Combi or unvented System?

[ianbuckley]

Hi I am new to the forum and looking for advice.





I want to replace a very old boiler that has not worked for years and have been relying on electricity for all our heating requirements. My wife and I live in a two bedroom, one bathroom bungalow. All the rooms are fairly large. I was looking to replace the whole central heating system and considering a Vaillant EcoTEC plus 835 combi, new radiators with replacement plumbing. We are both retired and don't have a requirement for lots of hot water at specific times. My biggest concern is that the boiler will be located in the laundry at one end of the property whereas the bathroom is at the opposite end, a distance of 70ft apart. Although we are not on a water meter I was wondering if an unvented Vaillant System would be a better option even at an additional cost of £2000 installed.





I have to way up the running cost of heating unused water, in the pipes with the combi and waiting a while for hot water in the bathroom, against keeping a tank full of hot water in an unvented system and the additional maintenance cost but having almost instant hot water.





I would value your thoughts and recommendations.

 

[SimonG]

Did I not read this one a few days ago?

Always unvented for me, as long as pressures and flow allow.

In your case I would want the combi or cylinder much closer to the bathroom.

 

[Chuck]

My biggest concern is that the boiler will be located in the laundry at one end of the property whereas the bathroom is at the opposite end, a distance of 70ft apart.

Hmm, that's 21m, roughly twice the rule of thumb limit for 15mm pipe. You're going to be wasting about a gallon of water each time you need any hot and waiting an annoying long time for it to come through. It's possible that a sharp-eyed surveyor may note this as an issue if you ever want to sell-up and move.

If you have space for an unvented cylinder somewhere closer to the points of demand, e.g. near the centre of the house, then that would be much nicer. If you have to have it 20 m away, discuss the pros and cons of having a secondary return and circlating pump on a timer with your installer.

Switching from electricity to gas is going to be a very significant saving regardless of the details of the installation.

One really needs to see the site to know what the best solution is going to be. Get a local independent firm with a good reputation to do you a couple of quotes for the various options.

 

[steadyon]

1. You could combine the two, using the hot water directly from the combi for the kitchen and laundry room, and using the unvented cylinder to feed the bathroom.
2. The cylinder would be heated from the central heating circuit of the combi, so you would need appropriate controls.
3. Heat loss from a modern hot water cylinder is fairly low. Depends on size , but can be as little as 1 kWh per day, approximately 8p if heated by gas.
4. You could probably get away without an unvented hot water cylinder if you were prepared to put up with a pump while filling bath / having shower.
5. A 35 kW boiler is pretty large for a two bedroom property. It might pay to do proper heat loss calculations to determine the heating load, then add, say, 2 kW for the cylinder heating load. If you plan to have the heating and hot water cylinder running at separate times you may be able to significantly reduce the boiler capacity you need.
6. It is not good practice to significantly over specify the boiler, as it can make it run in "on / off" mode rather than modulating for more or less heat. You can, on most boilers, turn the heating load down, but you are still paying the manufacturer's "extra" for a bigger boiler.

 

[ianbuckley]

Thank you all for spending time to reply. Following your advice I have spoken to my Gas engineer and taken yours and his recommendations. Change system from a combi boiler to an un-vented hot water cylinder and secondary return bronze pump located in the centre of he property Includes Vaillant EcoTEC plus system 24w, grundfoss bronze pump, 200litre mains pressure cylinder, Honeywell zone valves and hive smart controls.

 

[Scott_D]

If you have the space and money get an unvented cylinder with secondary return.
If not a combi and put up with a short wait for hot water.