Space heating back-up systems: For when it goes wrong (or you are refurbishing)

[Mark Siddall]

Apologies for the length of this one: –

The premise of this posting: When it all goes wrong and the Gold/PH targeted building does not perform to the design standard, the need for a conventional back-up heating system can be designed out, such a design will save time and money. ….at least that’s what I have come to believe.

Background
Given that, as a nation, we are unfamiliar with all-air heating systems it’s easy to develop blinkers when designing system that we are unfamiliar with (at least that’s my experience elsewhere). With regard to the Gold/PH standards the documents that I have read to date have seemed to suggest that, based upon the 50C heated air temperature limitation, a generic all air system can ONLY supply a heating load of <10w/m2. Thus the space heating standard of the Gold/PH standard was determined by the selection of an all air system and the space heating that such as system can provide.
It was as a consequence this observation that I believed that the PH system was liable to being very deterministic i.e. hit all the standards (U-value, airtightness etc) or the design will fail and occupants will suffer the consequences….unless you also have a more conventional backup system (rads or underfloor etc); thus incurring additional costs, materials, delays etc. One of the key aspects of the PH standard was its affordability, the concern is that such additional cost will inhibit the development of such projects.
Given that these standards seems to hinge on the success of the buildings airtightness* the risks associated with designing to these standards in the UK could be considered to be high. (The highest tested airtightness in the UK, that I am aware of (found on this forum), is 1.5m3/m2@50pa i.e. 50% greater than the Gold/PH standards require). The question then arises how can a least cost back-up system be implemented?

Experience Abroad
Based upon a Canadian example (see “Heating using MVHR” elsewhere on the forum) where heating demand far exceeds the PH standard, it would seem that all air systems are far more robust than the PH documents lead me to believe. Paul-in-Montreal has noted that in Canada they use all-air heating via mechanical ventilation pretty much consistently in residential properties. Given that they are using these systems successfully, and safely, to heat buildings that do not achieve the PH standard you can only conclude that that some other factor previously unconsidered is at play i.e. a generic all air heating system can supply a heating load of >10w/m2.

Proposal
If the heat load is greater than 10w/m2, and it is considered desirable to avoid a traditional back-up system, then it must be recognised that the heated air temp has to remain below 50C. How then can the heat supplied be increase? There seems to be two choices 1) increase velocity 2) increase volume. Increasing velocity has to be discounted, as it has to be maintained for comfort. On this basis the only conclusion that I can reach is to increase the volume of air that is to be heated. As a consequence the cross sectional area of the duct will need to be increased thus increasing the carrying capacity and the energy delivered /second.

It is on this basis that I tender that rather than resorting to back-up systems (traditional rads, underfloor heating etc) and incurring additional on site labour, materials and expense, you could simply re-size the ductwork to accept an increased heating load. Given that the primary heating system (heat pump, condensing boiler etc.) would need to be able to provide adequate energy for DHW/solar panel back-up, and that this peak demand is much higher than the space heating requirement, no major changes to the heating plant will be required (not knowing much about heating system I could be very wrong on this one, I presume you can “adjust the flow” some how).

The energy consumption will be greater than 10w/m2 but the fundamental technologies behind the PH have been introduced and installed without resorting to conventional back-up systems.

Does this make sense? Is the logic flawed? Does anyone foresee any problems in implimenting this strategy? Comments appreciated.

Mark

* refer to Energy-efficient terrace houses in Sweden Simulations and measurements, Maria Wall

[David Olivier]

Mark

But the airflow then exceeds what a normnal ventilation air supply can provide, needing more fanpower (and recirculation).

And convective heating may not be all that comfortable if the thermal envelope has defects in it. Radiant floor heating does compensate a little for downdraughts and for the pool of cold air which collects on the floor of normal UK buildings.

David.

[Mark Siddall]

Paul-from-Montreal has done another a sterling job and exposed, a little more clearly than David, the failing in my concept 🙁

Key statements in his email were:
“It seems that you're assuming an all-air heating system is required to supply new fresh air at the rate that is required for heat delivery (i.e. in my case, 1400CFM). This is not a valid assumption.”
“There is no need to introduce new air at anything like the rate that the heating system moves it around.”
He also felt that the installation diagrams on page 4 of this document: http://www.vanee.ca/literature/install/bronze_Install.pdf would make things clear.

So, if I understand correctly, the Canadian recirculatory system relies upon volume air circulation (compared the PH standard this is achieved by increased volume or increased velocity) for the all-air heating system thus allowing more energy to build up within a closed loop. This “air flow volume/second” can when required be greater than the min. fresh air requirement. As a consequence only a small proportion of the air flow volume is actually fresh air, the majority is recirculated air. By moving this larger volume of air around, and being able to heat it, suitable internal air temperatures can be achieved. By heating large volumes of air.

If, like the recirculatory system, large quantities of air were to be heated using a MVHR system designed to the PH standard (i.e. not recirculatory) then, due to its configuration, heat loads would increase significantly due to the fact that a larger proportion of cold external air would have to be heated to an acceptable internal temperature. This would result in exceeding the PH space heating standards.

In a nutshell; all has, I think, become clear. Paul was bang on when he observed my assumption that “an all-air heating system was required to supply new fresh air at the rate that is required for heat delivery.”
That's where I went wrong!

Thanks all. At least I can put that one to rest!

Mark

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