David Bleicher provides the answers to tricky technical questions posed by BSRIA Members. Problems solved this month include harvested rainwater and concerns with vending machines.
Q. Do you have any experience with non-return valves passing water in the wrong direction?
This is quite a common problem. Depending on the type of non-return valve, if there is insufficient back-pressure to close off the valve, you will get some back-flow. You could remedy the problem by using double check valves, or in the case of a thermostatic mixing valve (TMV), installing an additional non-return valve immediately upstream of the TMV. The dead-leg should be as short as possible to reduce the risks associated with warm water stagnating.
Cups versus mugs
Q. What is the energy consumption difference between using mugs and a dishwasher, as apposed to a hot drink vending machine with disposable cups?
Hot drinks vending machines can consume approximately 5 kWh per day according to research carried out by NUS Services Limited Reduced Energy Guide Action Sheet 6. A domestic dishwasher will consume 1.07 kWh per use at 55°C and 1.44 kWh at 65°C.
By managing the frequency of washes a dishwasher uses in the course of a day (for example, once in the evening or in the morning with a full load of mugs) the dishwasher will be more energy effective.
A step further with regard to disposable cups versus mugs, taking in to consideration the energy used to produce them, it would take 39 uses of a ceramic mug to break even with a paper disposable cup and 1006 uses to break even with a disposable foam cup.
Q. Is it permissible to locate a boiler in a high voltage intake room?
Part B; Fire Safety of England & Wales Building Regulations is supported by guidance. In Approved Document B, Volume 2 relates to buildings other than dwelling houses. This classifies both oil-filled transformer and switch rooms and boiler rooms as places of special fire hazard. It states that every place of special fire hazard should be enclosed with fire-resisting construction.
Furthermore, there is an access issue. Those who have keys to the HV intake room aren't likely to have keys to the boiler room, and vice versa. In any case, on matters such as these it's always best to consult with the eventual users of the building, the CDM co-ordinator, and the building control authority for the specific project.
Q. Can I feed harvested rainwater into a domestic hot water system?
There is no reason you shouldn't be able to do this. The British Standard on this is BS 8515:2009, available from the BSRIA library, and there's some additional guidance in BSRIA Guide BG 1/2008 The Illustrated Guide to Renewable Technologies which is available from the BSRIA Bookshop as a free download to Members, and available to purchase by Non-members.
Q. Why do the communal areas in my apartment block get so hot? It was 27°C outside my apartment last winter when it was 5°C outdoors.
This is a common problem, particularly in new developments with good airtightness and thermal insulation in the external walls. Temperatures can be even higher in summer. Heat is lost from buildings through the walls, roof, windows and floor and by ventilation. Communal areas often don't have any of these.
To make things worse, the pipes that take heating and hot water to apartments usually run through the communal areas. Heating pipes at 70°C to 80°C and domestic hot water at 60°C to 65°C always lose heat to the surroundings. This means that there will be a heat gain to the communal areas. This heat must go somewhere, so the temperature of the communal areas rises until the air temperature is high enough to lose heat to the surrounding apartments.
To solve this problem you need to either reduce the heat flow into, (or increase the heat flow out of) the communal area. In practice this means increasing the insulation on the pipes.
BS 5422:2009 specifies the maximum allowable heat loss from pipes of each common diameter. There is always going to be some heat loss from pipes, typically 10 to 18 W/m, even from pipes that meet this Standard.
The other way to reduce the temperature is to introduce fresh air ventilation. Table A1.5 in CIBSE Guide A: Environmental Design recommends an outdoor air supply rate of 1.3 l/s m² in corridors.
In a recent case investigated by BSRIA, with conditions like this the temperature was reduced from 27°C to 23°C in winter and 32°C to 30°C in summer just by adding the recommended ventilation rate. Increased ventilation can be used to reduce the temperature further, but this wastes energy so insulation is preferred.
Designers of heating systems can also try to minimise the heat gain to communal areas by keeping hot pipes out of the areas, using control systems that reduce the temperature when less heating is needed, or by incorporating ventilation.
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