Starting to think about MVHR

[Jeremy Harris]

I'll see if I can dig around for a suitable DC brushed motor speed controller later.

 

As for the smoke issue, I can confirm that it definitely does get sucked in through an MVHR intake and then efficiently spread through every room in the house.  Our neighbour lit a bonfire a while ago and the whole house smelt of smoke within a few minutes.  I shut the MVHR off, but the smell then stayed in the house for ages, and didn't really clear until the bonfire had gone out and I turned the MVHR back on again.

 

TBH, I can't see an activated carbon filter being that useful, as it would need changing very regularly if exposed to smoke regularly and would probably be pretty expensive in terms of additional running cost.  With a fairly regular source of smoke the capacity of the activated carbon would quickly get used up.  It might be OK for a situation where someone has a bonfire a couple of times a year, but I can't see it dealing with smoke on a near-daily basis, somehow.

[MikeGrahamT21]

mmm, I think i'd agree with you there! Perhaps BPC are just on the sell sell sell tactics. Plus the filter they recommended is a whopping £800 for the smallest one! And no guarantee of if it doesn't work can i have my money back.

 

I'm going to pay more attention to where we can smell the smoke this winter, it may be that we can only get it at the nearest point to the neighbours house, though i guess it probably depends on the wind direction! They are to our east, so more often than not the westerly's carry it away, but get a cold easterly, and it would definitely be on, and coming in our direction, so its not looking promising!

 

I'm really annoyed to be honest, as I've been dreaming all this up for years.

 

We do seem to suffer from quite high humidity in the house (if i leave the dehumidifier off we reach around 70% in occupancy, dropping back to 65% if we go out), which this would have cured, and currently resolve this using a smart dehumidifier, but obviously it takes a lot of electric, though does give us some heat back being a dessicant version, and this brings it back to a more reasonable 60%, though we can go to 50% if required.

 

We have an extractor in the bathroom which is PIR activated, so is on regularly, but perhaps isn't powerful enough. I don't even know whether to consider a single room MVHR for bathroom when this one breaks, as we'll be in the same situation, just on a smaller single room scale!

[Jeremy Harris]

I've just had a poke about on ebay, and it looks like one of these speed controllers should work OK with a normal electric car radiator fan: http://www.ebay.co.uk/itm/20A-Universal-DC10-60V-PWM-HHO-RC-Motor-Speed-Regulator-Controller-Switch-AM-/253089458063?epid=874428193&hash=item3aed4ea78f:g:W3wAAOSweNxZjSGv

[graeme m]

Did someone say you can't heat through MHVR? According to my PHPP designer (Oh yeah money to burn) you can.

[PeterW]

1 minute ago, graeme m said:

Did someone say you can't heat through MHVR? According to my PHPP designer (Oh yeah money to burn) you can.

 

A standard MVHR cannot heat incoming air more than the energy available in the outgoing air. 

 

Some advanced MVHR units such as the Paul ones and the Genvex ones have a separate air to air heat pump built into the unit that allows increased supply air temperature. 

 

They are expensive ...!!

[graeme m]

Thay'll be the one then. They're blooming expensive but it saves central heating.

[Jeremy Harris]

2 minutes ago, graeme m said:

Did someone say you can't heat through MHVR? According to my PHPP designer (Oh yeah money to burn) you can.

 

No, is the simple answer.  The air coming out of the fresh air vents will always be colder than the air being extracted from the extract vents, so the MVHR will always tend to cool the house down, but it will cool it down a great deal less in cold weather than normal ventilation.

 

If you fit either an active MVHR, with an air-to-air heat pump (like the Genvex unit we have) or if you fit a post-heater in the fresh air duct, then you can provide a very limited amount of heating, but this is really only suitable for passive house type heating levels.  For example, our Genvex can provide around 1.5 kW of heating, more than enough to keep the house warm, but it cannot heat the house quickly, as it only moves less than half a house full of air per hour.  As a consequence, it could take hours to heat the house from cold using this system, but it could maintain the temperature easily enough once the house is warm.  The reason we don't heat the house with it is that it tends to make the air very dry, so we only use the Genvex for comfort cooling in summer, not winter heating.

[ProDave]

I found the mvhr is a bit like a stove.  The mvhr unit is the cheap bit, it's all the duct, plenums and terminals that cost the money, (same as the flue will cost a lot more than the stove)

 

I only bought the ducting, plenums etc from BPC but could not find anywhere cheaper.

[joe90]

I also found the extras cost much more than the unit I got from fleabay but I did make my own plenums and manifold incorporating silencers so only had to buy their ducting ( which was expensive enough ?).

[jack]

Might be apocryphal, but I believe that the Passivhaus space heating standard (15kWh/M²/annum) was originally chosen based on the maximum amount of energy that could be delivered via duct heating without scorching dust.

 

That's one of the reasons the PH standard annoys me - they've chosen this number based on an arbitrary decision to assume warm air heating via MVHR.  The number could just as easily have been higher if they'd assumed, eg, underfloor heating.

[Jeremy Harris]

It's certainly around the right figure to have been derived that way.  Our Genvex unit could pretty easily maintain the house at a comfortable temperature in winter, but I would need to increase the ventilation rate in very cold weather just to get enough heat through the ducts.

 

I think we have to remember that when the PH standard was being put together all those years ago (around 35 years ago IIRC) there wasn't much data to go on.  I think that it was seen as the time as being a very challenging target, using the construction methods that were in general use back then, whereas now it's not at all hard to meet the standard, or just have a house built from one of the suppliers that offers PH level insulation and airtightness as part of a standard package.

 

One thing I've found is that both the PH and building regs Part F background ventilation level seems higher than needed to maintain good air quality, so I suspect that this may be another number that was pretty much plucked from nowhere.  I very much doubt if there are many, if any, non-MVHR fitted houses that come anywhere near meeting the background ventilation level that's in Part F for an MVHR equipped house, for example.

[jack]

1 hour ago, JSHarris said:

I think we have to remember that when the PH standard was being put together all those years ago (around 35 years ago IIRC) there wasn't much data to go on.  

 

I've no doubt you're right. Interesting that after 35 years of data and improving knowledge of how to build, we're still stuck with exactly the same figure, with no allowance for climate or house size.

 

I think there's decent basis for starting again based on what's now known, and developing a more realistic standard that allows some flexibility based on, eg, location and climate, while not punishing smaller houses. 

[graeme m]

I'm awaiting details but the architect and PhPP bloke say it is now possible to heat and yes the house will be to PH standard at least. I've also decided to go for full testing during the build, we may not want the plastic badge in the end  but, we will know how the house functions and if some naughty tradesman has made a cock up, so to speak, before it's all too late.

I may still include a radiator in each bathroom and one in the lounge. My architect says she intalled three in her PH and she's only ever switched one on once.

[Jeremy Harris]

Being possible to heat using a heat source in the MVHR and being desirable and practical to heat this way are two different things.

 

Firstly, an MVHR running at normal ventilation rate will take over 2 hours to change all the air in the house.  In practical terms, this means that heating the house from cold with such a system can take many hours.  This isn't too big a problem if you never let the house get cold.  However, even a passive house needs heating in very cold weather, and it's very likely that the background airflow from an active MVHR with duct heating will be too low to meet that heating requirement, so the air flow rate will have to increase.  This can be a nuisance, as increased flow rates tend to create a little bit of noise.  This noise is acceptable when the MVHR boosts when showering or cooking, but may be an issue at other times.  We use our active MVHR for comfort cooling, and the slight noise increase from the higher flow rate when it's in this mode is noticeable.  Not enough to be a daytime nuisance, but I would rather not have this slight noise at night time.  Luckily there's no need for active comfort cooling at night, as the house will have cooled down enough during the day, and the MVHR on full bypass is enough to keep the bedrooms comfortable.

 

Secondly, heating the air in the house tends to both make it a bit uncomfortable, and also tends to take a very long time to get the floor surface temperature up.  We've found that we only ever use the very low temperature ground floor UFH when heating is needed, and are convinced that heat coming from the floor is a great deal more comfortable than heat coming out of the air vents.  We have no heating on the first floor, except towel rails, on timers, in the bathrooms.  I did fit fused outlets for small panel heaters in the bedrooms, but never fitted the heaters, as they aren't needed.

 

The UFH is absolutely superb.  It runs at a very low temperature (typically 22 to 23 deg C) and can not only warm the ground floor slab in winter, but can cool it to around 18 deg C in summer.  For the very modest additional cost of 300m of pipe cast into the passive slab it was one of the best investments we made.  Not only is having a slightly warmer than ambient floor very comfortable, it also acts as a large storage heater, so in very cold weather the UFH comes on for maybe an hour each morning, sometimes every other morning, and the house stays at a nice, even, temperature.

Edited September 28, 2017 by JSHarris

[joe90]

As above from Jeremy, during the design stage of our build I discounted heaters wet or dry in the MVHR, have put in UFH downstairs from an ASHP but upstairs will have small electric heaters that can be plugged in IF it ever gets cold enough (which I doubt).

[jack]

45 minutes ago, graeme m said:

I may still include a radiator in each bathroom and one in the lounge. My architect says she intalled three in her PH and she's only ever switched one on once.

 

Definitely include provision for heating in bathrooms. Not including this was one of the biggest errors we made during our build.

 

We have a house with PH-level insulation and airtightness, but even when the air temperature is a consistent 21-22^oC, it's still slightly cooler than desirable when you step out of the shower in the middle of winter. Tiles on the floor, in particular, tend to suck heat from your feet, giving the impression that the room is colder than is actually the case.

 

Personally, I'd put UFH in the bathroom (low temperature) just to take the chill off the tiles in the morning, and wire for a radiator of some sort in case you find you need it.

 

We're planning on retro-fitting some infrared panels to address this issue. It would have been a lot cheaper and more effective to include UFH when the house was being built.

[Stones]

In our last house, we had an exhaust air heat pump. An expensive bit of kit to run so the less said about it the better.  However, it did heat the supply air up using a wet duct heater, and that did produce a very pleasant environment to live in.  A wet duct heater could be plumbed into the supply ductwork from an MCHR, although they are not cheap, and you do need a fairly large duct heater if this was going to be your sole source of heating.  An electric duct heater is far more compact and cheaper but would if run too hot, lead to the scorched / burnt dust smell referred to earlier.

 

2 hours ago, JSHarris said:

 

One thing I've found is that both the PH and building regs Part F background ventilation level seems higher than needed to maintain good air quality, so I suspect that this may be another number that was pretty much plucked from nowhere.  I very much doubt if there are many, if any, non-MVHR fitted houses that come anywhere near meeting the background ventilation level that's in Part F for an MVHR equipped house, for example.

 

You have to bear in mind that occupancy plays a large part in air quality.  If there are only two of you in the house, vs four in mine, you are producing less 'pollutants' and using less fresh air.  Our ventilation rate is set to suit the four of us.  If we have people round for any length of time, I would increase the ventilation rate to compensate.  I'm guessing the rates set are based in some way on the notional occupancy levels?

[joe90]

I tend to agree on occupancy levels. I used to advocate running MVHR like central heating, I.e. It only runs when needed ( needs co sensors as well as rh) with our build site ( middle of nowhere so no pollutants) and our love of open windows I am not sure how often ( in the summer) it will be required. We need to suck it and see as they say.

[ProDave]

We are having wet UFH in our bathrooms. the bedrooms are having electric heater points to fit a small panel heater if it proves necessary.

 

Our house is "designed" (as far as BC are concerned) for 5 people, though there will normally only be 3 of us in the house. One could argue that to replace the air we breath, we only need 3/5 of the BC required ventilation rate?

[joe90]

Forgot to add I am installing electric UFH in our ensuite and electric towel radiators in both bathrooms on timers. As our build is three bedrooms we had to have a six person treatment plant but most of the time it will only be two persons and that is the reason I chose the vortex plant as that is the only one that can be "turned down" to match the occupancy level.

[Stones]

6 hours ago, ProDave said:

We are having wet UFH in our bathrooms. the bedrooms are having electric heater points to fit a small panel heater if it proves necessary.

 

Our house is "designed" (as far as BC are concerned) for 5 people, though there will normally only be 3 of us in the house. One could argue that to replace the air we breath, we only need 3/5 of the BC required ventilation rate?

 

That's my thinking.  Given how sewage treatment requirement is calculated  (you and I both had to fit a system to accommodate 6 people because of the number of notional bed spaces in our respective house), it wouldn't be a great leap to suggest a similar logic was applied to ventilation requirements.

[Jeremy Harris]

Part F could be clearer, I think, as the minimum continuous ventilation rate allowable with MVHR still seems much higher than the likely natural ventilation rate without MVHR.  It seems fairly common to hear of people with passive houses turning down the background ventilation rate once they have a completion certificate, on the basis that the rate in the regs is higher than needed.  I'm not convinced that this is wholly down to the number of occupants in every case, either.

 

Getting back to heating using ventilation air, then if deciding to do this the system has to have significantly greater air flow capacity, which means bigger ducts and terminals.  Air can only move a tiny fraction of the heat that water can.  Very roughly, for a given amount of heat transfer you need to shift around 3,400 times more air than water.  This explains why air heating systems use big ducts and water heating systems can get away with using small pipes.

[MikeGrahamT21]

On 27/09/2017 at 17:19, JSHarris said:

I've just had a poke about on ebay, and it looks like one of these speed controllers should work OK with a normal electric car radiator fan: http://www.ebay.co.uk/itm/20A-Universal-DC10-60V-PWM-HHO-RC-Motor-Speed-Regulator-Controller-Switch-AM-/253089458063?epid=874428193&hash=item3aed4ea78f:g:W3wAAOSweNxZjSGv

Thanks, i've added that to my watch list.

[j_s]

I have a leaky house that I am working on to make air tight. All my external bedroom walls are having the plasterboard removed for me to find holes in the blockwork. Luckily on the one bedroom so far I've found minimal issues but they are there. Wall plate is a major issue with some gaps. Already sealed externally now as had all new soffits and fascias and guttering added at considerable expense. Whilst the scaffolding was up I did all the sealing I could and we sorted out the insulation so it wrapped around from cavity to loft. All tied in so it wouldn't move in the future with bent wall ties. I am working on the loft as well to seal from above and below. 

 

Mvhr is fitted to upstairs and we just switched it on today to make sure its running ok. 

 

New extension added as well and making serious improvements downstairs at the same time. Again plasterboard off on external walls and my time and effort in sealing all gaps, holes etc. Will also be using the paint on blowerproof liquid on floor to wall, wall to ceiling whilst the plasterboard is off as well as the wall corners. It's costing me a fortune for the extension and all new bits and pieces so I decided to put my own serious time in to improve everything else. The builder is great and understands what I'm trying to do. I really hope all my effort makes a difference.

 

 

[Jeremy Harris]

The cheapest way to make brick or blockwork airtight is to just use a standard cement wash parge coat.  Mix it up so it's thin enough to apply with a soft broom and you can do a whole wall in minutes, for very little cost.  It used to be the standard way to seal up a wall and prepare it for wet plastering years ago, but it is every bit as good for just making leaky brick or blockwork pretty airtight, and it's dirt cheap, too!

Edited September 29, 2017 by JSHarris