Котельную проверили, с тем чтобы теплосеть работала весь отопительный сезон.

Because it is the direct object of проверили.

The dictionary form is котельная. In this sentence it appears as котельную, which is the accusative singular form.

• котельная = boiler house / boiler room / boiler plant
• проверили котельную = checked the boiler house

Since котельная is a feminine noun ending in -ая / -яя, its accusative singular usually ends in -ую / -юю:

• новая котельная → новую котельную

So Котельную проверили literally has the structure [The boiler house] checked, but in natural English we would say They checked the boiler house.

This is a very common Russian pattern.

Проверили is past tense plural of проверить. Russian often uses the 3rd person plural / past plural with no stated subject to mean something like:

• they checked
• someone checked
• the staff checked
• it was checked

This is an indefinite-personal way of speaking. The sentence does not care who exactly did the checking; the important thing is that the checking happened.

So:

• Котельную проверили = They checked the boiler house / The boiler house was checked

Because the sentence presents the checking as a completed action.

• проверить → perfective → to check, to inspect, and complete the check
• проверять → imperfective → to be checking / to check repeatedly / to check as a process

Here the idea is that the inspection was carried out successfully as a finished step before the heating season.

So:

• Котельную проверили = they completed the inspection
• Котельную проверяли would focus more on the process, repetition, or ongoing action

In this context, perfective is the natural choice.

С тем чтобы is a purpose expression. It means:

• so that
• in order that
• with the aim that

So the sentence means that the boiler house was checked for the purpose of making sure the heating network would operate throughout the heating season.

This expression is a bit more formal and bookish than plain чтобы.

Compare:

• Котельную проверили, чтобы теплосеть работала...
  = They checked the boiler house so that the heating network would operate...

• Котельную проверили, с тем чтобы теплосеть работала...
  = They checked the boiler house with the aim of ensuring that the heating network would operate...

The second version sounds more official, technical, or bureaucratic.

Because с тем чтобы introduces a subordinate clause of purpose.

The main clause is:

• Котельную проверили

The purpose clause is:

• с тем чтобы теплосеть работала весь отопительный сезон

Russian normally separates this kind of subordinate clause with a comma.

So the comma works much like in English:

• They checked the boiler house, so that the heating network would operate...

Even if English style may vary, in Russian this comma is standard.

That is one of the most important things to notice.

After чтобы (and therefore after с тем чтобы), Russian usually uses the past-tense form of the verb, but the meaning is not ordinary past tense. It is part of a subjunctive / non-indicative construction.

So:

• чтобы работала = so that it would work / so that it might work
• not simply it worked

The form работала agrees with the subject теплосеть, which is feminine singular.

Compare:

• чтобы теплосеть работала
• чтобы двигатель работал
• чтобы системы работали

So yes, it looks like past tense in form, but here it expresses purpose, desired result, or intended functioning, not straightforward past time.

Because its subject is теплосеть, and теплосеть is a feminine singular noun.

• теплосеть = heating network / district heating network

Russian past-tense verb forms agree in gender and number with the subject:

• masculine: работал
• feminine: работала
• neuter: работало
• plural: работали

Since теплосеть is feminine singular, the correct form is:

• теплосеть работала

Теплосеть is short for тепловая сеть and usually means a heating network, especially a district heating system/network that distributes heat to buildings.

It does not mean just warmth or heating in a general sense. It refers to the infrastructure or system itself.

In technical or municipal contexts, this is a very normal word.

So in this sentence:

• котельная = the boiler facility
• теплосеть = the heating network supplied by that facility

This is the accusative of duration of time.

Russian often uses the accusative case to show how long something lasts:

• весь день = all day
• всю неделю = all week
• весь год = all year
• весь отопительный сезон = the whole heating season

So here:

• работала весь отопительный сезон = operated throughout the whole heating season

This is not the direct object of работала. It is a time expression telling us the duration.

Because after весь in this kind of time expression, Russian uses the accusative:

• весь сезон
• весь отопительный сезон

Since сезон is a masculine inanimate noun, its accusative singular looks the same as the nominative singular:

• nominative: отопительный сезон
• accusative: отопительный сезон

So the form does not change visibly, but grammatically it is accusative.

Yes, absolutely.

A simpler version would be:

• Котельную проверили, чтобы теплосеть работала весь отопительный сезон.

That means basically the same thing.

The difference is mainly style:

• чтобы = neutral, everyday, very common
• с тем чтобы = more formal, official, administrative, or technical

In a report, inspection summary, or municipal document, с тем чтобы sounds very natural.

Yes, it is natural, especially in a formal or technical context.

It sounds like something from:

• a utility company report
• an inspection note
• a municipal services document
• a maintenance summary

It is not very conversational, because words like котельная, теплосеть, and especially с тем чтобы belong more to official or technical language.

A more everyday-style version might be:

• Котельную проверили, чтобы теплосеть работала весь отопительный сезон.

But the original sentence is perfectly normal in formal Russian.

It has two parts:

1. Main clause
   Котельную проверили
   = The boiler house was checked / They checked the boiler house

2. Purpose clause
   с тем чтобы теплосеть работала весь отопительный сезон
   = so that the heating network would operate throughout the heating season

So the logic is:

• first, they completed the inspection
• second, this was done for a purpose
• that purpose was continuous operation during the whole heating season

A very literal breakdown is:

• Котельную = the boiler house
• проверили = checked
• с тем чтобы = so that / with the aim that
• теплосеть = the heating network
• работала = would operate
• весь отопительный сезон = throughout the whole heating season

Yes. Depending on context, English might prefer either an active or passive translation.

Possible translations:

• They checked the boiler house so that the heating network would operate throughout the heating season.
• The boiler house was checked so that the heating network would operate throughout the heating season.

Russian often leaves the agent unspecified with forms like проверили, and English can respond in two ways:

• active with vague they
• passive

Both are reasonable here.