temp at one point when it was 39C externally.

 

This make sense because the efficiency of say 85% gives:  supply temp = 24 + (39-24)*0.15 = 26.25C.  Is this correct?  We also have a enthalpy exchanger, but I dont see why this woudl have an impact.. in fact ERV generally has lower efficiency than HRV.

 

 

[Marvin]

  On 20/07/2022 at 22:32, MJNewton said:

As is mentioned from time to time here the heat transfer in an MVHR is bidirectional insofar that heat transfer occurs in whatever direction the temperature delta is. Thus, if the outside air is warmer than inside then heat will be transferred from the intake air to the air extracted from the house and in doing so cool the supply. I normally only see a degree or two tops in practice but the recent heatwave gave some more extreme figures producing just shy of 5°C of cooling on occasion:

 

 

Of course, with the low air volumes and heat capacity it likely didn't make all that much difference but every little helps - even if only psychologically! I think I've hit refresh on my monitoring charts and Met Office forecast app more times in the last two days than the last two years!

 

Also evident from the chart though is that even with the summer bypass activating when appropriate (extract < intake and all relevant thresholds being met) there was still some heat transfer taking place and raising the incoming air by a couple of degrees. In my unit the bypass diverts the extracted air around the side of the heat cell and so I suspect there's still a chance some heat transfer will take place from the close proximity, and not to mention the 40°C loft space that's going to eventually heat the unit up somewhat despite its insulation.

Expand  

The 40C+ heat in our loft caused the inlet air temp to increase dramatically. I am working on shortening the inlet pipe and increasing the insulation on it. Secondly I am ready to instal 2 thermally controlled fans to extract the heat from the loft once I can work up there. We will see what difference that makes..  

[MJNewton]

  On 20/07/2022 at 22:54, Russdl said:

Really interesting that you mention that.
 

Over the last hot spell we saw a difference of 9 degrees for exactly the reasons you’ve explained, but why is our temperature difference so much larger than yours? We have an enthalpy heat exchanger, does that make the difference or are we just lucky?!?

Expand  

 

I guess it could be our house!

 

We're in a 2007 Persimmon house (boo hiss!) and so might not have the levels of insulation and detailing that you've got? We've got some large sliders and a roof lantern downstairs - both unshielded from solar gain - and indeed the sliders were open most of the time due to constant child traffic and so we were getting high-20s downstairs at least and as we were only(!) seeing 36°C outside our -5°C of cooling sounds like about the right ballpark for a notional 85% efficiency?

Edited July 21, 2022 by MJNewton