Author Topic: Heating by MVHR system  (Read 14265 times)

Alan Clarke

  • AECBmember
  • Posts: 193
    • View Profile
Heating by MVHR system
« on: January 24, 2006, 06:33:47 PM »
Does anyone have experience of heating a very well insulated house just by heating the supply air of a mechanical ventilation heat recovery (MVHR) system?  The Passive House people say this works for a heat load of 10W/m², which makes sense for a 50°C maximum supply temperature and a vent rate of 0.4 air changes per hour.  I presume for a thermally massive house this heat load would be after allowing for occupancy and incidental gains.

The only heating I’ve seen offered on MVHR kit is electric heating – not where we want to be going.  Is there an off-the-shelf standard water based system?  Alternatively could get a hot water coil made up specially. 

Alternatively, has anyone seen the Viessmann combined air-water heat pump, hot water cylinder and MVHR combination unit (Vitores 343) in action?  Looks neat, but is it wise acoustically to put a heat pump in the same box as the ventilation system?  (And what would your local plumber say if you called him out look at it?)

Nick Grant

  • Moderator
  • Posts: 1312
    • View Profile
    • Elemental Solutions
Re: Heating by MVHR system
« Reply #1 on: January 25, 2006, 08:26:05 AM »
Alan

Didn't BEDZED do hot water coils of the district CHP? Don't know if this is the only heating used.

David OLIVIER

  • AECBmember
  • Posts: 792
    • View Profile
Re: Heating by MVHR system
« Reply #2 on: January 25, 2006, 04:24:59 PM »
I thought the original Bed Zed plan was underfloor heating from its biomass CHP plant, which has had serious problems (the heat source not the distribution pipes).

Yes, it surely has to be a plumbing coil from the boiler or (better in towns and cities) from a CHP system.

D.

Alan Clarke

  • AECBmember
  • Posts: 193
    • View Profile
Re: Heating by MVHR system
« Reply #3 on: January 26, 2006, 10:32:22 AM »
At Bedzed I remember seeing one natural convector in the stairwell, near the hot water cylinder.  With the compact layout of the terraced houses there this may have worked OK.

davecraig

  • AECBmember
  • Posts: 11
    • View Profile
Re: Heating by MVHR system
« Reply #4 on: January 27, 2006, 12:44:02 PM »
I had a look at the veissman device-heatpump built in to the MVHR. I was quoted £3300 for the pump and £1400 for the ducts. This is to extract heat from the incoming fresh air and the extract air, and circulate the heated air via the fresh air system. The rep says it works well!

Nigel Bishton

  • *
  • Posts: 31
    • View Profile
    • The Pump House
Re: Heating by MVHR system
« Reply #5 on: January 28, 2006, 08:44:48 PM »
Genvex manufacture a combined MVHR and air to air heat pump.

Do a google search and you will find the uk distributor.

David OLIVIER

  • AECBmember
  • Posts: 792
    • View Profile
Re: Heating by MVHR system
« Reply #6 on: January 29, 2006, 09:57:22 PM »
But the COP of air to air heat pumps falls at the very time that you need more heat.

D.

Nigel Bishton

  • *
  • Posts: 31
    • View Profile
    • The Pump House
Re: Heating by MVHR system
« Reply #7 on: January 30, 2006, 09:28:18 PM »
I think the idea is that they extract more of the heat from the exhaust air. They do not use the external ambient air which clearly would give you a very low cop during cold weather.

I expect these devices are only useful if you have a very highly insulated and air tight building and hence a very low heating requirement.

David OLIVIER

  • AECBmember
  • Posts: 792
    • View Profile
Re: Heating by MVHR system
« Reply #8 on: January 31, 2006, 10:40:29 PM »
I know a modern low-energy house (by UK standards) with a GENVEX system and in cold weather the owners top it up with electric resistance heat or a woodstove. The total electricity consumption & CO2 emissions aren't that low although it's tightly-built for the UK.

I feel we're debating the wrong topic since if we can't achieve airtightness standards much better than Shouses around World War 2 

D.

davecraig

  • AECBmember
  • Posts: 11
    • View Profile
Re: Heating by MVHR system
« Reply #9 on: February 02, 2006, 08:17:57 PM »
have you had your new house tested for air tightness david?

Mark Siddall

  • Moderator
  • Posts: 1214
    • View Profile
Re: Heating by MVHR system
« Reply #10 on: February 15, 2007, 01:39:23 PM »
Another old thread deserving a dusting off...

One question that I have about the Gold/Passive House standard that I have not yet fully resolved in my mind relates to the interrelationship between heating demand and an all-air-heating system.
The standards seek to achieve a heating demand of 10w/m2 which, along with other economic factors, has led to the mainland European specification of an all-air-heating system i.e. the traditional radiant heating system is not required/has been designed out. So as to ensure a good indoor air quality the restriction placed on an all-air-system is that the air is not to be heated above 50C (to avoid dust etc. being burned). What I want to know is; what is the maximum heat load that an all air system can satisfy (w/m2)?
To develop an appreciation of why this question arises here is a little scenario: -

Imagine that a design, based upon the standards, fails to achieve the standard in practice i.e. as built/refurbished project, the consequence of this failure is that the heating demand is higher; if this exceeds the 10w/m2, and an all air system can not satisfy this additional demand, then a whole new un-planned heating system is required adding substantially to construction costs. Obviously this is problematic and hence the need for me to develop a greater understanding of the matter.

So the underlying question to what is the maximum heat load that the an all air system can satisfy? is really how much tolerance does the all-air-system have?

If anyone is able to answer this question I would be grateful if you could let me know. As noted above this would be particularly relevant in certain refurb/conversion projects.

Mark

David OLIVIER

  • AECBmember
  • Posts: 792
    • View Profile
Re: Heating by MVHR system
« Reply #11 on: February 15, 2007, 02:54:20 PM »
Mark

On a current project, a new house actually, the M&E engineer wanted underfloor heating *as well as* the plumbing coil. This was felt to be OTT. The insurance policy, if there is to be one, is likely to be two blanked-off pipes so that the area in question (very heavily-glazed) could in future be fitted with a tiny radiator if need be. However, the design of that area of the building is subject to revision and if so the insurance policy may not be used.

Another project fails to achieve the 10 W/m2 despite good U values and needs three small radiators in addition to the warmed air.

HTH


David.


Mark Siddall

  • Moderator
  • Posts: 1214
    • View Profile
Re: Heating by MVHR system
« Reply #12 on: February 15, 2007, 09:15:36 PM »
Thanks David, sadly it doesn't quite hit the mark though. Back-up strategies I can figure, up to a point. I still would like to know how much tolerance an all-air-system has.
..... fiscally speaking the less plumbing and extras the better, provided you don't end up shooting yourself in the foot of course.
 
Mark

Mark Siddall

  • Moderator
  • Posts: 1214
    • View Profile
Re: Heating by MVHR system
« Reply #13 on: February 16, 2007, 04:56:36 PM »
Our Canadian non-member, part time contributor, Paul in Montreal, emailed me the following rather useful pointers on the tolerance of all-air-systems.

In my renovation property in Montreal, heat load on a -23C day is about 16kw - our forced air heating system has no problem delivering this amount of heat. The house area is around 180m^2 - so this is 88W/m^2. The system uses a GSHP which has a capacity of around 10kw (55w/m^2) as well as auxiliary resistance heat of 10kw for a total of 110W/m^2. Forced-air heating is very common in Canada where, of course, the heat loads are much higher than in the UK - the past 3 weeks we've had an average differential of 35C from inside to outside (with as high as 45C on some cold nights and days).

The key to making it work is a properly designed delivery system (i.e. ductwork). I don't know if you read the Greenbuilding forum, but I just made a post there that describes the requirements of forced air heating systems:

http://www.greenbuildingforum.co.uk/forum/index2.php?DATEIN=tpc_fygfuitdc_1171486438

There is a formula, Q=1.08 * Delta-T * CFM which gives the amount of heat a forced air system can deliver for a particular temperature differential above ambient. The equation is in imperial units but can easily be converted to metric; Q is in BTU/h; Delta-T in degrees F and CFM is cubic feet per minute.

From Paul's statement, by all accounts it sounds like alternative back-up systems can be done away with, instead the capacity of the heating system can be upped to suite and ducts re-sized, thus minimizing cost by doing away with additional plumbing/labour etc. (i.e. if you fail the airtightness requirements you can re-calculate the heat loads to suite and design out the problem.)

The only thing that is not overtly clear in the email is whether the air temperature at-point-of-heating exceeds 50C, thus increasing risk of reduced IAQ (this said I dont get the impression that IAQ suffers in this manner even in the extremes of the Canadian winter.)

Thanks Paul.

Anyone have any thoughts to add?

Mark
« Last Edit: February 16, 2007, 05:43:13 PM by Mark Siddall »