Straight answers

Guides from our actual engineers.

Real questions we get asked, answered plainly — including when the answer is “you don't need us for that.” No fluff, no jargon, no upselling.

Boilers

How much does a new boiler cost in Hertfordshire?

A like-for-like combi swap in Hertfordshire typically costs £2,310–£3,010 installed — including the boiler, flue, system flush, magnetic filter, controls, labour and VAT. For comparison, iHeat's fitted prices start at £1,945 including VAT and its August 2026 installs averaged £2,820, with the middle half between roughly £2,525 and £3,247. A conversion (say, system boiler to combi) runs higher — typically £3,200–£4,300 with us, and around £4,000 at iHeat — because pipework and cylinder removal are involved. A quote far below these ranges is worth questioning: ask what has been left out of the job or the price.

DDavid · Founder, Datum · 3 min

Boilers

Why is my boiler losing pressure?

A cold boiler that sits in the green zone on its pressure gauge is normal; your manual gives the exact range for your model. An occasional top-up is ordinary. If you're topping up every few weeks, something is letting water out — most often a tiny leak on a radiator valve or joint, a weeping pressure-relief valve, or an expansion vessel on its way out. None of those fix themselves, and repeated topping up masks the symptom while the cause quietly gets worse.

DDavid · Founder, Datum · 3 min

Landlords

Do I need a CP12 as a landlord in Hertfordshire or London?

Yes. If you let a property with any gas appliance, flue or pipework, the law requires an annual Landlord Gas Safety Record — the CP12 — carried out by a Gas Safe registered engineer. You must give tenants a copy within 28 days of the check (or before they move in), and keep records for at least 2 years. Good condition is no exemption, and the HSE says breaking the regulations could mean a substantial fine and/or a custodial sentence.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

Air con

How much does home air conditioning cost in the UK?

A single-room air-to-air system (one indoor wall unit, one outdoor unit) typically costs £1,800–£2,600 installed in the UK — 2026 market guides put the spread at roughly £1,500–£3,000 (BestBuilders; WarmZilla gives £1,000–£2,500 for installation) depending on unit and job, with Daikin and Mitsubishi Electric single rooms listed at £1,800–£3,200. Multi-room systems start around £3,400 and scale with each room (BestBuilders: £2,800–£4,500 for two rooms, £3,500–£7,000 for three to four). Because these systems are air-to-air heat pumps, they heat in winter as well as cool in summer — which is the half of the value most people miss when comparing quotes.

TDThe Datum team · Heating, cooling & plumbing engineers · 4 min

Boilers

Combi or system boiler — which is right for my home?

For most homes with one bathroom, a combi wins: instant hot water, no cylinder, no tank in the loft. For households running two or more showers at once, a system boiler with a hot-water cylinder wins — Worcester Bosch says a combi can struggle to keep up with demand in a busy household, while system boilers suit homes needing hot water at several outlets at once. The right answer is about how your household actually uses hot water, not which boiler is 'best'.

DDavid · Founder, Datum · 3 min

Boilers

What should a proper boiler service include?

A proper annual service is done by a Gas Safe registered engineer, who checks the boiler's main components for wear and checks for carbon monoxide, as Baxi describes. The Gas Safety (Installation and Use) Regulations also require the flue, the supply of combustion air, operating pressure or heat input and safe operation to be examined. Baxi says a service takes 40 to 60 minutes on average. A five-minute look and a sticker is not a service, whatever the invoice says.

DDavid · Founder, Datum · 3 min

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How much does air conditioning cost to run in the UK?

At Ofgem's 26.32p per kWh price cap, a typical split unit costs roughly 15–20p an hour at rated power: the Mitsubishi Electric MSZ-AY25 datasheet lists 0.6 kW cooling input, the Daikin FTXM35A 0.76 kW. A hundred hours of cooling costs about £16–£20. In winter the same unit delivers several times more heat than the electricity it uses (datasheet SCOP 4.8–5.2 for those two units).

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

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Do I need planning permission for air conditioning in the UK?

Usually no, if the system provides heating as well as cooling: the Planning Portal says air conditioning may then be permitted development, within limits on size, siting and number of units. Cooling-only units need planning permission, and listed buildings and conservation areas need consent. Flats may also need freeholder consent under the lease. We check all of this at the survey.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

Boilers

How long does a boiler last — and when should I replace mine?

Baxi says a boiler lasts 10 to 15 years if it is serviced annually and looked after properly — some soldier on longer, but parts can get harder to find past that point. The rule of thumb: under 10 years old with a single fault, repair; beyond 10–15 with rising bills, repeat faults or discontinued parts, replacement often wins on total cost. The worst option is the one most people choose: waiting for the fatal breakdown on the coldest week of the year.

DDavid · Founder, Datum · 3 min

Boilers

What size boiler do I need for my house?

For a combi, size is driven mostly by hot-water demand: EnergyPages puts 24–30kW as suiting most 3-bed homes and 30–35kW a 4-bed with two bathrooms; beyond that, a combi may be the wrong type entirely (see system boilers). EnergyPages also says oversizing burns more gas per cycle and shortens boiler life. Proper sizing counts your radiators, bathrooms and mains flow rate; it isn't read off the house price.

DDavid · Founder, Datum · 3 min

Boilers

Why is my boiler making banging, gurgling or whistling noises?

Most boiler noises have a recognisable cause. Vaillant says kettling, banging and whistling come from water heating too quickly and reaching boiling point, often where limescale builds up; gurgling is trapped air, or Worcester Bosch says a blocked condensate pipe. Kettling can be dangerous and should be inspected by a qualified professional. The pattern (when it happens, where it's loudest) tells most of the story, and a short video with sound lets us tell you which you have before anyone books anything.

DDavid · Founder, Datum · 3 min

Heat pumps

Heat pump or new boiler — what should I actually do in 2026?

Honestly: for a well-insulated home with generous radiators (or underfloor), an air-source heat pump is a serious option — and government grants exist for qualifying installs, claimed through MCS-certified installers. For a typical draughty UK home with microbore pipework and standard radiators, a modern boiler is still often the pragmatic swap today — with the house prepared (insulation, radiator upgrades) so a heat pump is the natural next step. And there's a middle path most people miss: air-to-air heat pumps (air conditioning that heats) can take over room-by-room heating now at low cost.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

Boilers

Why is my boiler leaking water?

Water under or inside a boiler is never normal, but the cost swings enormously with the cause. The common, cheaper ones: worn seals, or a pressure-relief valve letting water out because system pressure is too high — Vaillant calls excessive pressure one of the most common causes, and lists a faulty expansion vessel or passing filling loop. More serious is corrosion, and Vaillant says a heat exchanger can fail or crack over time, especially in older boilers. If water is actively dripping, especially near the electrics, switch the boiler off at the fused spur, catch the water, and book an engineer before a slow leak finds your floors and ceilings.

DDavid · Founder, Datum · 3 min

Boilers

Why does my boiler keep turning off or cutting out?

A boiler that fires then cuts out is protecting itself from something — the trick is which something. The most common causes are low system pressure (check the gauge — if it has dropped below the range in your manual, it may lock out), a frozen condensate pipe in cold weather, poor water circulation from a stuck pump, sludge or a closed valve, or air in the system. Those are often fixable. A boiler that ignites with a bang and then drops out, or throws the same fault code straight after every reset, needs an engineer promptly — repeatedly resetting a boiler that's failing to fire safely is the one thing not to do.

DDavid · Founder, Datum · 3 min

Boilers

Why has my boiler got heating but no hot water (or vice versa)?

When one works and the other doesn't, the fault is usually in the part that decides where the heat goes — not the boiler's ability to make heat. On a combi, Vaillant, Viessmann and Baxi all name the diverter valve, which sends hot water to the radiators or to the taps: stuck one way, you get one without the other. On a boiler with a cylinder, other components in the hot-water circuit are involved. It's rarely a whole-new-boiler problem, but it needs an engineer to pin down which component.

DDavid · Founder, Datum · 3 min

Boilers

How much does a new boiler cost in a Victorian or older house?

A like-for-like boiler swap in a period property typically lands in the same £2,310–£3,010 range as any combi swap, but older houses more often need the extras that nudge it toward the top of that band or into a conversion — because of what's behind the walls, not the boiler itself. What moves it: microbore or lead pipework, a flue that has to route through an old chimney, no easy filling loop or condensate route, and years of sludge that make a proper system flush essential. We give you the honest fixed price after a free survey — no guesswork off a postcode.

DDavid · Founder, Datum · 3 min

Boilers

How long does it take to fit a new boiler?

A straightforward like-for-like combi swap is almost always a one-day job — out with the old, in with the new, flushed, tested, controls set and handed over, often with your heating back on by evening. It stretches to two or three days when the job is more than a swap: converting from a system boiler to a combi (or vice versa), moving the boiler to a new location, upgrading pipework, or fitting a cylinder. A proper installer tells you the realistic timeline at the quote, not on the morning.

DDavid · Founder, Datum · 3 min

Boilers

Should I repair or replace my boiler?

The rule of thumb: under about 10 years old with a single, non-major fault — repair. Beyond the 10–15-year mark, or facing a big-ticket part (heat exchanger, PCB, fan) on top of rising bills and repeat call-outs — replacement often wins on total cost. The two questions that settle most cases: is the failed part a major one, and are parts still available for your model? A discontinued boiler needing a heat exchanger is a replace; a five-year-old boiler needing a diverter valve is a repair. We quote both, every time, and let you choose with the real numbers.

DDavid · Founder, Datum · 3 min

Boilers

What is a power flush — and does my system need one?

A power flush pumps cleaning chemicals through your radiators and pipework to shift the black sludge (corroded metal) and scale that build up over years. Signs you may need one include radiators cold at the bottom, cold spots, a noisy boiler or slow-to-warm rooms. Approved Document L says heating circuits should be thoroughly cleaned and flushed before a new heating appliance is installed. If your system is clean and protected, you don't need one, and we'll say so.

DDavid · Founder, Datum · 3 min

Boilers

How do I choose a boiler installer — and what should a good quote include?

Start with the non-negotiable: the engineer must be Gas Safe registered, and you're entitled to see the card. Beyond that, the difference between a good installer and a cheap headline is what's written down. A proper quote is fixed, in writing, and spells out the boiler and flue, a full system flush (not a rinse), a filter, controls set up and explained, making good, removal of the old boiler, the warranty length and what keeps it valid, and any finance terms in full. If a quote is vague on those — or the price seems too good — that's usually because something on that list is missing.

DDavid · Founder, Datum · 3 min

Air con

Why is my air conditioning not cooling (or blowing warm air)?

Start with the checks you can do safely. Daikin's checklist for a unit that won't cool is: air filters not dirty, air inlets and outlets of both units not blocked, a sensible temperature setting, doors and windows closed and airflow settings appropriate. Confirm the unit is in cool mode, not heat or fan-only. If it's still blowing warm, it needs an engineer: GOV.UK says working on air-conditioning refrigerant needs F-gas qualifications, so it isn't a DIY or top-up-bottle job.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

Air con

What size air conditioning do I need for a room?

Allow about 0.1 kW per square metre as a starting point. Daikin's guide puts a well-insulated 20 m² bedroom at about 2 kW and a 35 m² living room at about 3.5 kW, with large south or west windows adding 10–20%. BestBuilders says an undersized unit runs flat out and never cools the room, while an oversized one short-cycles and wastes money. We confirm the size at the survey.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

Air con

Can air conditioning heat my home in winter?

Yes — and it's the half of the value most people miss. A home air-conditioning unit is an air-to-air heat pump: in winter it runs in reverse and heats the room, and because a heat pump moves heat rather than burning fuel to make it, on the datasheets for the units in our range you get roughly 4.7 to 5.2 kilowatts of heat per kilowatt of electricity (standard test conditions). For heating the rooms you actually live in — a home office, a bedroom, a snug — it's far cheaper to run than an electric radiator. It cools in summer and heats in winter from the same unit.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

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Is home air conditioning worth it in the UK?

Often, yes — but for a reason most people miss. The Met Office says summer 2025 was the UK's warmest on record (16.10°C, beating 15.76°C in 2018), and a bedroom that stays cool is a real quality-of-life issue. But the honest case for air conditioning isn't just the hot weeks: a modern unit is an air-to-air heat pump, so it heats efficiently in winter too. That turns it from a seasonal luxury into a year-round system that cools in summer and heats the rooms you use most in winter. Where it's not worth it: if you only want it for a handful of nights and won't use the heating side, a good fan may do.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

Air con

What's the best air conditioning for a bedroom?

For a bedroom the priorities flip from a living room: it's about how quiet and how gentle, not how powerful. The best bedroom units are quality inverter models (Daikin, Mitsubishi Electric and similar) that run quietly on sleep mode — the datasheets for the units in our range give 18–19 dB(A) for the indoor unit's quietest setting — and hold a steady temperature without the on-off blasts of a cheap unit. Size matters the other way too: Daikin puts a well-insulated 20 m² bedroom at about 2 kW (2.5 kW if poorly insulated or sun-facing), and siting is half the job — positioned so the airflow never blows directly onto the bed. Get those right and you barely know it's there.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

Air con

Single-room or multi-room air conditioning — which do I need?

It comes down to how many rooms you genuinely want to control and how they're laid out. A single-room (single-split) system is one indoor unit and one outdoor unit — simplest, ideal if you mainly want one space sorted. A multi-room (multi-split) system runs several indoor units from one outdoor unit — tidier outside and great when you want a few rooms independently controlled. But multi-split isn't automatically cheaper or better: BestBuilders puts a single room at £1,500–£3,000, two rooms at £2,800–£4,500 and three to four at £3,500–£7,000. We size it to your actual rooms.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

Hot water

What is a thermal store, and how does it work?

A thermal store flips the usual cylinder idea around. Instead of storing your hot tap water, it stores a tank of hot 'heating' water, and when you turn on a tap the incoming cold mains passes through a heat exchanger and comes out hot, at mains pressure. Gledhill, a thermal store manufacturer, says the stored water is not fed to the taps and that a store can take several heat inputs, such as a boiler, solar thermal or a solid-fuel appliance. That flexibility is its whole point.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

Hot water

Thermal store or unvented cylinder — which should I choose?

Both deliver mains-pressure hot water — the difference is what they store and how they're installed. An unvented cylinder stores your hot tap water directly, under mains pressure, heated by the boiler; it's simple and suits busy households, but it's a pressurised vessel, so Approved Document G treats installing one as notifiable building work needing at least two independent safety devices, and Heatrae Sadia recommends annual maintenance. A thermal store stores heating water and warms your tap water on demand through a heat exchanger, and takes several heat sources easily. For a standard gas home with high demand: usually unvented. For a multi-source or renewables setup: often a thermal store.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

Hot water

Which hot water cylinder do I need — vented, unvented, or a thermal store?

There are three broad routes. A vented cylinder is fed from a cold tank in the loft, so Vaillant says it relies on gravity rather than mains pressure — cheap and simple, common in older homes. An unvented cylinder is fed directly from the mains, which Vaillant says means better pressure, but it needs a good incoming supply and a competent installer. A thermal store heats water on demand and suits multiple or renewable heat sources. Which you need comes down to your incoming mains pressure and flow, how much hot water you use at once, your heat source, and space.

TDThe Datum team · Heating, cooling & plumbing engineers · 3 min

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