This time, it’s a paint job and solar heaters.



Architect Navneet Malhotra is a perpetual student. He loves to break myths and unmask the true cause for bad work by constantly sharing experience… “Path to gaining knowledge is through sharing,” is his motto.




I have just bought an apartment which was used by its earlier owner for about eight years. This is a middle income group society and the wall surfaces of the apartment are both uneven and flakey. Two walls in the living room have old and outdated wallpaper. I am confused whether I need to strip it off before painting those walls. I think that the walls are not in good shape, and that removing the wall covering will damage them further. The uneven plaster behind will be exposed and I do not have the funds to do a coat of plaster of paris.
Harsukh Patel, Mumbai

Start by rubbing or dabbing a lightly damp cloth wad on a corner of the wallpaper. See if the ink smudges or spreads. If so, you won’t be able to apply paint on top. Also see if the area becomes a little moist and appears to be  darker than the surrounding. Appearance of this dark patch will indicate that there is no vinyl (water repellent) coating on the surface (some special wallpapers have a water repellant coating that does not allow the paint to get a grip on the surface). Therefore, if you have wallpaper that has a ‘fast’ print and its surface is not coated with a vinyl film, then you could theoretically paint it over. However, before you do this, you will still need to address a few issues.

Paper that is loose, ripped or has holes cannot be painted over and will have to be removed.

The moisture in the paint can also dilute the wallpaper’s adhesive, causing sections to wrinkle, bubble or detach from the wall. In case this happens after the first coat of paint, I suggest you don’t proceed further and work towards the removal of the wallpaper instead. By continuing to paint, you would be investing effort and money into a project that won’t last.

Most papered surfaces have a bold textured pattern. There are chances that this texture or print may show through the new coat of paint. I suggest you paint a small section of the wallpaper before starting.

Painting over a wallpaper has outcomes that are hard to predict. For example, all wallpapers are available in 10-mt-long rolls that are about 0.5 m to 0.7 m wide. These long strips of paper are stuck next to each other without overlap to achieve a seamless, joint-free surface. However, on painting, these invisible seams or joints could start to show through and may appear as fine cracks on the surface. The best way to prevent some of these problems is to apply a coat of oil-based primer on the surface (wall putty) before painting, and follow this by sandpapering it to get rid of undulations. The base primer will dampen the impact of joints as well as make the surface seem more flat and smooth. Avoid paints that have low viscosity or use excessive water.

Just in case you choose to remove the wallpaper, you will still need to understand that, depending on how and when the paper was applied, the work could be difficult, messy and time-consuming. In ‘old’ papered surfaces, the glue can become one with the plaster and may form a solid mass. So large clumps of loose plaster may detach from the main brick surface along with the wallpaper…

According to me, removing the wallpaper will be a better option, since repairing the plaster and applying a base primer will become inevitable, irrespective of whether one chooses to paint or simply re-paper the surface.

Solar water heating seems to be quite popular. I am planning my home and would like to incorporate such a system right at the outset to avoid making changes while retrofitting. Can you please tell me if this is a good choice to make, in terms of cost effectiveness and efficiency.
Harit Singhania, New Delhi

Solar water heating uses the energy from the sun to heat water. Solar water heaters (SWH) with a capacity of 100 to 300 lts are available for residential use in India. The temperature of water achieved is about 60 oC to 80 oC. A 100-litre SWH can replace an electric geyser and may save up to 150 units of electricity monthly and thus reduce the carbon footprint, a small but significant contribution.

The main components of a SWH are a collector to harness solar energy, an insulated tank to store hot water and connecting pipes. Two types of SWH are available. The Flat Plate Collector (FPC) SWH, as the name suggests, consists of flat plate collectors covered by an insulated, glass-topped metallic box that receive the sun rays. These systems have a long life of 15 to 20 years, and are suitable for non hilly regions with good water quality. They are available in multiple of 100 LPD (litres per day): 100, 200 and 300. The second type is the Evacuated Tube Collector (ETC) SWH where the collector is made of double-layered evacuated or completely empty borosilicate glass tubes having a selective coating on the outer surfaces of the inner tube. These systems are less expensive (by 10 per cent to 15 per cent), and can work efficiently in hilly regions where availability of flat ground is limited. This system is available in any size: 50, 75, 100, 125 and 150 LPD. The life of this system may, however, be less than that of a FPC system as its glass collectors are prone to damage and are high on maintenance. This is why most vendors as well as government-published journals recommend this type over the ETC system for domestic use.

A good SWH should have a sufficient collector area for the required capacity. For example, on a sunny winter day in North India, 1 sq m of FPC area produces approximately 50 lts of water with temperature of upto 30 °C to
40 °C. FPCs made in the country typically have an area of around 2 sq m and are thus capable of heating around 100 lts of water in a day. An ETC system of 14 tubes covering an area of 1.5 sq m is able to provide 100 lts of hot water in a day. The most efficient way to determine the required capacity of a SWH is to make an estimate the daily requirement hot water. An SWH is capable of heating a fixed quantity of water and is most efficient during sunny days. The temperature of water is determined by the combination of the collector area and the tank capacity. The water temperature in the storage tank could easily reach 50 °C to 60 °C, much hotter than what we require for bathing (around 40 °C). It may be mentioned that a 100-litre system is considered adequate for a family of four adult members.

Some of the issues that need to be considered while planning a SHW system:

Cloudy days makes the system inefficient. The temperature of the water does not reach the comfort zone. Many manufacturers have begun adding a back-up heating element (a metal rod) inside the storage tank to ensure availability of hot water all year round. This takes care of those days when the sun does not shine or demand of water goes up.

The water that is heated during the day is stored in an insulated storage tank that may experience cold winds during the night. Though these tanks are well-insulated, the temperature of the stored water may drop below the comfort zone by the time you use it in the morning. The heating element comes to the rescue here, but defeats the purpose of saving energy and reducing the carbon footprint. Thus, insulating the supply pipes becomes absolutely necessary. Nitrile rubber tubes with a wall thickness of 15 mm are an excellent option, but they fail at junctions (pipe joints). Here, you could introduce insulated rope along with insulation tape.

The distance between the storage tank and the bathroom increases the time the hot water takes to reach the outlet, while the stagnant cold water in the pipes further reduces the temperature. Ideally, the outlet (tap) should be connected back to the storage tank, making it a closed-loop circuit. This way, by installing a small temperature sensor and a low-capacity automatic pump in the loop, you could circulate the water every time the temperature drops below a pre-defined level (say 18 °C). That will ensure the presence of hot water in all locations and no time lag every time you open the tap.

Paying attention to a few areas will improve the efficiency of the system. For instance, if an electrical back-up is provided in the tank, set the thermostat at the lowest acceptable temperature. Dust on collectors reduce their efficiency, so try to get them cleaned at least once in a week. In case the hot water supply pipes are connected to a reverse cycle (closed) loop and pumps, install additional SWH collectors to account for the extra water stored in the pipes.

Typically, domestic SWH systems are installed on the roof to take advantage of unobstructed sunlight. The collectors have to face the sun, and hence should be oriented towards the south for maximum reception. There should be no hindrance in the path of sunlight towards the south, west and east (an arc of about 120°, ie 60° both sides of the southern direction, should ideally be free of shadows). As a thumb rule, the requirement of a shadow-free area is around 3 sq m for each 1 m x 2 m collector used. As far as possible, the area should be flat, away from rainwater drains and close to the bathrooms. Cold water should be available at a height of around 2.5 m from the base of the system.

You can save on electricity bills considerably by using SWHs. Where regular use is concerned, you can save up to 150 units of electricity per month, and if the electricity rate is ` 5 per unit, it equates to ` 750 per month, assuming 100 lts are consumed per day. Considering we use hot water for six months in a year in North India, you can save
` 4,500 annually!

An SWH of 200-litre capacity can serve a family of 4, with two bathrooms and a kitchen comfortably. The cost of the SWH varies across vendors. An FPC SWH of 200-litre capacity with 4 sq m of collector surface will cost about ` 30,000, while an ETC system may be cheaper by 15 per cent to 20 per cent. This cost may vary from state to state as the subsidies offered by various state governments are different. It covers the collector, tank, basic piping and the electrical back-up only. A typical payback period of an SWH is 5 to 6 years.

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