i bought one unit of "tsunami water pump" at a stall in tesco puchong (next to JCO Donut i think)

so far so good, 0.65hp able to support 2 showers & 4 taps...at least the water still runs the shower whenever someone else's using the taps...

is it necessary to get a water pump for the whole house, or only heater with pump will do for a normal DSL house?

Using Grundfos- CM3 5PT
Bought from 
https://www.facebook.com/builders.hardware?fref=ts

No complaints and cheapest if you are in Klang Valley

depends on your requirements..

Case 1: some would have problem pumping water from the main source (supplier/well & etc) to their tank, so they'll need a pump to pull it from the source & feed to the main tank..

Case 2:some would have no problem getting water from the source to the tank, but having problems with the pressure of the house piping. One of the symptom would be the loss of water pressure whenever all taps & showers are being used simultaneously... in this case, they'll need a water pump to regulate the water pipe pressure for the whole house..

brother (in toilet):
"hey, who's using the water,.... i am bathing here..got shampoo in my eyes..arrgghhh..!"

sister (at the kitchen):
"i am doing my dishes...shut up or come here & do it for me..!"

before getting a water pump, first have to check if there's a leak at any point of the piping too...look for "swollen" walls or any constantly-damped surfaces with molds...things like that

IMO, water pump of your heater helps to pull water from your tank, or direct from the source to ensure there's enough amount of water to be heated...it still depends on the overall internal piping of your house to get optimal pressure..

Before you buy a pump you need to consider what you are using it for. Is it because:
1) the overall incoming water pressure is low? In large well-established areas with high population density this can be a problem
2) or, there is significant and consistent pressure loss inside the house, especially on the higher floor?
3) or, your water pressure is so-so, but you feel that you want to higher pressure especially for rain shower installations?

You also need to understand the piping system of your house, i.e., the quality of the pipe, and how the water flows, that is, which faucets/outlets are getting water from the mains and which are getting though the tank. Typically cisterns and toilet sinks, as well as showers on higher floors may be supplied from the tank, while showers on the ground floor and kitchen sink might get the supply direct from the mains.

After understanding your needs and consulting with your plumber, you might find that you do not need a pump at all if you just do a little bit of rerouting.

scenario 1, i think it would be safe to say you really need a pump (outdoor would be fine).
scenario 2 and 3 you need to understand the piping system first to decide the optimum placement of the pump, i.e., indoor (rooftop) or outdoor (near the mains).

Remember that pumps can add anywhere from 1 bar to 2 bar boost to your water pressure. If you have old or poor quality pipes, they might burst!

Other Factors to consider for pump:
variable vs fixed boost, safety features.

boost level: how many bars can you get out of the pump? You need to consider your incoming pressure and the height of the house. For normal 2-3 storey house, more than 2 bar boost is excessive.
Variable boost: means there is a controller setpoint that varies the pumping speed to maintain a set pressure value. E.g. set to maintain pressure at 2 bar.
Fixed boost: means it gives you the full 2 bars (on top of your incoming pressure) when it turns on. Quite risky in my opinion.

Safety features:
-sediment trapping (prevents incoming sand or dirt from clogging or damaging your pump)
-dry run safety (auto cut off in case there is no incoming water, in order to prevent the pump from heating up and getting damaged.)

Other factors:
Noise: quieter is better..so you don't disturb your neighbours
Placement: having a pump indoors is more discreet, if you place outside, you might get in trouble with the council or water authorities if they catch you putting it too near the mains.

Pump brands: There are Japanese, German and China-made brands out there. In general Japanese and German brands are better engineered and last longer.

Things to remember:
water pumps are mechanical devices that need to be maintained. It's quite normal to hear of the cheaper China made brands breaking down after 1 year. The safety features I mentioned above help prolong the life of the pump.

Lastly, do you really need a pump? In my case I learned the hard way I didn't really need it. I wanted to get better shower pressure on my 2nd floor so that I could have a rain shower. I made the mistake of assuming that the shower on the 2nd floor was getting water from the mains (I confirmed the ground and first floor ones were) and got a outdoor pump. When I turned it on, the sudden water pressure broke my riser and I got a flooded living room.    In the end I discovered that I could live without the pump if I just made a bypass from the mains direct to my 2nd floor shower head. The pressure from the rooftop water tank combined with the mains was enough to get me a decent shower pressure.

BTW, I am using Grundfos CM 3-5 PC currently. I found it is much more noisy compare to the Tsunami pump at the same power rating!