Heat loss calculation check

[SBMS]

13 minutes ago, Gordo said:

Here’s an extract form HETAS guidance regarding MVHR. For JohnMo benefit so he knows I’m not just making it up. There so many reference materials out there

Probably gone a bit off topic here, but my two cents was when I looked into this it did sound like MVHR and solid fuel stoves weren’t an ideal match and we therefore steered away from them for our build (although I am a sucker for a real fire). Be interested to hear how others with airtight houses, with MVHR run their wood burner?

[Gordo]

3 minutes ago, SBMS said:

MVHR and solid fuel stoves weren’t an ideal match

Agree 100%. But you got to love a log stove

Edited Monday at 20:26 by Gordo

[dpmiller]

I didn't find any reason not to have a roomsealed woodstove in an MVHR-equipped house, but I *did* understand that I could not have extraction in that room; our stove is in a room with only a supply vent, the kitchen end of the house may get a gas stove some day as that's OK in a room with extraction.

 

I also chose to run the house at a slight positive pressure to reduce the chances of smoke being pulled into the room if the stove door is opened to rapidly etc.

 

 

[Gordo]

26 minutes ago, dpmiller said:

I didn't find any reason not to have a roomsealed woodstove in an MVHR-equipped house, but I *did* understand that I could not have extraction in that room; our stove is in a room with only a supply vent, the kitchen end of the house may get a gas stove some day as that's OK in a room with extraction.

 

I also chose to run the house at a slight positive pressure to reduce the chances of smoke being pulled into the room if the stove door is opened to rapidly etc.

 

 

As a rule a room sealed stove in a room with a supply vent should be safe  but even safer if overall the house is slightly pressurised  I would be more concerned with an non room sealed stove and look for a "flue gas interference test" as proof. There were concerns raised about the risks when refueling which is OTT IMO.

 

Similar risk with an open flue solid fuel effect gas appliance. But your gas fitter should check draft 

Edited Monday at 22:08 by Gordo

[Gus Potter]

1 hour ago, Gordo said:

As a rule a room sealed stove in a room with a supply vent should be safe  but even safer if overall the house is slightly pressurised  I would be more concerned with an non room sealed stove and look for a "flue gas interference test" as proof. There were concerns raised about the risks when refueling which is OTT IMO.

 

Similar risk with an open flue solid fuel effect gas appliance. But your gas fitter should check draft 

Now here is the rub.

 

Personally I would live in a house that has some element of air heat recovery in terms of ventilation at a push. But not having windows that open is beyond comprehension. What happens when you have wet dog, want to dry some extra washing on the bannister..or just want a blast of outside air. This is not a home to me.. it's an extension of a crappy sanitized office.

 

Ok you want to have MHVR and a wood burning stove. But I will bet my last cent that you won't change the rope seals every year on the stove door! A rope seal kit is about 12 -15 quid.. but most folk won't do it.

 

As @SteamyTea says.. these wood burning stoves can be a killer. Always have a CO alarm in the room. 

 

Design your house and systems to last and look at think how you are going to maintain it and the cost of that.

 

Be practical and not just aim to meet the regs. That is good design that will protect the value of your property and make your effort worthwhile and rewarding in the long term.

 

Remember when you come to sell, some hard arsed surveyor is going to look at all this and may well write down the value if they can't understand what you are up to!  This is not Fred Dibnah.. folk need to get a mortgage.

 

1 hour ago, dpmiller said:

I also chose to run the house at a slight positive pressure to reduce the chances of smoke being pulled into the room if the stove door is opened to rapidly etc.

Does any one think the same calculations and controls will work the same way in 20 years time? Any takers?

 

Edited Monday at 23:02 by Gus Potter

[Gordo]

21 minutes ago, Gus Potter said:

Now here is the rub.

 

Personally I would live in a house that has some element of air heat recovery in terms of ventilation at a push. But not having windows that open is beyond comprehension. What happens when you have wet dog, want to dry some extra washing on the bannister..or just want a blast of outside air. This is not a home to me.. it's an extension of a crappy sanitized office.

 

Ok you want to have MHVR and a wood burning stove. But I will bet my last cent that you won't change the rope seals every year on the stove door! A rope seal kit is about 12 -15 quid.. but most folk wont do it.

 

As @SteamyTea says.. these wood burning stoves can be a killer. Always have a CO alarm in the room. 

 

Design your house and systems to last and look at think how you are going to maintain it and the cost of that.

 

Be practical and not just aim to meet the regs. That is good design that will protect the value of your property and make your effort worthwhile and rewarding in the long term.

 

Remember when you come to sell, some hard arsed surveyor is going to look at all this and may well right down the value if they can't understand what you are up to!  This is not Fred Dibnah.. folk need to get a mortgage.

 

Does any one think the same calculations and controls will work the same way in 20 years time? Any takers?

 

Wise words. All this stuff is supposed to be maintenance checked including MVHR & stoves. A lot of stoves get once over every few years I’m sure. HETAS etc say MVHR is supposed to be have flow rates checked annually and ducts cleaned (lol dead on). I doubt many are checked following commissioning. I wonder how sanitary they are after 10 years. Interesting as I never thought much about this before.

 

TBH the MVHR controls are mechanical dampers that are adjusted and fixed in position so shouldn’t need adjustment I guess. Ducts are bound to be lined in bacteria laden dust. Maybe just the extract ducts as filters should protect supply ducts

Edited Monday at 23:21 by Gordo

[Gus Potter]

44 minutes ago, Gordo said:

Wise words. All this stuff is supposed to be maintenance checked including MVHR & stoves. A lot of stoves get once over every few years I’m sure. HETAS etc say MVHR is supposed to be have flow rates checked annually and ducts cleaned (lol dead on). I doubt many are checked following commissioning. I wonder how sanitary they are after 10 years. Interesting as I never thought much about this before.

Often I may come over as a bit of a crusty / sceptical. In mitigation I designed and installed my first UF heating system some 30 years ago.. long before this came more main stream. I'm an SE and designer now.. still learning lots.

 

But many of my Clients are folk that want to build say an extension (some are big mind) the odd new build and some really big barn / church type etc conversions. Most don't have a bean to rub together.. every pound is a prisoner! They have a mortgage and want to protect their asset. Often I say.... hey look you can have fancy controls, systems but lets also look at the value of these when you come to sell. Lets also look at the maintenance costs.. the reality of sticky valves, ducts getting clogged.. who can mantain all of that and at what cost.

 

For example I have some Hive controls... nice graphs on my phone etc but if I want to move house it's difficult to transfer the ownership.. in fact a nightmare. Now many modern systems have software, controls that are difficult to maintain after five or so years.

 

Now if you have plenty of cash to splash and want to make an environmental statement or just like techy controls then on you go as far as I'm concerned. If you are happy with the maintenance and potential loss of value when you come to sell. If that is what you want then I'll get that into the design. Mind you during the design process you may change your mind and come round to my way of thinking! Especially once we start to look at build cost /benefit in detail.

 

1 hour ago, Gordo said:

I wonder how sanitary they are after 10 years. Interesting as I never thought much about this before.

 

This is a good point.

 

Old crusty ends.

 

 

 

 

[JohnMo]

8 hours ago, Gus Potter said:

But not having windows that open is beyond comprehension

Why would you do that, even in a passivhaus. Opening windows can be draft free so no impact on an air test 

 

@Gordo text attached that was provided to hetas to do their note on MVHR. Doesn't say running an unbalanced MVHR system, as seem to keep saying.

RVA_position_on_MVHR_and_open_flue_appliances_Final_4th_July_2022.pdf

[dpmiller]

I guess the question is, if the MVHR is "unbalanced", where does the excess air normally go...

 

I commissioned my system on a still day with doors and windows open- intake and extract flows set to nominal figures separately- I then closed the house up and ramped the supply fan a touch to get positive pressure on a manometer. Minimal change to flows noticeable.

 

Losing a small amount of heated/ preheated air is a price I prefer to pay over cold air infiltration through any unavoidable leaks.

[joth]

14 minutes ago, JohnMo said:

Why would you do that, even in a passivhaus. Opening windows can be draft free so no impact on an air test 

 

@Gordo text attached that was provided to hetas to do their note on MVHR. Doesn't say running an unbalanced MVHR system, as seem to keep saying.

RVA_position_on_MVHR_and_open_flue_appliances_Final_4th_July_2022.pdf 92.32 kB · 1 download

 

Opening windows is generally a requirement for building regs. 

But Non-opening windows do tend to score better on phpp, not because of airtightness but because they have less frame. If the frame has a lower U value this is a big deal. We went with one non-opening picture window 2x2 m for this reason, don't think opening would have been practical anyway. But the room has plenty other fenestration too 

[Gordo]

4 hours ago, JohnMo said:

 

@Gordo text attached that was provided to hetas to do their note on MVHR. Doesn't say running an unbalanced MVHR system, as seem to keep saying

I think you should read over this a few times then make your own conclusions. I would advised from research and common sense to slightly pressurise house from MVHR, but you do what you like based on your research. The carbon monoxide and particulate matter in your room won't affect me.

Edited 19 hours ago by Gordo

[Gordo]

4 hours ago, dpmiller said:

where does the excess air normally go...

Think of it as a sealed bottle. If it can't escape the air pressure just increases some until an opening is created. If there is some leakage (there always will be) through imperfections, the excess air escapes. Better that than negative pressure that sucks in cold unfiltered drafts, IMO.

[JohnMo]

1 hour ago, Gordo said:

I think you should read over this a few times then make your own conclusions. I would advised from research and common sense to slightly pressurise house from MVHR, but you do what you like based on your research. The carbon monoxide and particulate matter in your room won't affect me.

We have stove in house which has a balanced MVHR system (supply and extract flows equal).  The stove is fully room sealed with external air supply supplying both primary and secondary air.  The lounge is fitted with two supply terminals. Have carbon monoxide alarm also in room. Fire is fully serviced yearly and also monitor thermal camera to check for any small leaks when on and when off.  MVHR is serviced every 6 months, and we have extract filters at every extract terminal to ensure ducts remain clean.

[dpmiller]

what happens if you quickly open the stove door on a windy night/ before the chimney heats? A wee bit of pressure in the room will assist ensuring the smoke goes the right direction at times like this.

[ProDave]

If an MVHR us unballanced, it will either suck air in, or expell air through the leaks.  Even a house with a good air tightness test result will have some leaks.  That imballanced and unwanted air flow will of course not benefit from heat recovery.

 

Nobody is suggesting a house without opening windows, building regs would not allow that, and there is nothing wrong with opening a window in a house with mvhr.  For much of the year our bedroom window is open a bit as even with no bedroom heating it can be hard keeping the bedroom cool enough.  As a general rule, you can open one window, or one door in an air tight house without getting much of a through draught from that even on a windy day.

 

Do check your MVHR from time to time.  Our inlet vent became blocked.  I sort of sensed something was wrong when I started to notice the bedroom becoming a bit stuffy (when the window was shut) and I confirmed the house was being depressurised.  The inlet vent is a common type with a coarse gauzze mesh to stop flies getting in, and that had become very clogged indeed.  You can't get to the outside of the mesh to suck the debris off so I got the compressor air line on it to blow all the debris away from the inside.  Normal operation restored and it is now an annual maintenance job.  As a secondary indicator, annual heating usage had crept up by about 5%.  Now the blockage has cleared it has gone back down.  That shows the extra heat loss as the MVHR was not operating properly.

[Gordo]

3 hours ago, JohnMo said:

We have stove in house which has a balanced MVHR system (supply and extract flows equal).  The stove is fully room sealed with external air supply supplying both primary and secondary air.  The lounge is fitted with two supply terminals. Have carbon monoxide alarm also in room. Fire is fully serviced yearly and also monitor thermal camera to check for any small leaks when on and when off.  MVHR is serviced every 6 months, and we have extract filters at every extract terminal to ensure ducts remain clean.

That's good, you have made your choices and with proper maintenance and a balance MVHR with room sealed stove there is unlikely to be a problem. 

[SteamyTea]

Regarding pressure imbalance.

If your house is at a higher pressure than outside, moist air is being forced into, and through, the structure, where it can condense.

 

I doubt if it is a real problem, just something to put out there.

[Gordo]

31 minutes ago, SteamyTea said:

moist air is being forced into, and through, the structure

Good point, there is that to consider. Hopefully the airtight VCL and breathable layer does it's thing 😁

Edited 13 hours ago by Gordo

[SteamyTea]

10 minutes ago, Gordo said:

Hopefully the airtight VCL and breathable layer does it's thing

Well only where it is properly installed.

If the weekends happens to be near some untreated timber, that is on the cold side, then it could cause problems.

[Gordo]

19 minutes ago, SteamyTea said:

could cause problems

Potentially but unlikely, obviously a condensation risk analysis should be done to avoid this problem. Like anything I guess this may not be done or not to spec or with poor workmanship. Over 12 month cycle (unless the moisture is trapped) if the element make up can breath it shouldn't rot. Persistent damp is the concern IMO 

[Iceverge]

6 hours ago, SteamyTea said:

Regarding pressure imbalance.

If your house is at a higher pressure than outside, moist air is being forced into, and through, the structure, where it can condense.

 

I doubt if it is a real problem, just something to put out there.

 

This is the real benefit with airtighess. 

 

Keeping the structure safe from all the dampness we make inside. 

 

Although with mechanical ventilation the leaks will rarely cause issues as the internal air will generally be of a safe humidity level anyway.