Blocked or broken derby drains. A combi boiler is the most compact, space saving hot water and central heating system available today, which is why it’s also the most popular choice for modern homes. Operations include full commercial, residential and heating and plumbing service departments. If you’d like a little more information about what type of system might be best for you please call us on. Do you suspect a problem with your drains? faults within your drainage system that are not addressed.