"Can't we just throw a couple of cheap heaters in there?"
In November 2021, I stood in a draughty commercial unit with a client who had just received a quote to replace their failing boiler. I could see the number bothering him before he even said anything. Then he asked the question I've heard at least forty times since I started handling heating orders back in 2016:
"What if we just grab a few of those Lasko heaters for the office and a diesel heater for the workshop? That'd get us through winter."
I agreed. Not enthusiastically, but I agreed. And that decision cost us around £2,300 by the following March.
I'm not writing this to sell you on any particular product — the quote he'd rejected was for a Vaillant combi boiler, but the specific brand wasn't the point. The point was that I let a false economy through the door because I hadn't done the real math. I've now made six significant mistakes like this over nine years, totalling roughly £6,800 in wasted budget. I keep a list. This is the one I keep coming back to.
The cheap option isn't the cheap option
The thing about portable electric heaters and diesel space heaters is that they look cheap on the invoice. A Lasko heater costs £40-£80. A diesel heater runs maybe £120-£250 depending on the model. Compare that to a proper system install and of course the gap looks dramatic.
But the invoice isn't the cost. It's just the first line of it.
Here's what actually happened over that winter:
The Lasko units handled the office fine — that was never the issue. The problem was the workshop, which is where the diesel heater came in. We calculated the fuel burn based on the manufacturer's spec, which claimed something like 0.3 L/hour on low setting. What we didn't account for was the duty cycle in a poorly insulated unit. The heater was running flat-out for 9-10 hours a day, five days a week, because the building couldn't hold heat. Fuel consumption nearly tripled.
The diesel units also need venting if you don't want your team breathing exhaust fumes. I should add that we'd assumed the existing ventilation ports would be sufficient — they weren't, and we ended up rigging temporary flexible ducting that the client really should have had a proper install team fit.
By January, the client had spent more on diesel and electricity than the original combi boiler quote. By March, they'd agreed to book the full install anyway — plus we had to remove and dispose of the temporary heaters, plus there was water damage in the corner of the unit where condensation had built up because the heating was inconsistent.
Total rework and replacement: £1,450. Extra energy cost versus a properly sized system over the same period: roughly £720. And that's before we count the time I spent in January and February doing site visits to fix things that shouldn't have needed fixing.
Why this keeps happening — and it's not just cost
Honestly, I'm not sure why the diesel heater misconception persists so strongly in trades. My best guess is that a lot of us — myself included, early on — think about heating as a temperature problem. "It's cold, so I add heat." That framework makes cheap heaters feel rational.
But heating isn't a temperature problem. It's an efficiency and consistency problem. A cheap heater can add heat. What it can't do is deliver that heat efficiently across a space, at a controlled temperature, without secondary costs piling up.
There's a bigger issue underneath that one, though. The real problem with the "just grab a cheap heater" approach is that it treats heating as an emergency solution instead of a system decision. And once you're in emergency mode, you stop thinking about maintenance, lifespan, and integration — the things that actually determine what you pay over five or ten years.
This is the point I wish I'd made to my client back in 2021. It wouldn't have been about the Vaillant combi boiler specifically. It would have been about whether any properly sized, properly installed system beats portable alternatives once you stretch the timeline out.
Where I actually messed up
Let me be specific about my failure here, because this is the part I now train new team members on.
I once lost us a £4,200 job by specifying a Vaillant arotherm plus 15 kW for a building where the emitter system was undersized. On paper, the capacity matched the heat loss calculation. In practice, the existing radiators couldn't dissipate 15 kW at the flow temperatures we were designing around, so the actual system efficiency tanked. The client noticed the short cycling immediately. We ended up reducing the pump speed and adding two additional radiators — at our cost — to make the system behave. That mistake taught me that equipment specification and system specification are two different jobs.
The 2021 heater disaster taught me a different lesson: never quote a cheap alternative without running the full TCO comparison first. Not because the cheap option is always wrong — sometimes a temporary solution genuinely is the right call for a business with cash flow constraints and a two-month window. But because quoting the cheap option without showing the client the full cost picture is professional negligence, and I did exactly that.
How I now think about heating TCO
Here's the framework I use now. It's not perfect, but it's saved us from at least four repeat disasters since 2022.
Total cost of ownership = purchase price + installation + energy over expected lifespan + maintenance + risk cost + integration cost.
That last one catches people out. "Integration cost" means the cost of making this heating system work with your existing electrical, your existing emitters, your existing controls, and your existing building structure. A cheap system that doesn't integrate cleanly can triple the effective install cost.
And maintenance — this is where I see clients consistently underestimate. Any hydronic system, whether it's a vaillant combi boiler or a budget alternative, needs periodic attention. Hot water cylinders need flushing. Radiators need balancing. If you don't know how to drain a hot water heater properly — or don't have someone on-site who does — you're accumulating scale and efficiency loss that quietly costs 5-10% of your energy bill every year. That's not an emergency checklist; that's routine operations, and the TCO math only works if you include it.
For heat pump installations, add another line item: commissioning quality. A Vaillant arotherm plus 15 kW can be a very efficient unit, but only if the installer actually commissions it properly — checks refrigerant pressures, confirms flow rates, and sets the weather compensation curve correctly. I've seen installations where none of that was done, and the efficiency was worse than a well-maintained older gas boiler.
I'd rather install a slightly underspecified system with a good installer than an oversized system installed badly. The second option always costs more over time.
The specific things I check now
Before I sign off on any heating comparison — cheap or expensive — I run through four questions:
- What's the real heat demand? Not "how cold does it feel?" but an actual heat loss calculation for the building. Without that, any recommendation is guesswork.
- What's the energy cost over 3 years? Not the invoice cost — the fuel or electricity cost, based on realistic usage patterns, not manufacturer-spec ideal conditions.
- What maintenance will this need, and who's doing it? If the answer is "nobody knows," that's a warning sign.
- What's the failure mode? If the cheapest option fails in January, what happens? If the answer is "we lose £2,000 in productivity," that needs to be part of the cost analysis.
There's something satisfying about running this comparison properly and seeing the client actually get it — the moment when they realize that the "cheap" option isn't cheap. After my 2021 mistake, it felt like a small redemption to build that into our process.
A note on where this doesn't apply
I can only speak to what I've worked with — commercial and light industrial units in the £15,000-£80,000 annual energy spend range. If you're looking at a much smaller office or a large industrial distribution centre, the numbers will be different. The TCO framework holds, but the specific cost structure changes.
I've also never fully understood why diesel heaters hold such strong loyalty among certain trades, despite the maintenance overhead. If someone's got insight on that, I'd genuinely like to hear it.
What I would say — without reservation — is this: don't let anyone quote you a cheap alternative without showing you the running cost, the maintenance cost, and the failure scenario next to it. If they can't put those numbers together, they're not giving you a quote. They're guessing.
