The heatwave has triggered a surge in prices for seasonal items, with the cost of one inflatable hot tub nearly doubling in a week, while an industry expert said air conditioning units had risen by about 17% since April.

The Guardian looked at popular items across a range of websites and examined their prices on PriceRunner, an independent price comparison service. One of the biggest price increases was for the Bestway inflatable hot tub Lay-Z-Spa Cancún AirJet, which was available for £160 on 21 May but now retails for a minimum of £299.

Of the 11 heatwave-related items examined by the Guardian, six were at their highest price in the last three months, while the remaining five were unchanged. The Dyson Cool Tower fan was priced at £299 on Amazon, up from a low of £249.99 during the period examined. The Morphy Richards Flexi Freeze 12K BTU portable air conditioning unit rose to £410, from £389, in a matter of weeks after 4 May.

The De’Longhi Pinguino Gentle Jet air conditioner cost £689.95 at Tiny Lux, up from a low of £659.99 a few days earlier.

Nick Glynne, the chief executive of Buy It Direct Group, one of the UK’s largest independent online retailers, said: “It’s really hard to get pricing strategy right. Look at hotels, where prices change based on demand and supply. The instinct is always to help customers by offering low prices, but when demand rises and there is limited supply, the question becomes to what extent you let prices increase and where you cap them.”

He said price was sometimes controlled “algorithmically” based on how much interest there was in a product, and that there was no such thing as a “fixed price” but that there were price caps and collars.

Glynne said: “We are buying on supply and demand all the time. Shipping rates are entirely driven by supply and demand. If you’re bringing products over at peak times of the year, shipping costs can be two to three times higher.

“Raw material prices are also dictated by supply. Chinese factories produce millions of units every year, so even if I order a thousand containers it may only make a small difference. Input costs are constantly changing depending on constraints and fluctuations in raw material prices … oil prices, for example, have a massive impact on plastics … Our mantra is everyday low pricing, and more often than not prices actually go down.”

Glynne said another aspect was “deciding where to pitch your pricing in a retail environment from a customer demand perspective. Is it based on profit margins, rising demand, or a lack of demand?”

He said: “At the moment, if we were to rebuy air conditioners from east Asia, the increase [in price] in air conditioning units is around a 15-17% compared with a few months ago. That is largely down to shipping costs and the price of raw materials, particularly oil and plastics, as well as demand for both the raw materials and shipping capacity.”

Martyn James, a consumer expert, said: “As soon as the sun comes out, prices increase on countless hot weather products, from ice-cream to air conditioners. Because many businesses increase prices in advance of demand, like when the Met Office forecasts a heatwave., it can be hard to know if you’re being ripped off.”

He added: “Some businesses actually mark up prices to artificially high levels one day, then reduce them the next. So it’s better to assume that the 25% discount on a new fan isn’t quite as good as it might appear – you might still be paying more that you would have done last week.”

James suggested starting with the manufacturer’s recommended retail price (RRP). “These prices are usually the top end that you might be expected to pay and are often artificially high so they can be discounted. Next, pop the item and brand you are thinking of purchasing into a search drive and look at the ‘shopping’ options. You’ll get a good idea of what the current going rate is for what you want to buy. Watch out for shops you haven’t heard of offering suspiciously low prices though.”

He recommended using price tracker websites such as CamelCamelCamel for Amazon products and PriceSpy for other retailers. “These sites will tell you if you’re getting a good deal on an item in the sale, or whether it was cheaper in the past. You can even set price alerts on some sites so you get notified when the price drops to your ideal range.”