Storage Combi or not - advice and help please

[GraemeHM]

Bit of background.... we are just completing the structure of our single story extension to our two bedroom, two bathroom (one shower, one bath/shower) house. This will add an extra bedroom and extra shower. We are on LPG and currently have a Glow-worm 30cxi combi boiler, which is in good working order and meets our needs more or less. We will be relocating the boiler into the ground floor extension as part of this project.

 

One of the things we dislike about our current setup is that every time you turn on a hot tap, even for a short period, the boiler fires up. My impression (and I'm sure I have read this somewhere) is that firing up the boiler often and for short periods is inefficient and costly in fuel. We are on LPG so fuel is expensive. One thing we do like about our current combi is that we don't run out of water in the tank.

 

Although the existing combi has been well maintained I have been considering our options.

 

One option is replacing the boiler with a system boiler and tank. Tight for space, but possible. Another option I have also been looking at is to replace the combi with a storage combi eg Glow-worm Energy 35 Store combi. My hope is that the HW tank in the storage in the combi will stop the constant start/stop of our existing setup, is that true?

 

Any thoughts and advice welcome.

 

[Nickfromwales]

2 hours ago, GraemeHM said:

One of the things we dislike about our current setup is that every time you turn on a hot tap, even for a short period, the boiler fires up. My impression (and I'm sure I have read this somewhere) is that firing up the boiler often and for short periods is inefficient and costly in fuel. We are on LPG so fuel is expensive.

If you turn the hot tap then you’re telling the boiler you want hot water ?!?  Of course it’s going to fire up. It’s an instantaneous hot water heater so it’s doing exactly what it’s supposed to do. If that upsets you, open the cold tap

 

There are negligible inefficiencies in that cycling as that’s what a combi is designed to do for its working life, eg supply hot water on demand, when there is demand, and for however long it’s demanded for.

 

Changing to a cylinder will give you far greater losses as you’ll be heating a 210-250 L cylinder, at least twice a day, and the latent / standing losses of that will be much higher than the whiff of gas used when firing a combi sporadically for small / frequent pockets of hot water.

 

The burner in the combi only lights ( consumes gas ) for the period of time that the tap is open. It runs on ( pump only) after that to rid the primary heat exchanger of the residual nuisance high grade heat that is left behind ( so the boiler doesn’t kettle ) and that only uses a tiny amount of electricity for a minute or so. 
 

Only change to a cylinder if you require more DHW, but for your situation you’d need a minimum of 210L of stored water, heated on demand. The boiler would fire sporadically to keep that topped up after an initial heat up cycle, for however long the timer is set to do so.

Edited June 12, 2021 by Nickfromwales

[MortarThePoint]

1 hour ago, GraemeHM said:

Although the existing combi has been well maintained

 

Replacing it could be a lot more expensive than the gas saved.

 

If what really bugs you is the noise from just washing hands, the cold water tap isn't a bad shout.

 

I've seen small Ariston undercounter electric tanks that may be of interest.

[markc]

MyGF has a small under sink electric water heater and it’s fantastic, I reckon it holds about 5 litres and perfect when cooking and hand washing 

[Nickfromwales]

3 minutes ago, markc said:

MyGF has a small under sink electric water heater and it’s fantastic, I reckon it holds about 5 litres and perfect when cooking and hand washing 

Mine too, but the wife does nothing but pick holes in it ?

[Nickfromwales]

Adding a multipoint such as the Ariston would cost a good few hundred quid. 
Stick with the combi, and relocate it as central to the hot water outlets as practicable. 

[SteamyTea]

On 12/06/2021 at 07:42, Nickfromwales said:

latent

Sensible when talking about thermodynamics.

Latent is to do with phase change.