Dear Mr. XYZ
I am the tenant of your flat situated in the Army Officers Colony. I am writing to inform you that the hot water or heating system for our flat has not been working for the last two weeks due which we are facing a big problem.
We have been your tenants for the last five years and we have never given you a chance of reason to complain as we paying the rent on time. However, but this time we are really upset at you.
You seem to be avoiding present perfect patterns, even though it is the only correct option. Remember that present perfect grammar is taught at elementary level. You definitely need to know how it works.
Also, revise <em>a</em>/<em>the</em>, and prepositions.
Beware of writing too much. I removed some phrases because they were unnecessary.
I didn't remove it, but you should avoid direct personal comments ("we are really upset at you") in formal writing. There are methods and phrases which you can use to express your feeling in a formal register.
Although the handyman you sent the works man last week tried his best, he was but unable to fix the problem because, he suggest that according to him, the boiler is seriously damaged and cannot function anymore.
There were some inaccurate choices of words, which made the sentence sound awkward.
Therefore I request you to re-send someone else, or the same handyman, who can replace the boiler. and I also request give us a 20% reduction in rent amount for this month because we have spent almost two weeks without hot water or heating system.
You need to split this sentence in two, otherwise it sounds as if the handyman is giving the compensation.
I removed "or heating system" because it sounds as if you don't know what the problem is. The hot water or the heating?
I hope you will take act on our request to resolve this matter as soon as possible.
The sentence sounded awkward. Perhaps it was the informal register in a formal sentence.
Yours faithfully
Awais
In general, it's an OK effort but there is some basic grammar which is clearly missing. Some sentences were too long and the information became confusing. Because you switched from <em>I</em> to <em>we</em>, There was also a little confusion about who lives in the flat. You, you and some colleagues or you and your family?
I hope this feedback is a little helpful, and that your next effort will have these errors solved.