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HeatGeek vs Octopus surveys: puzzling disparity - any ideas why?

 
Air Source Heat Pumps (ASHPs)
Last Post by Mars 1 hour ago
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 baroquebank
(@baroquebank)
New Member Member
Joined: 6 hours ago
Posts: 1
Topic starter 26/07/2026 12:11 pm   [#3114]

I'm embarking on the ASHP road, hopefully with care, to replace the gas boiler on our 4 bed detached property. Initially had a HeatGeek survey which seemed to go well, liked the installer, all made sense and price seemed OK. But in the spirit of being cautious decided to have Octopus do a survey as well.

There are some differences between their heat loss assessments (HeatGeek 7.1kW (I feel probably better) vs approx 6kW), HeatGeek proposed 7kW Vaillant @ 45 degrees flow vs Octopus 9kW Cosy @ 50 degrees, the Octopus cost is noticeable lower than the HeatGeek but both tolerable.

The thing which is really puzzling me though is a discrepancy in their radiator replacement requirements:

HeatGeek - we must upgrade 2 upstairs bedroom radiators, both in rooms where the calculation shows existing radiators exceed heat loss

Octopus - we must replace 2 downstairs radiators, both in rooms where I do feel current radiators are marginal. Indeed one of these, in the kitchen, Octopus are saying must be significantly upgraded (or I must provide an additional heat source) in order to qualify for the government grant.

I am bemused by such a discrepancy for what should be similar heat pump solutions and am concerned that one of them might be wrong in a way that could be significant.

So I'm turning here to ask whether anyone with knowledge can provide an explanation for this discrepancy. Thanks in anticipation!



   
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Mars
 Mars
(@editor)
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Joined: 5 years ago
Posts: 4762
26/07/2026 3:34 pm  

@baroquebank welcome to the forums!

First of all, well done for doing exactly what more homeowners should do... getting more than one design and asking questions before signing anything. 

What you've highlighted is actually one of the biggest challenges in the world of heat pumps. People often think the heat loss calculation is the critical part of the design. It is important, of course, but once you're within about a kilowatt of each other, I'm actually more interested in how each installer intends to deliver that heat. That's where the design philosophy starts to diverge. The modulation and output of that unit at different temperatures also plays a part.

The radiator discrepancy doesn't necessarily mean one of them is wrong. They may be designing around different assumptions. For example, one installer may be happy with a slightly higher average flow temperature, another may be targeting lower flow temperatures for better seasonal efficiency. They may also be making different assumptions about room temperatures, radiator outputs, balancing or even how conservatively they want to size emitters.

What I'd encourage you to do is ask both installers the same question:

"Can you explain why these particular radiators need replacing, and show me the calculations behind that decision for my home?"

A good installer shouldn't have any problem walking you through it. They may point to radiator outputs at their chosen design flow temperature, explain their target delta T or justify why they're comfortable leaving one room as-is but not another.

One thing that also caught my eye is the choice of heat pump. A 7 kW Vaillant at 45C versus a 9 kW Cosy at 50C suggests two quite different design philosophies. Neither is automatically right or wrong, but I'd certainly want to understand why Octopus feels the extra output and higher design flow temperature are necessary when their own heat loss figure is actually lower than Heat Geek's.

One final thought. Don't become fixated on whether the answer is "Heat Geek" or "Octopus". Focus instead on which designer gives you the most confidence in their reasoning. Ask them to explain their decisions. The installer who can clearly justify why they've made each choice is usually the one I'd be inclined to trust.


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