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Best type of boiler and cylinder for property

We're looking to replace our old boiler and cylinder. I'm trying to understand the options available before getting quotes and would really appreciate any guidance based on the below, especially if I'm making any wrong assumptions or missing something I should be considering.

The property is a 4 bed detached with two bathrooms. Current hot water use is limited, but it would be nice to be able to have an acceptable experience running two showers for the few occasions it would be useful.

Current setup is a Glow Worm Space Saver 80BR Mk2, and a vented direct tank which both come from the land time forgot, combined with a noisy incorrectly fitted positive head pump that sounds like a jet engine.

The boiler and tank work correctly, but given the property size the inefficiency is ridiculous (the utility and airing cupboard where the units are could double as dry saunas at times) so I'd like to have them replaced.

A 24kw combi boiler with high flow should be sufficient for our needs. Downsides include whether the central heating can handle being pressurised, work needed to get cold water on mains supply, and risk second shower performance won't be great (our mains pressure and flow is good even with two outlets though).

Conventional with unvented cylinder.
+ Avoids pressurising central heating, avoids need for pump
- Will require cold supply to be moved to mains, assume other changes needed to change cylinder type, will it handle two showers?

Conventional with vented cylinder (like for like)
+ Least work to change, will definitely run two showers
- Requires a pump for showers
? With current cylinder being direct I don't know if getting an indirect cyliner is problematic to retrofit

Thanks for any help you can provide,
John G

4 Answers from MyBuilder Gas Engineers

Best Answer

Hi John,

Trust you are safe!

With regards your needs.

A combination boiler for two bathrooms is not the way forward for you. Bear in mind that water pressure will reduce for every tap used - this includes toilets. A 24kw is especially small, and you would require at least 28-35kw to make shower work.

A heat only boiler would be a great option, with an unvented cylinder. They store more pressure and would cope with your water demand. I would suggest a 300l cylinder for your needs given the size of property. This is CIBSE guidance. You will also gain the most energy saving with this setup.

I would personally stear clear of the older vented cylinders. They will lose a lot of heat, and counters the energy saving of a condensing boiler.

Without seeing your pipework, it would be hard to gauge its condition, but copper pipes can take at least 10bar of pressure, and a pressuried system normally operates at around 1 bar. Copper is generally designed to last for 50-70 years.

The other option I would consider is a System boiler. This is pressurised, and contains the pump. They are generally more energy saving, as its more economical to heat pressurised water. You also remove the risk of an open vented system sludging up due to air ingress.

Please feel free to ask again should you wish to clarify anything - advice is free!


Answered 11th May 2020

I would say a System boiler would cover all your needs and is pressurized, contains everything you need.. You can then remove the vented system plenty of makes on the market. silent boilers great shower pressure no tanks in the loft.


Answered 12th May 2020

if you have adequate pressure and volume (test at a time of high local usage 6am to 9am ,4pm to 9pm ) then system boiler indirect un vented cylinder,you may have to alter and improve your pipework,but if main not adequte your left with a vented system.


Answered 12th May 2020

Please don’t go for a 24kw boiler in a 4 bed boiler will be well underrated you need to carry out a heat loss Calculation to work out the Btu of how many units you need per room
Feel free to contact us


Answered 12th May 2020

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