MVHR Air - Ground Heat Exchanger?

[Conor]

During one of my many lunchtime build project online research sessions (procrastination!) I stumbled across the concept of ground - air heat exchangers for ventilation.

 

It seems simple, air passes through several meters of buried pipe prior to entering the ventilation unit. This ensures that the inlet air is always 6-12c all year round. I can see an advantage for summer cooling, along with reducing heat losses from the exchanger in winter nights as the incoming air will be warmed slightly. Would also mitigate any frost problems in the exchanger, no pre-heater required, so a cost saving there.

 

Off-the shelf kits seem to be £3k upwards... and without doing any actual calculations, I'm assuming this is significantly more than any winter heating or summer cooling savings.

 

With my experience in hydraulic engineering, I'm sure I could replicate a system for a few hundred pounds using 3x 110mm PVC drain pipes (would need to calculate losses of course), laid in parallel around the perimeter of the house.

 

We are building a basement and have ~50m long, ~3m deep and ~1-2m wide excavation that will be back-filled with red clay and fine stone. So there would be no excavation cost if I were to do this. I'd just need to install the network of pipes, a condensation chamber (simple enough as land drain would be directly below and I'd have enough cover for a constant fall in the pipes), inlet stack (would be hidden and sheltered behind the side door porch return), and manifold (would either re-purpose a standard 200mm manifold or have something manufactured) connection to the 200mm inlet duct on the unit, and some sort of bypass for when air temp is preferable to soil temp (12-18c in spring and autumn).

 

Am I missing anything? Am I mad??!!!

 

Would look very similar to this, but my 110mm pipes would be stacked vertically from 3m-2m depth of cover.

 

 

 

Edited August 7, 2020 by Conor

[ProDave]

I am sure this has been discussed before and there is a real risk of mould and bacteria forming on the inside of the underground pipes and causing serious problems with your air quality.

 

I believe the recommendation for something like this is silver plated pipework that stops mould growth?

[Conor]

28 minutes ago, ProDave said:

I am sure this has been discussed before and there is a real risk of mould and bacteria forming on the inside of the underground pipes and causing serious problems with your air quality.

 

I believe the recommendation for something like this is silver plated pipework that stops mould growth?

 

Ahhh, good point. That would explain why the commercial kit pipes are so expensive... but good falls, drainage and anti-bacterial paint should solve that issue. Will search for previous topics.

 

Edited August 7, 2020 by Conor

[pdf27]

Have a read of http://passivehousepa.blogspot.com/2012/12/earth-tubes-heating-and-air.html : they used a system which actually makes a lot of sense to me but nothing like it is available commercially. They use a corrugated pipe (of the type used for field drains) slit along the bottom over it's entire length. It has to be in free-draining soil, but if it is then any condensate will drain straight out cleaning the walls and presumably giving a relatively inhospitable environment for mould/bacteria when compared to a sealed tube which is at risk of spots of standing water.

Best of all, it's really cheap - about £1.50/metre plus the cost of digging and back-filling the hole - and provided you ensure that you have an alternative air inlet if it all goes horribly wrong then it looks like a good gamble to me. It's totally dependent on soil conditions though - needs to be free draining and well above the water table (mine is neither!).

[Conor]

19 minutes ago, pdf27 said:

Have a read of http://passivehousepa.blogspot.com/2012/12/earth-tubes-heating-and-air.html : they used a system which actually makes a lot of sense to me but nothing like it is available commercially. They use a corrugated pipe (of the type used for field drains) slit along the bottom over it's entire length. It has to be in free-draining soil, but if it is then any condensate will drain straight out cleaning the walls and presumably giving a relatively inhospitable environment for mould/bacteria when compared to a sealed tube which is at risk of spots of standing water.

Best of all, it's really cheap - about £1.50/metre plus the cost of digging and back-filling the hole - and provided you ensure that you have an alternative air inlet if it all goes horribly wrong then it looks like a good gamble to me. It's totally dependent on soil conditions though - needs to be free draining and well above the water table (mine is neither!).

 

Good find. 150mm twinwall pipe would be a better option as I'd only need two of them, rather than four 110mm PVC pipes.

 

Not so sure about the slit... we're above the water table at the minute and the ground will be well drained and pipes will most likely go in gravel or sand. But I'd still be worried about ingress or standing water. Think I'd prefer to have Tees every couple of meters with down pipes to the drains below. 

[A_L]

@Conor Take a look at 'brine earth heat exchangers' as an alternative

 

https://www.sole-ewt.de/PDF/en/Sole-EWT-Info-en.pdf

[Gone West]

5 hours ago, Conor said:

I stumbled across the concept of ground - air heat exchangers for ventilation.

I looked at the Rehau system eleven years ago when I was researching for my build. I discounted it on the grounds of cost and complexity relating to requiring a pumped sump and an lot of silver lined pipework. There were also a lot of scare stories around at the time about contaminated ventilation air supply.

[pdf27]

3 hours ago, Conor said:

 

Good find. 150mm twinwall pipe would be a better option as I'd only need two of them, rather than four 110mm PVC pipes.

 

Not so sure about the slit... we're above the water table at the minute and the ground will be well drained and pipes will most likely go in gravel or sand. But I'd still be worried about ingress or standing water. Think I'd prefer to have Tees every couple of meters with down pipes to the drains below. 

As soon as you do that you've either got a direct air path to the drains, or you've got standing water in the bottom of the tees. You're also starting to add complexity and hence cost.

The real problem is that in a UK climate it saves very little energy - you don't need the defrost heater on an MVHR, and it provides a little bit of comfort cooling in summer. That means it's got to be seriously cheap for it to make any sense - if not you're better off spending the same money elsewhere, for example on a couple of extra PV panels. The running costs are also non-zero - either a longer inlet duct with a bigger pressure drop across it or a circulating pump for the brine loop - which needs to be set against any energy saving.

For other climates - e.g. in the US - it makes much more sense: they regularly spend extended periods of time well below freezing in winter and have extended periods of high heat and humidity in summer, when it will help very significantly with the dehumidification load.

 

[Dan F]

The commercial options I have seen use brine loop and heat exchangers, which avoids some of the concerns but adds to price.

 

For our build I modelled the use of a gound brine loop in both winter and summer:

- In the winter the benefit (given very high level of insulation and high MVHR efficincy) was very low.

- The benefit in the summer was more significant, but given we were already planning PV/ASHP/UFH, we felt it made more sense to do UFH/MVHR cooling in the summer with free PV, rather than add another system and additional cost.

 

[Nickfromwales]

19 minutes ago, Dan F said:

The commercial options I have seen use brine loop and heat exchangers

Not overly expensive if you have a machine on site

[Conor]

10 minutes ago, Nickfromwales said:

Not overly expensive if you have a machine on site

That's not bad at all.

[Nickfromwales]

6 minutes ago, Conor said:

That's not bad at all.

The one I worked on seemed a robust setup, but will only work as well as it is installed eg buried the correct depth and in suitable ground.

I also introduced a PHE and a couple of zone valves to introduce heated water from the heating system into the same BPC duct heater / cooler. Effects were instantly notable in 'heating mode' and I expect it would work well as a passive unit also. At that price, nothing stopping you doubling up and fitting 2 lots of the kit with the duct coolers in parallel for larger properties or higher yield. 

[Nickfromwales]

Quote:

"The area where the undersoil tube is laid should not be built over so it is guaranteed that seeping rainwater can contribute to the thermal regeneration of the soil"....

so anyone thinking 'kill two birds with one stone' on the dig-out for the dwelling, nope, it has to be outside the build envelope. One client I had simply dug extra deep around the periphery where rainwater systems were to go and economised that way.

[pdf27]

36 minutes ago, Conor said:

That's not bad at all.

They've got some data on power consumption too...

Antifreeze pump is 3-45W, assume 15W.

Air resistance when cooling is 70Pa at 250m³/hr (about right for a 200m² house if I'm understanding correctly) -> 5W additional fan power @ 100% efficiency, so allowing for real life efficiency total system draw will be about 30W.

 

Using their example for cooling, with air into the MVHR at 16°C and ambient at 28°C the cooling power you get (assuming no dehumidification) is (1010 J/kg.K) x (1.2 kg/m³) x (250/3600 m³/hr) x (28-16)= 1kW of cooling for a COP of about 40.

 

Cooling power is 84W x temperature difference attributable to the ground loop (16°C in this case - their figures). DegreeDays.net gives 73 degree-days above 16°C in the past year at Aldergrove (closest station I could find to you) - 84 x 73 x 24 = 147,168 Watt-hours (147 kWh) of cooling. In reality this will be a bit better because it will also do some dehumidification.

Same calculation for heating power gives 103 degree-days below 5°C: a 208 kWh saving.

 

To me the numbers don't stack up as a means of energy saving - 400kWh/year is the output of a couple of PV panels, and in reality probably overstates things since night venting will probably deal with a big chunk of the cooling requirement for free and the frost protection on most MVHR systems probably kicks in significantly below 5°C.

The only times I think it makes sense is if you're trying to hit a very strict energy requirement for some sort of standard when the very high COP is of value, or if you have a restriction (planning or similar) preventing you from using an ASHP for summer cooling.

[Conor]

13 hours ago, pdf27 said:

They've got some data on power consumption too...

Antifreeze pump is 3-45W, assume 15W.

Air resistance when cooling is 70Pa at 250m³/hr (about right for a 200m² house if I'm understanding correctly) -> 5W additional fan power @ 100% efficiency, so allowing for real life efficiency total system draw will be about 30W.

 

Using their example for cooling, with air into the MVHR at 16°C and ambient at 28°C the cooling power you get (assuming no dehumidification) is (1010 J/kg.K) x (1.2 kg/m³) x (250/3600 m³/hr) x (28-16)= 1kW of cooling for a COP of about 40.

 

Cooling power is 84W x temperature difference attributable to the ground loop (16°C in this case - their figures). DegreeDays.net gives 73 degree-days above 16°C in the past year at Aldergrove (closest station I could find to you) - 84 x 73 x 24 = 147,168 Watt-hours (147 kWh) of cooling. In reality this will be a bit better because it will also do some dehumidification.

Same calculation for heating power gives 103 degree-days below 5°C: a 208 kWh saving.

 

To me the numbers don't stack up as a means of energy saving - 400kWh/year is the output of a couple of PV panels, and in reality probably overstates things since night venting will probably deal with a big chunk of the cooling requirement for free and the frost protection on most MVHR systems probably kicks in significantly below 5°C.

The only times I think it makes sense is if you're trying to hit a very strict energy requirement for some sort of standard when the very high COP is of value, or if you have a restriction (planning or similar) preventing you from using an ASHP for summer cooling.

That's a brilliant breakdown, thanks.

 

Would be even lower return for us here, we're by the sea so temp range is lower than at aldergrove, which is inland and at altitude. temp only goes below zero a couple times a year, and air temp is never above low twenties. 

 

Our 5m wide bifolds will be our primary summer cooling device! 

 

Having an ASHP and PV panels... May as well use those for cooling if needed.

Edited August 8, 2020 by Conor

[pdf27]

7 minutes ago, Conor said:

That's a brilliant breakdown, thanks.

 

Would be even lower return for us here, we're by the sea so temp range is lower than at aldergrove, which is inland and at altitude. temp only goes below zero a couple times a year, and air temp is never above low twenties. 

 

Our 5m wide bifolds will be our primary summer cooling device! 

 

Having an ASHP and PV panels... May as well use those for cooling if needed.

Under those circumstances I really don't think even a DIY system makes sense - night venting via the MVHR will probably provide all the cooling you need even without opening the windows, and the power used by the MVHR defrost heater will be minimal.

If you're only trying to provide a small amount of cooling and have an ASHP anyway, it's almost impossible to beat for a well-insulated house. Small total heat flow and very high COP (from the small temperature difference) means virtually no running costs and zero installation costs.

 

It's very easy to get carried away by a good idea which almost works. Solar Thermal does this for me - I really want it to work, but because we would have an ASHP then more PV is simply a better and cheaper solution to achieve the same thing. Both systems are very similar - they turn a small amount of electricity into a lot of heat: however, that is the wrong problem to solve because it ignores the impact of capital costs and the ability to spend the same money generating electricity.