All About the Frogs : Solid Walls Aren’t Always What They Seem!

Every heat pump install should start with a room by room heat loss calculation. This enables us to size the heat pump and the radiators correctly. Oversizing costs you money and efficiency, under sizing leaves you cold in the winter, so it's important to get it right.

We recently carried out a heat loss calculation for a 1930s house with a newer extension and the results pointed to a heat loss of 12kW. Seemed reasonable, but we wanted to double-check before committing to such a large system with a lot of radiator upgrades. Since it was heating season, we installed data loggers throughout the property for a month. These measured room temperatures and recorded heating energy use, giving us a clear picture of the home’s actual heat loss.

The result?

7kW.

That’s right, nearly half the predicted heat loss. A huge difference. To try and work out the cause of the problem we decided to bring in Richard from Build Test Solutions to measure the u-values of the two wall types. U-values are essentially a measurement of how quickly heat moves through an object.

Richard walked us through the process. It’s a clever setup - some surface-mounted targets, a thermal imaging camera, and 45 minutes of passive monitoring. By the end of the session, we had accurate, real-world U-values. We plugged the new figures into the same heat loss software, and the result? A predicted heat loss of 7.7kW. Not exactly the data logger figure, but close enough.

The likely cause? Well all house bricks have frogs - indents in the top of the brick that help it bond and make it a bit lighter weight. The impact in a wall can be a lot of trapped air and air is a great insulator, slowing the progress of heat through the wall. The u-values often used in these calculations don't allow for the frogs!!

This was a bit of an eye-opener for us. Installing a 12kW heat pump when an 8kW one would do isn’t just inefficient—it can drive up installation costs, often unnecessarily upgrading radiators and pipework. While it wouldn’t have been a disaster, it definitely would’ve been a missed opportunity to do things right. And that’s not how we like to work.

The takeaway? Standard U-values are a great starting point, but when accuracy matters - especially in retrofits - measured data can make all the difference.