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General: MVHR with pre-warmed supply air through ground pipe

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    MVHR with pre-warmed supply air through ground pipe

    I'm looking at a new build possibly with MVHR. I've seen the suggestion of pre-warming (or cooling in summer) supply air through a ground pipe laid around the foundations.

    The only drawback to this I have seen stated is concern about damp buildup and mould forming in the ground tube and leading to poor quality air coming into the system. Has anyone actually used this type of system, and if so what are your experiences?

    Thanks very much,
    Mark.
    • CommentAuthortony
    • CommentTimeFeb 2nd 2009
     
    I am proposing to collect heat via a bore hole with hydronic collection pipe to pre warm or cool incoming air to 13 C before it enters my MHRV

    Cant see the point of taking heat from near the foundations as this would be counter productive -- further away yes.
    • CommentAuthorSally M
    • CommentTimeFeb 3rd 2009
     
    We looked at this as well but the cost to put the pipe in, was so expensive, that it was not a viable option.
  4.  
    Posted By: Sally MWe looked at this as well but the cost to put the pipe in, was so expensive, that it was not a viable option.


    I'm guessing that the bulk of the cost was the installation, rather than the materials - is that correct?

    If so then I think that was part of the point of putting it in near the foundations - the hole is already dug, you just need to put the pipe down.
  5.  
    Posted By: tonyCant see the point of taking heat from near the foundations as this would be counter productive -- further away yes.


    Is this because you're concerned about cooling the ground near your foundations and therefore increasing the heat loss from the building?
    • CommentAuthortony
    • CommentTimeFeb 3rd 2009
     
    Yes
  7.  
    Try this:
    http://mb-soft.com/solar/saving.html

    In Germany they spiral a 8-12" pipe around a house foundations. Easy and cheap to do when building. Pretty common there.

    Look at this:
    http://tinyurl.com/bn4tyw

    The best Earth Pipe is one where the house is on a hill and the pipe runs away from the house, so any condensation runs away too.

    Google Earth Pipe and Earth Tube on groups and web.
    •  
      CommentAuthorPaulT
    • CommentTimeFeb 3rd 2009
     
    I have looked at this for our own HRV systems and have come to the conclusion that it is a huge investment for very little return...

    If your HRV system recovers 90% of the exhaust air heat (relative to the incoming air) then the heat loss through ventilation is small.

    Here are the calculations:

    Outgoing air 20C
    Outside air 0C

    Supplied air temperature 80% temp difference (to allow for duct losses) = 16C

    If you could pre-warm the incoming air (prior to the heat exchanger) Supply air = 17.6
    A huge amount of effort for a small difference in incoming air temperature.
    - This improvement is equivalent to imporving the overal system efficiency to 90% (better detailing/insualtion on ducting would achive this)


    We have a moderate and very wet climate, which in my view is not appropriate for this type of system - athough it would be good for somebody to set up a system and monitor it!

    It is a nice idea for hot climates to couple the air supply to underground temperatures, but like Ground source heat pumps the piping needs to be oversized.
  9.  
    Thanks for the links Water Systems.

    Unfortunately we're on pretty flat ground, so there isn't really any natural drainage to take into account. The soil is also mainly clay and tends to get quite wet. However, we are looking at a basement, which will almost certainly include a sump-pump for drainage.

    So, following the MB-Soft site we could arrange a series of pipes that drain down towards the house, and then allow for drainage down to the basement sump. We'd need to be careful that the system didn't try to suck air back out of the sump, but I guess this isn't that hard to do.

    Based on the Altechnica document, we'd need around 100M of 8" pipe , which seems a lot, especially if I want to avoid laying it near to the foundations. We do have a big enough plot to do this though.

    Are most MVHR systems capable of sucking air down such a long pipe, or would additional fans be needed?
  10.  
    90% MHRV heat recovery? Maybe that is by frigging it by putting the electric motor in the supply duct. Good cheating though, although some put in motors which may generate heat to boost figures, so beware.

    I did see a web site a few years ago (couldn't find it today, but never looked that hard) where the 8" plastic pipes were spiraled around the "inside" walls of the basement. Intake from outside. The basement generally is a stable temperature.

    The great thing about an Earth Tube is that is heats and cools. So you get both, so well worth doing the sums on the investment. While the foundations are uncovered and diggers on site, the cost must be minimal. Although a retrofit like the Chicago system in the garden is quite easy to do.
  11.  
    Posted By: Water Systems The great thing about an Earth Tube is that is heats and cools. So you get both, so well worth doing the sums on the investment. While the foundations are uncovered and diggers on site, the cost must be minimal. Although a retrofit like the Chicago system in the garden is quite easy to do.


    Yes, in fact it's the cooling that interests me at least as much as the heating, especially in those rare hot summer nights we have....
  12.  
    If there is a concern about humidity or mould growth, consider running the air through a heat exchanger in the house to avoid this.

    Then you could have only 10% fresh air from outside and rest re-circed. Run this internal air through the earth pipe heat exchanger. So, still only one heat exchanger used, but maybe two fans.
  13.  
    I don't think I understand this suggestion. If I only take in 10% or fresh air, then won't the house become stale?

    I thought the MVHR was designed to provide (a safety margin more than) the minimum number of air exchanges needed to keep the air fresh? (I even recall seeing one system with CO2 sensors to vary it's flow rate depending on demand...)
  14.  
    PaulT - thanks for your calculations.

    It seems to me that if the MVHR is much less than 90% efficient then the ground pipe might be worthwhile for heating in the colder months. Above that then it's probably dubious.

    However, I'm more interested in the potential for cooling in the summer. A nice influx of 12-14C air (heat exchanger bypassed) mixed with ambient internal air could make for much more comfortable summer days/nights for a potentially relatively small investment, or am I dreaming?
  15.  
    Posted By: MarkBennettI don't think I understand this suggestion. If I only take in 10% or fresh air, then won't the house become stale?


    I should have said from 10 to 100%. It depends on the amount of air being shifted.

    In commercial HVAC, 10% minimum fresh air has to be introduced. Most MHRV systems are quite small and don't shift too much air.

    However if I was building a well insulated eco house, I would consider up sizing the ducting a little and insert a copper heat coil battery and have the MHRV system do the heating as well. That is fine if only using an oil or gas boiler - fossil fuel to maintain high temps.

    The problem is that copper coils require a high flow temperature and if using predominately heat pumps and/or solar then lower temps are needed then UFH may be the answer.

    Many fit gas or oil boilers and have MHRV and UFH, which is a crazy expensive setup when a means of heat distribution has already been fitted - the ducting. The UFH can be dispensed with and the money spent on insulation and air-tightness.
  16.  
    Here is the Earth Pipe run around the basement slab - under the house. Worth reading:
    http://www.thenaturalhome.com/earthtube.htm
    •  
      CommentAuthorPaulT
    • CommentTimeFeb 3rd 2009
     
    Just typed a long reply and lost it!

    So now a summary:

    We do get 90% heat recovery and have test data to prove it! The Passiv Haus institute have tested and certified several system as values even higer than this - so not a cheat!

    Water S - you are desacribing a Passiv Haus concept - already working in 100's of homes.

    HRV's supply 100% fresh air (by design).

    Do not overheat supply air - it will become very dry and cause dust problems.

    Better to have a secondary heating system for absolute peak loading.

    Oversized ducting - we duct individualy to each room from the central unit.

    Small HRV systems - I agree - so we oversize everything apart from the fans - big heat exchange, big grilles & big ducting means smaller fans!
    (simple really, just not as convenient for the builder).

    One of the reasons why we have not gone down the earth supply route (although we have been asked to).
    "Cleaning earth tubes is an easy matter - simply lay nylon cord in the pipes while constructing and use it to pull a bleach or disinfecting solution soaked towel through the tubes" - earth tube web site.

    Mark - summer cooling is the biggest potential benefit - but consider a warm and humid night - the reduction in air temp will cause condensation in the pipes! It would be good if you went for it and could provide monitoring data. You would need temperature senors anyway to monitor indoor, outdoor and pipe temperatures to switch in-between supplies. I think there was mention of seperate heat exchanger to dump the underground air and cool fresh air prior to supplying - this may be the safest option.

    With good shading a home could be cooler inside than out during the day in summer, so a HRV system would then work in reverse - keepin gthe home cool.

    Key to all of the above is shading as it does not need electrical power!
  18.  
    So, just by coincidence an email was sitting in my inbox with information about the upcoming ECOBuild show in London. One of the topics mentioned in the linked flyer was very relevant:

    http://www.rehau.co.uk/building.solutions/civil.engineering/ground.heat...geothermal.energy/awadukt.thermo.shtml

    It seems that Rehau are promoting a full product for ground air pre-warming/cooling. I'm dubious, and it's bound to be expensive but I'll be at the show and will try to talk to them.

    I'll let you know what I think after the show in March.
  19.  
    Paul, overall it sounds like we're in general agreement.

    Posted By: PaulTWith good shading a home could be cooler inside than out during the day in summer, so a HRV system would then work in reverse - keeping the home cool.


    Do you mean shading of the house, or shading of the ground pipe collection area?

    Hopefully the former, since the latter is surely irrelevant?

    With good insulation and limited south facing windows I guess lack of shading of the building wouldn't be so bad. Besides, if the building was shaded the solar collectors on the roof wouldn't work so well.

    P.S. sorry you lost the long email!
    •  
      CommentAuthorPaulT
    • CommentTimeFeb 3rd 2009
     
    Sahding of the house.

    Interestingly South facing windows are OK:

    The reason is that in summer the sun is at a high angle durring the day and so can be effectively sahded with relative ease.

    In the winter the sun is lower an light penetrates through them providing Solar gain (That is why inside walls facing South windows are the the onse to buiold using high thermal mass rather than your outside walls).

    West facing windows are bigger problem in summer - as the sun drops it can get underneath shading and warm up the home at the end of the day (bad timing).

    My prefered solution for windows is to copy our Continental cousins - shutters (roller). simple raly - extra shelter on a winters night, shading in summer and wide open to let light in at other times.

    I have seen the System you mention and it was several 1000's, so even though we are in the HRV business I advise people to spend money elsewhere (we are really consultant's who sell key system that I believe every home should have (Indoor air quality being my big issue, actualy more important than energy as my son has Asthma and has been criticaly ill at times - one of the reasons why I am so passionate about environmental building).
  21.  
    Posted By: PaulTI have seen the System you mention and it was several 1000's, so even though we are in the HRV business I advise people to spend money elsewhere (we are really consultant's who sell key system that I believe every home should have (Indoor air quality being my big issue, actualy more important than energy as my son has Asthma and has been criticaly ill at times - one of the reasons why I am so passionate about environmental building).


    I had a feeling it was going to be expensive and if it's as expensive as you suggest then it's not worth it. It will be interesting talking to them though.
    • CommentAuthorralphd
    • CommentTimeFeb 3rd 2009
     
    A heat pump HRV makes more sense. In summer it can provide dehumidification while an earth tube does not (or does so little it can be dismissed).

    HRVs aren't really needed in most houses; even tight houses get plenty of natural air infiltration. Only if you are in PassivHaus territory (<0.6ACH@50) would I consider mechanical ventilation. In colder climates that cause greater stack effect air infiltration you get a natural air infiltration rate of ~0.2ACH with a house that tests 1.5ACH@50Pa. You only need ~10cfm per person for healthy indoor air.
    http://eetdnews.lbl.gov/nl16/ASHRAE.html

    Here in Canada HRVs are common (most provincial building codes require them), yet I've found they are not necessary and can reduce indoor humidity to unhealthy levels.

    According to Hot2000, the F326 Required continuous ventilation rate for my house is 211.9 cfm ( 0.24 ACH). My last blower door test for my ~54,000ft^3 house was <1.3ACH@50 and my HRV installer tested the flow rate of my HRV as 87cfm on low speed. My research indicate healthy wintertime humidity levels in the range of 40%-50%RH with at a 20C indoor temperature. Over 50% increases risks of mold/fungus growth along with dust mite proliferation. Under 40% there are comfort and health issues as well.
    http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0030151

    At 40%RH & 20C there are 6.9 g of water vapor per cubic meter of air (a dew point of ~5C). According to Environment Canada data, the average outdoor dew point for the heating season in Truro, NS is below -5C(actually close to -10C). A -5C dew point = 3.3g/m^3. After 1 air change without added moisture the relative humidity will drop below 30% (1 air change looses 1.8g/m^3). For my house, that equals 2.75kg of water lost per air change. At .24ACH, 15.8kg of water vapor needs to be added to the air each day to maintain 40%RH. With 2 adults and 4 children, up to 7kg of water vapor should be produced from breathing, cooking, bathing, etc.

    I did not have an issue with low humidity last year (my first winter of occupancy). This is easily explained by the drying out of the house. I measured the wood moisture content in my house before occupancy at 13-15%. Equilibrium moisture content for softwood at 20C/40%RH is 8%. The ~6 tons of wood inside my house vapor barrier has to loose ~300kg of water to go from 13 to 8% moisture content. My recent measurements are ~10%MC, indicating significant drying has
    occurred.

    According to CBD 110, as little as 3CFM per person of fresh air is needed to maintain healthy CO2 levels.
    http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd110_e.html

    So I'd say either put in a heat-pump HRV so you can get summertime air condition, or go with the cheapest HRV you can get and just use it when you need bathroom/kitchen exhaust.
  23.  
    Posted By: ralphdA heat pump HRV makes more sense. In summer it can provide dehumidification while an earth tube does not (or does so little it can be dismissed).

    HRVs aren't really needed in most houses; even tight houses get plenty of natural air infiltration. Only if you are in PassivHaus territory (<0.6ACH@50) would I consider mechanical ventilation.


    That is a very valid point. Many people are venting their homes twice when using a MHRV. However the MHRV does prevent household smells from infiltrating the building fabric giving a permanent fresh feel to the house.


    So I'd say either put in a heat-pump HRV so you can get summertime air condition, or go with the cheapest HRV you can get and just use it when you need bathroom/kitchen exhaust.


    In the UK a superinsulated home of high thermal mass with passive solar orientation is the ideal starting base. Then expensive heat pumps and whatnot will not be required and just a small LPG, natural gas oil boiler with DHW solar assist. Those with cheap wood around may want to consider a wood burner.

    The heating requirements of the house would be so small the boiler would rarely cut in. Metal penetrating the building fabric (flues) can cause havoc with cold bridging. A house with so little heating requirements would be better being all electric, which is service free. Then no service charges for gas or oil boilers to negate the higher running costs and also no gas meter or LPG tank standing charges. The capital cost of installing electric heating is also far less too.

    Depending on the air-tightness, as you have mentioned a MHRV system may not be required, just extract at source: cooker hob and bathrooms with heat recovery fans.

    Grey water re-cycling/rainwater capture is usually worth it.

    Nice post.
    • CommentAuthorralphd
    • CommentTimeFeb 4th 2009
     
    Wood is OK as a main heat source, but in cold climates you need a backup. Otherwise when you go someplace warm for vacation in the winter, you can come back to a house with frozen pipes. Passive solar can work against you too; the more glazing you have then the greater your peak heat load is. On a cloudy day when the temp is -20C you loose more (net) heat from the window in 24hrs than you would from a well-insulated wall of the same size.

    Getting off topic though...

    To tie it back in, if you have a normal wood stove (not sealed combustion) a HRV is a good idea.
  25.  
    Posted By: ralphdPassive solar can work against you too; the more glazing you have then the greater your peak heat load is. On a cloudy day when the temp is -20C you loose more (net) heat from the window in 24hrs than you would from a well-insulated wall of the same size.


    You install movable insulation. Internal shutters, which also are security aids as well. In France they have the shutters between two pains of glass and a handle winds it down.

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