Condensing or combi boiler

[Tully]

I need to change the boiler and have a conventional boiler water tank/header feeder in loft and cylinder in airing cupboard. Its been suggested I get A combi boiler and do away with tanks. Its a four bedroom house with 14 rads and two bathrooms with just me and two children. I like a bath but I have a shower room for the children. Is this a good idea? Ive been told to go for a Worcester Bosch greenstar classic ? Advice please.
Tully

 

[bernardgreen]

Keep the tank and cylinder and avoid a combi boiler.

All boilers have to be condensing, combi, system or heat only they are all condensing,

Only exception is a building ( such as a Listed Building ) where a condensing boiler could not be installed due to restriction on flue and / or condensate drain.

 

[Tully]

Thank you bernardgreen, two of my neighbours have changed over to combi one loves it one hates it oddly its the young couple with three children that think its great and the mature couple who hate it. I love a hot bath and from what I have read thats unlikely, thanks again.
Tully

 

[Doggit]

Part of it will depend on the incoming cold water flow rate, and it's pressure. If you've got a good flow rate, then with a 32 or 39Kw combi, it'll fill the bath dead quick. And if you've got a hot water cylinder, um, it'll also fill the bath dead quick. But, if you only have a 28Kw combi with a low flow rate, then it'll take longer. The current hot water cylinder may struggle to handle the bath being filled and the shower going at the same time, but with a good flow rate, and an unvented cylinder, you may get both going at the same time. It's possible that one neighbour has a sensibly sized combi, and the other doesn't.

 

[bernardgreen]

It's possible that one neighbour has a sensibly sized combi, and the other doesn't.

It is difficult if not impossible to get a truly "sensibly sized" combi. The difference in heat required to heat flowing hot water ( as much as 36 kW or more ) and the heat required to maintain the house at a sensible temperature ( as low as 3 kW or less ) requires a boiler that can modulate down 12 to 1 ( or greater ). The boiler may be very efficient at one end of the modulation range and far less efficient at the other end.

 

[chrisw99]

In early December we changed from a standard to a combi set up. Had the house nearly 20 years, and first time with a combi so wasn't sure what to expect.

So far so good, I'm very impressed with it, and my December gas bill was a lot less than my November one, when traditionally it's a lot higher, so looks like I'll save quite a bit too.

My only negative point is the amount of time it takes the bathroom basin tap to get hot water, although the bath tap is fine. But I've realised the basin taps are trendy ones with a spray pattern, so they are severely limiting the flow. Will get that changed to a standard tap and I think it'll sort it.

 

[bernardgreen]

You might fond the gas bills in summer are higher as combi boilers can be very inefficient when supplying small quantities of hot water.

 

[chrisw99]

You might fond the gas bills in summer are higher as combi boilers can be very inefficient when supplying small quantities of hot water.

I doubt it - previously to get hot water in the summer we had to run our boiler at full whack for at least 90 minutes over night every night to get a decent amount of water to last the day!

 

[bernardgreen]

in the summer we had to run our boiler at full whack for at least 90 minutes over night every night

time it takes the bathroom basin tap to get hot water,

When there is a demand for hot water a combi has to heat its heat exchangers, pump and a couple of litres of water before any heat reaches the water flowing to the taps (*). When the demand for hot water ceases the heat exchangers, pump and a couple of litres of water are still hot. They cool down and that heat is lost to the environment, some of it may warn the room but a lot is lost via the flue. That is why a combi supplying several small quantities of water can be ( much ) less efficient than a lower powered boiler heating a larger volume of water in a cylinder.

(*) not all boilers have to heat exchangers, pump and a couple of litres of water. Some boilers have a single heat exchanger but this means the heat exchanger has a large thermal mass, some boilers use a tube in a tube construction with very small thermal mass.

 

[John D v2.0]

My only negative point is the amount of time it takes the bathroom basin tap to get hot water, although the bath tap is fine. But I've realised the basin taps are trendy ones with a spray pattern, so they are severely limiting the flow. Will get that changed to a standard tap and I think it'll sort it.

Modern basin taps are usually water saving ones. You can change to a larger one to let you purge the cold quicker, but a better solution would be to change the pipe to 10mm. Then less water to come through and sitting in the pipes.
Just don't serve the bath tap or shower with the same pipe!

 

[chrisw99]

When there is a demand for hot water a combi has to heat its heat exchangers, pump and a couple of litres of water before any heat reaches the water flowing to the taps (*). When the demand for hot water ceases the heat exchangers, pump and a couple of litres of water are still hot. They cool down and that heat is lost to the environment, some of it may warn the room but a lot is lost via the flue. That is why a combi supplying several small quantities of water can be ( much ) less efficient than a lower powered boiler heating a larger volume of water in a cylinder.

(*) not all boilers have to heat exchangers, pump and a couple of litres of water. Some boilers have a single heat exchanger but this means the heat exchanger has a large thermal mass, some boilers use a tube in a tube construction with very small thermal mass.

I suppose a good rule of thumb in the summer is to educate ourselves to use the cold tap rather than the hot if just rinsing hands. Even now since we got the combi, I've noticed the kids just quickly wash their hands in seconds with the hot tap, well before any hot has even started, so they may as well have used the cold tap. Seems a waste the boiler firing, or diverting, for a few seconds needlessly.

 

[chrisw99]

Modern basin taps are usually water saving ones. You can change to a larger one to let you purge the cold quicker, but a better solution would be to change the pipe to 10mm. Then less water to come through and sitting in the pipes.
Just don't serve the bath tap or shower with the same pipe!

Good point, although apart from a couple of feet the pipe goes behind the tiles and under a tiled floor so a bit tricky to change now.

It's such a fine spray on the hot tap that the flow through it quite often doesn't trigger the boiler to detect a flow and start heating, unless it's turned up to full and then it sprays and splashes everywhere all over your clothes. And a few of the spray holes seem to be clogged as well. Definitely need a new tap methinks.

 

[JohnD]

I suppose a good rule of thumb in the summer is to educate ourselves to use the cold tap rather than the hot if just rinsing hands.

I haven't got a combi, but my gas usage in summer (hw only by cylinder) costs in the region of 20p a day, so the waste from washing hands is insignificant.

 

[Doggit]

I doubt it - previously to get hot water in the summer we had to run our boiler at full whack for at least 90 minutes over night every night to get a decent amount of water to last the day!

The other side of the coin, is that in the winter, it (virtually) cost nothing to supply the hot water. Swings and roundabouts.

Add a Combisave to the boiler outlet, and that'll cut down on wasted water whilst the combi gets going.

 

[JohnD]

we had to run our boiler at full whack for at least 90 minutes over night every night to get a decent amount of water to last the day!

I wonder if that was an electric immersion heater. About one tenth the power of a modern gas boiler.