1 Dec 2010

Low energy homes - they don't always work

I've been banging on about low energy homes on this blog for years, and promoting the concept to all who come here. Whether it's the full blown PassivHaus standard, or a less taxing version with maybe SIPS, MVHR and a wood burning stove, it seems to me to be a no-brainer.

But sometimes it's good to pause and reflect. And it has to be admitted that building a low energy house is not always as plain sailing as advocates would like to admit. They don't all work as planned, and sometimes it's hard to put a finger on exactly what has gone wrong.

Yesterday I took a phone call from a selfbuilder in Oxfordshire who has just built a massively insulated, near airtight timber frame house which is heated by underfloor heating on the ground floor (powered by a heat pump), a wood burning stove in the living room and an MVHR ventilation system to distribute the heat around the house. All indications were that the house was well built and it had achieved an airtightness score of 1.7a/c/hr@50p (which is extremely good). Yet whilst the heated downstairs of the house was basking in temperatures in the 20°Cs, the upstairs rooms were stuck at 14°C. All that acoustic insulation put in the floors to meet Part E of the building regs was stopping the heat rising into the bedrooms, and the MVHR system simply wasn't redistributing the heat as had been hoped for. The ventilation system even had a 1kW post-heater installed in the inlet ducting, and yet still the upstairs was still freezing.

And that wasn't all. The wood burning stove was also causing problems. Despite having an airflow ducted directly to the stove, it wasn't burning efficiently and was smoking through the vent hole. And when the door was opened to re-fuel, large amounts of smoke were escaping into the living room - the so-called spillage problem already discussed on this blog. And the heat generated from the wood stove wasn't finding it's way into the bedroom upstairs.

This wasn't as it was planned. The finger of suspicion would seem to point at the commissioning of the MVHR ventilation, and the suspicion must be that the system hasn't been balanced correctly. But the installers were adamant that it wasn't their fault and were proving difficult to deal with. "We told you it wasn't a heating system, " they said, "you are expecting far too much from it." Which leaves the selfbuilders here in a bit of a hole, because what they have designed is in effect quite a complex, interactive building which depends on several different elements working well with one another. And when the outcome doesn't meet expectations, then it's damn difficult to know what to do about it.

The fallback is always to install a central heating system throughout the house. But a lot of low energy designs sell themselves on the fact that they will cope with the severest of conditions without recourse to conventional heating, and it's difficult not to be tempted by this logic. I've spoken to many selfbuilders who have done away with conventional heating systems and been delighted by the performance of their homes - incidentally the Denby Dale PassivHaus is just such a case, performing really well during this prolonged cold snap - but it's good to bear in mind that not everyone who tries this route is delighted with the results, and that if things don't work out as expected, then it leaves the client in a hole.

27 comments:

  1. The house is suprisingly cold upstairs. Airtightness good, could the problem be with the site fixing of the insulation. What "u" value did they hope to achieve

    ReplyDelete
  2. I do not have a explanation for the
    MVHR ventilation system and the poor heating upstairs. However, the builder should of invested in a norwegian stove jotul and insulated chimney blocks. Many houses in norway use wood burning stoves with no such ploblems

    ReplyDelete
  3. Interesting article. I can relate to some of this. Our house doesn't have MHVR but we do have a similar pattern of upstairs/downstairs temperature. That glass fibre between the floors was a pain to install and not even that good at sound deadening but it certainly stops the bedrooms getting warm.

    Our new Morso 1648 woodstove has taken some getting used to after older stoves we have had(like the wonderful Jotul 602). It is a modern stove that meets all sorts of standards and gets you lots of eco-points but it needs to be handled very differently. Essentially it burns a small amount of wood quickly rather than a large amount slowly. Having the tertiary air vents permanently open is great for air quality and overall efficiency but it does reduce the effective draw from the chimney and seems to make it harder to control. (We have fancy insulated flue too so it's not the chimney at fault)

    Getting it to light from stone cold can be a challenge - the key seems to be to get it blazing as soon as can possibly be done. I light it with very loosly compressed newspaper and lots of very finely chopped kindling and leave the door ajar to get is roaring for a few minutes. These stoves need to burn fairly fast to work well, especially when cold. I suggest that if the fire is smoking when refueled, it's not hot enough. When the fire and chimney are really warm they are less picky.

    ReplyDelete
  4. On a broader front it makes me wonder how many of these ventilation and heat recovery systems are being switched off because the occupants either do not like them or do not understand what they do.

    And why in this case has this set-up not been commissioned properly? Presumably because it has never been fully tested in winter and the problems are now only coming to light. Perhaps the smoke from the spillage in the living room could be used to track the ventilation paths elsewhere!

    ReplyDelete
  5. Thanks for all the helpful comments. I don't know what the fabric U values are, but I understand that they beat the current regs.

    In theory, it should all work like a dream.....

    ReplyDelete
  6. Dave HoworthDecember 02, 2010

    "In theory, it should all work like a dream"

    I'm not sure why you think so? There are two issues:

    (1) An MVHR system can't transport enough heat to keep a house warm unless the house is insulated to at least Passivhaus level. That's basically how the Passivhaus insulation level was decided. Since there doesn't seem to be any claim that this house is a passivhaus, I wouldn't expect an MVHR system to work well for heat distribution, as the supplier indicated.

    (2) Even if the house is insulated to passivhaus level, the supply air needs to be heated to about 50 C in order to be able to deliver sufficient heat. That's why the air is heated within the ducts. If the heat comes from a room source, then that room needs to be heated to 50 C, which would make it pretty uncomfortable.

    So to me, it sounds like a basic design/specification error.

    ReplyDelete
  7. Dave's comments are spot on just what I was going to write.

    ReplyDelete
  8. The finger of blame points to whoever said that a MHVR system can distribute temp differences around the house. It moves half the house's volume of air per hour - how much heat does that contain and move? not a lot - and it only returns 85% of that. Add to that, the natural chimney effect - heat rising - doesn't work because there's no lower inlet and upper outlet in an airtight house. With floors stuffed with insulation, that leaves only buoyant convection (should work in the stairwell) and mixing. Do they leave the doors open?

    ReplyDelete
  9. I would tend to agree with many of these comments but I have come across several people building SIPs homes who have used MVHR to distribute heat around the house and have been extremely happy with the results. Possibly this has been more by luck than judgment!

    ReplyDelete
  10. That is what is supprising, it works with Sips, the only reasons I can see for this house not performing are poor u Values, excessive thermal bridging, and the floor insulation. But you would still expect the heat to move up the stairwell.

    ReplyDelete
  11. MVHR can't heat large, low-occupancy detached houses. Heat loss is too high for a low air flow to cope. Also 1.7 ac/h is three times as leaky as Passivhaus. It doesn't meet the 30 year-old R-2000 standard either. I'm sad to hear that they're so unhappy but not surprised. They should have fitted small radiators.

    Added to that the real test of a heating system isn't normal or mild winter weather as we had for a decade, but severe winters; e.g., 2009-10 and the current cold spell. At a daily temperature of -10 degC (Scotland) or -5 degC (Midlands), heating systems sized for -1 degC are unable to cope. If their present heat loss is 3 kW (24/7) and they're putting in 2 kW, mostly downstairs, from wood and electricity, upstairs rooms which aren't directly-heated will inevitably be cold.

    This problem occurs to some extent even in low energy concrete-floored buildings with badly; i.e., overoptimistically, designed heating systems. Insulated floors are definitely set to make it worse.

    ReplyDelete
  12. I agree with many of the comments previously made. An additional factor may be that SAP may have been used in the design process - and SAP has been shown to be a very poor at modelling low energy homes.

    In contrast, the PHPP software (intended for Passivhaus design) is known to be an excellent tool for modelling low-energy homes, and IMHO should be used even if there is no desire to achieve Passivhaus standards. Using it now would also be a very good and way of determining exactly what is going wrong in this case.

    ReplyDelete
  13. It's not a surprise to me. Any HVAC professional would have done a calculation to determine the flow rates required to distribute sufficient heat for each room. The problem is, HRV systems are designed to move fresh air, not heat as their flow rates are about an order of magnitude too low. For example, the ventilation requirements for a bedroom are of the order of 10 l/s. Assuming the total house requires 60 l/s and recovers 85% of the heat, then the supply air would be at 17C. If 1kW is added to this then the air would be at about 31C. Now that 10 l/s supply to the bedroom would only be bringing in around 135W. I'd be surprised if the heatloss of the room was this low, hence the low temperature.

    Maybe with passivhaus levels of insulation and airtightness one can get away with such a low heat supply, but it should be noted that even passivhauses have heaters in their HRV ductwork, precisely because there isn't sufficient capacity to move enough heat around (and the fact that the passive gains in a passiv house are also insufficient in cold weather).

    Forced air heating systems can work well, but proper duct sizing must be performed to give sufficient volumes of air for heat transport as well as sizing the ducts to keep the air velocities reasonable to prevent noise.

    As for Dave Haworth's comment about heating air to 50C, one problem with such temperatures is that they are on the verge of "dust burning", which is one reason poorly designed forced-air heating systems are despised. Also, a very high supply air temperature can cause drafts.

    Just to put this all in context, I have a forced-air heating system that's heated by a GSHP - the supply air is at around 34C and the total supply rate is about 660 l/s. This is sufficient to keep all the rooms at the same temperature (I'm not in a passivhaus so it's neither well insulated nor air tight).

    Paul.

    ReplyDelete
  14. Sounds like the new soundproofing between floors is very effective.

    I wish I could float on the ceiling as that's where all the warm air seems to hide in this room!

    I quite fancy some heat recovery air brick vent style things but suspect they'll just be fairly useless and quite annoying in the bedroom.

    ReplyDelete
  15. Interesting discussion. We promote Haus houses to our clients so it is worth being kept updated simply for them.

    ReplyDelete
  16. I have experience of a couple of houses which do work better - though maybe rely mostly on natural convection up the staircase, which is open to the living area. We put in pipework for rads, as precaution, but never used.
    1.7 ach is pretty good by a lot of people's standards, but indicates there are some real holes somewhere. If all that leakage is from the bedrooms it may not be as good as thought, ie 3ach. Also, maybe the roof U-values aren't being achieved - due to thermal bypass, and/or poorly fitted insulation.
    The point about SIPs is interesting as these generally have consistent U-values and airtightness all round, and no thermal bypass issues (for the fully foamed type).
    Finally alarm bells should be ringing about that stove - I wonder if someone thought would be good idea to extract from the room containing the stove to "capture" the heat? Definitely not a good idea as depressurising the stove will lead to carbon monoxide entering the living space. However the system has been designed that is what is appearing to happen, and this needs to be reliably sorted before the stove is lit again.
    Meanwhile a few thermostatically controlled elec rads in the bedrooms won't use much power if the roof really is insulated...

    ReplyDelete
  17. And another thing: "The ventilation system even had a 1kW post-heater installed in the inlet ducting, and yet still the upstairs was still freezing"
    Do you mean "inlet ducting"? If so then this is a frost protection heating and serves no useful purpose in heating the house (the heat exchanger transfers any heat input to the exhaust - that's the idea, to keep the exhaust above OC).
    Or do you mean supply ducting?

    ReplyDelete
  18. Personaly know few people who were trying really hard in order to achieve the result of low energy homes, but finally they ended up paying larger bills than i did without any done to save energy...

    ReplyDelete
  19. Agree with most of the above. MHRV can only redistribute small heat loads.
    Added to this assuming the problems were occuring last November. MHRV and Heating System may have been designed for -2degC ext. 20degCinternal.
    Which given the cold November will have left "tightly" designed i.e NOT oversized systems struggling.
    November 2010 degree days were 298 compared to a twenty year average of 216

    ReplyDelete
  20. Sometimes getting low energy house can make you spend more money before you can meet the installation of energy-reduced home.

    ReplyDelete
  21. There is no reason that this house couldn't have been designed and constructed to work pretty much as designed. However - the skills and understanding to carry this off successfully are still in their infancy in the UK. This doesn't mean that we can't fast track getting and employing those skills - but we need to be realistic and manage our own and our clients expectations. We need to focus on th elow energy houses that do work and understand why they work. I encourage people to use this database (new and refurb projects) in that spirit.It will be further improved and added to over the next 12 - 18 months, but is useful even now. see www.retrofitforthefuture.org and for an example of a low energy refurb that work for a normal(ish) family of five and over 2 very cold winters (warm and low energy - as measured)enter 'Enerphit' into the search box. For designers - download the carbonlite documents including the passivhaus/gold and silver design detailing guidance.

    ReplyDelete
  22. There is no reason that this house couldn't have been designed and constructed to work pretty much as designed. However - the skills and understanding to carry this off successfully are still in their infancy in the UK. This doesn't mean that we can't fast track getting and employing those skills - but we need to be realistic and manage our own and our clients expectations. We need to focus on th elow energy houses that do work and understand why they work. I encourage people to use this database (new and refurb projects) in that spirit.It will be further improved and added to over the next 12 - 18 months, but is useful even now. see www.retrofitforthefuture.org and for an example of a low energy refurb that work for a normal(ish) family of five and over 2 very cold winters (warm and low energy - as measured)enter 'Enerphit' into the search box. For designers - download the carbonlite documents including the passivhaus/gold and silver design detailing guidance.

    ReplyDelete
  23. You have given us useful information regarding the low energy homes and why they don’t always work. The information us very useful

    ReplyDelete
  24. This is definitely a topic that's close to me so Im happy that you wrote about it. I'm also happy that you did the subject some justice. Not only do you know a great deal about it,

    ReplyDelete
  25. This is great because just like any other time you'e burning wood, you are going to get ash and soot on your stove and with all of the pieces contained to see more..wood furnaces Buy a Greenwood wood boiler that saves 70% on your home heating bills. We offer the world-class wood gasification heating appliances on an affordable budget.

    ReplyDelete
  26. This insulation is covered with a protective mesh and tight delivery that is extremely durable and has a high impact resistance. External Wall Insulation is a more expensive way to insulate the walls of your home, but it is highly effective.Our isolation comes in a wide variety of finishes and colors, which means that you improve the look of your home !

    ReplyDelete
  27. The finger of suspicion would seem to point at the commissioning of the MVHR ventilation, and the suspicion must be that the system hasn't been balanced correctly.

    splendid group review

    ReplyDelete