Why would they rent the hardware to you if it would generate more coins than you are paying?
I'm not saying I will pay less than their costs.
But Would be great to make an investment for achieving the ROI in four or five months.
Why would they rent you a miner that will roi in 4 or 5 months instead of just mining themselves. If you give them x and the machine will generate x coins in 4 or 5 months, it doesn't make sense for them to do this because they will just get more than x bitcoins in 6 months.
I agree. It makes sense...
Well... I have some old computer which still turned on all the day. They work as a kind of server...
Is there some way to keep them mining?
I don't want lots of profit. Just some satoshis a day. The propose here is to help me to support my faucet balance.
Since they are old, they probably won't be able to produce lots of hashrates. Turning them on would essentially yield you 0 profit as mining pools have relatively high payout threshold and it would take years and years for you to even withdraw. Since it is also being used as a server, the high heat could cause hardware degradation and potentially cause hardware failures.
Hashnest is the potential legit one as they are backed by bitmain. Notice I used potential. Even with actual hashrates, the owner can still run away with the funds, look at friedcat, he was one of the more trusted people on Bitcointalk but now people are accusing him of scamming tens of thousands of dollars from his mining contract.
That's outdated, its last updated on 9 March 2015. See how they listed AMHash s legit? Yes, they scored the best and still turned out to be scam.