Cost / kWh saved calculations

[Mark Siddall]

I am trying to calculate the cost / kWh for an MVHR system, everything was going ok until I go as far as the number of air changes/hr. For others with more experience than me with these calcs I have a couple of questions: –

· Using a B.Regs standard Noddy box as the datum I intend to use 1 ach/hr (in accordance with Part F). Should I try to include air changes resulting from infiltration/exfiltration in this calc, or should I leave it for another calculation?

On one hand it seems sensible as this form of background ventilation rate is considered, however, on the other hand it makes the whole calculation procedure more complex than may be necessary, furthermore, it could undermine a proposed airtightness cost/kWh saved calc. (I have a tendency to believe that infiltration should be left for another calc, thus keeping things a little simpler, not only for me but for anyone that has to digest the document.)

· Whilst I am on the subject of airtightness and performing a cost/kWh saved calc, does anybody know the average air change rate for a pressure tested B.Regs Approved box i.e. 10m3/m2@50pa, and similarly Silver and Gold standard buildings? Will the air change rate vary with building size i.e. house, school, office, hospital etc? Once I have this range of data I can calculate, and subsequently justify to a range of clients, the benefits of moving from B.Regs and an improved airtightness standard.

Any help appreciated.
Thanks,
Mark

[Mark Siddall]

Has nobody got anything to add on this topic?
David O is there something you could contribute?
I still want to perform this calc but a few cues on how to proceed would be greatly appreciated.

Thanks,
Mark

[David Olivier]

Mark

A Bldg Regs semi. probably has c 13 m3/ m2 hr @ 50 Pa. Note – the denominator is the thermal envelope area.

People are, however, calculating this number using the total envelope area, including separating walls or even floors. A very mistaken procedure and allows the thermal envelope to leak more if the separating walls are moderately airtight (they need to be to meet Part E).

A detached house would be 10 if it met the Regs.

An office would be more than 10 as Paul Jennings (also on this forum) tests buildings for a living and says they routinely fail, although peole use varying procedures to temporarily improve themn so that they pass.

In Canada and Germany, realistic per capita fresh air rates are being used. 5 ltr/sec seems to be the consensus in Germany. Their experience since about 1996 is that rates of 8-10 ltr./sec.cap make the internal air too dry and therefore unhealthy from about Dec to March.

Sorry to diverge from any advice given by Part F, but a figure of 1 ac/h is meaningless as it could be

1. too low (unhealthy) for a densely-occupied flat and
2. too high (also unhealthy) for a 160 m2 detached house containing one occupant and ten cats (yes, this latter situation has actually arisen on a real project!).

Sorry for delay in replaying but I have to do real work too!

HTH

David.

[Anonymous]

The calcs for my building go as follows:
The current “approved” 10 m3/m2 @ 50pa represents 6.5 ach/hr.
The Gold standard 1 m3/m2 @ 50pa sees this fall (not surprisingly) to 0.65.
But what does this all mean in reality? What is 50pa?

[ … ]

How much energy is just floating out of the house on a normal relatively still day, never mind a gusty day when wind pressures can reach in excess of 2000pa? The mind boggles.

Interesting figures. I renovated a 109 year old house in Montreal and got the air leakage down from 12.5ACH@50Pa to 6.7ACH@50Pa – around the current UK approved figure! I still think my house is quite leaky!

Running some figures through hot2000 for our heat load, the current house has a nett heat load of 158923MJ per year (44145kwh). If I put could get the air leakage down to 1ACH@50Pa (R2000 standard) and also put in a HRV, the nett load goes down to 123171MJ per year (34214kwh) but the HRV consumes 1095kwh per year – this represents a saving of 8835kwh per year. In our case, since we heat with a GSHP the actual heating consumption would go down from 16268kwh to 12000kwh, so a rather smaller saving. Still, just goes to show how much heat is lost through ventiation.

As for wind pressures of 2000Pa, this really is meaningless – that much pressure inside a house would cause it to explode! The 50Pa test is a depressurization test. For the 1ACH@50 case, an airflow of 61l/s is required to depressurize by 10Pa.

I guess the main point is the UK target figures of approximately 6.5ACH@50 are indeed dismal.

Paul in Montreal

[Mark Siddall]

Paul,
Intersting stuff. FYI All I was trying to get at with the mention of 2000pa external wind pressure was that infiltration/ exfiltraction rates would increase far beyond the normal background rate causing increase heat loss.

Mark

[Anonymous]

In my scenario for a 5 person house an MVHR system, based upon the Gold standard, supplying 4.5lts/sec, will give an air change rate of about 0.3/hr (this would be less with

4.5l/s is very low. In my modeling, 61l/s were required to meet the F326 ventilation rates.

As for the 2000Pa wind load, I know what you mean. Wind does affect air leakage and make a bad situation even worse, but it's very hard to quantify. I know that, subjectively, the house feels colder when it's windy, everything else being equal, but it doesn't feel as cold as it used to when it was even leakier! The new house, which is to R2000 standards feels the same no matter how windy or cold it is outside – I've been there when it's -28C outside and is a pleasant 20C inside. Not too many properties in the UK are subject to that sort of thermal gradient!

Hot2000 modeling does a great job, though, of cost/benefit analysis of various improvements and has models of HRVs built in. The model I have of my old house has proved to be quite accurate over the period of about 18 months since we did the improvements.

Paul.

[Mark Siddall]

Paul,
4.5lt/s/person is based upon the CIBSE standards, and seems to dove tail with David O's comments above where he notes concerns that greater rates can lead to the air becoming to dry i.e. the low RH of from outside getting inside. I think the key to low rates lt/s/person is avoid the need to humidifiers.

Mark

[Anonymous]

Paul,
4.5lt/s/person is based upon the CIBSE standards, and seems to dove tail with David O's comments above where he notes concerns that greater rates can lead to the air becoming to dry i.e. the low RH of from outside getting inside. I think the key to low rates lt/s/person is avoid the need to humidifiers.

Ah, it was 4.5l/s/person – I didn't see the /person denominator. F326 uses 5l/s/person per room. Over here in Canada, dryness in winter is certainly a problem. We have a house with a HRV system and it's on a humidistat to prevent excessive dryness – though all the bathrooms have a button that provides for 20minutes of operation at maximum flow rate.

Paul.

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