Search with Wattson

Notifications

Clear all

Joining the Renewable Heating Hub forums is completely free and only takes a minute. By registering you’ll be able to ask questions, join discussions, follow topics you’re interested in, bookmark useful threads and receive notifications when someone replies. Non-registered members also do not have access to our AI features. When choosing your username, please note that it cannot be changed later, so we recommend avoiding brand or product names. Before registering, please take a moment to read the Forum Rules & Terms of Use so we can keep the community helpful, respectful and informative for everyone. Thanks for joining!

Should I change my cylinder when changing from a gas boiler to a heat pump.

Air Source Heat Pumps (ASHPs)

Last Post by David 13 minutes ago

1 Posts

1 Users

0 Reactions

4 Views

RSS

David

(@david-2)

Active Member Member

Joined: 2 months ago

Posts: 8

Topic starter 12/11/2026 11:37 pm [#3222]

I have 178 L Gledhill unvented cylinder which was fitted in 2018 , which Google tells me has a heat exchanger surface area of 0.78 m².

I don’t really want the expense of changing it.

We have a fairly low demand for hot water, there are just the 2 of us, and my wife showers at the gym most of the time.

Over the summer I used 6 kWh of gas per day to heat the water.

I appreciate that it will take quite a lot longer to heat up the water, in Graham Hendras book there is a graph showing that it will take an hour to raise the water temperature from 20° to 40°C, as opposed to 20 minutes with a heat pump cylinder, this is with a flow temperature of 55°C.

I assume doing it this way would not cost any more, but if the flow temperature was increased to speed things up, then this would cost more.

An hour or more would be too long to leave the house heating off. Can the system be programmed to say heat the hot water for 15 minutes in every 60?

I intend to get solar panels and a battery, so apart from midwinter I would have my “own” electricity to heat the cylinder.

Depending on which installers I choose the heat pump will either be a Viessmann Vitocal 150A 12 or a Vaillant. Heat Geek would be installing the Vaillant. and they wanted an extra £2000 to change the cylinder. The installer who would fit the Viessmann said on the phone that it would be an extra £1000 to change the cylinder, he’s coming around next week to do the survey and give a firm quote.

I would be interested to hear the experiences of anybody who has had a heat pump installed and kept their old cylinder.

Quote