Combi boiler, or condensing?

alexthecheese

alexthecheese

alexthecheese

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Looking to get a shower fitted in our upstairs toilet, which means that we're looking at removing the water tank to make space for a shower cubicle.

Our choices currently are move the water tank to the loft, probably at quite some expense, or ditch the water tank and get a combi boiler.

There's two of us in a 4 bedroom house, but I want to to this in a way that wouldn't damage the appeal of the house. Given that this is really a family house we live in, is a combi boiler better or worse than a condensing boiler?

I know condensing boilers are more efficient, but it depends on usage. So in certain situations a condensing boiler could be better than a combi, and vice versa.

Our gas bill is tiny, we only have hot water on for an hour in the morning inthe summer, then a few hours of heating in the evening. Well below average I'd imagine.

What are your initial feelings?
 
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Worcester Bosch don't have the 15ltr water store option, not essential, had the 937 at my last place, & it was nice having instant store hot water on tap first thing in the morning.

See if your mate can get a deal on the Valliant, as to fitting, is it same location, basically a straight swap, flue might need some work (venting via external wall or through roof), or a new location.

Probably around £300 - £350, but it all depends on pipework & ease of installation,without seeing, it's difficult giving an accurate price, but it's only a day's work to install.
 
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Same place. Gonna be in the kitchen but hidden by a cupboard.

What exactly does the 15ltr storage tank give you? A bit of instant hot water before the boiler kicks in properly?
 
As a prospective buyer I'd look at a condensing boiler as an additional expense, as it's one of the first things I'd change in a new house.
 
How much would I notice not having a storage tank built in? Do these things work on a timer, or do they just kick in as and when? If I went for a boiler without one and then got in the shower in the morning, how long would I have to wait before it got hot?

Seems to be about £300 extra over the Worcester Bosch offerings to have a Vaillant with storage tank built in.

Looking at the video guide on youtube of the Vaillant one with the storage tank - it looks ruddy massive! :p I don't think we'd be having it for that reason alone, we'd have to move it out of the kitchen.
 
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alexthecheese said:
How much would I notice not having a storage tank built in? Do these things work on a timer, or do they just kick in as and when? If I went for a boiler without one and then got in the shower in the morning, how long would I have to wait before it got hot?

Seems to be about £300 extra over the Worcester Bosch offerings to have a Vaillant with storage tank built in.

Looking at the video guide on youtube of the Vaillant one with the storage tank - it looks ruddy massive! :p I don't think we'd be having it for that reason alone, we'd have to move it out of the kitchen.
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I have a 15 year old Vaillant (at a guess, was in here when I moved in). The shower warms up in about 10 seconds. It's insignificant and I guess it's probably better now.

The most annoying thing is when you want to quickly rinse your hands at the sink, you have to wait for it to warm up and when cooking, it's a pain.
 
Condensing combi from atag. 5 years warranty. Small. Super efficient. As unbreakable as it gets.
 
There are options on the Vaillants to keep the water hot, but then it fires up periodically to keep the system ready to produce hot water. Mostly it depend on how cold your boiler is, how long your pipes are and if you're always using little bit of hot water all the time. I have short pipes and can get hot water in about 10-20 seconds from a cold boiler. My father in law has long pipes and something on the end can take ages to get the hot water around to it even though he has a tanked system.

Condensers should be better than combis, although if they are set up incorrectly, they can be no more efficient than combis. You should spec a condenser to the flow rate and heating requirements of your house (there are calculators online). Make sure your boiler is set to output what your radiators can use, and that the hot water coming back to the boiler is cool enough to condense the steam out of the condenser and reclaim that heat energy.

Excess heat that can't be shed by the radiators just builds up in the system and boiler. Any heat isn't reclaimed is lost out the chimney, just like and old style combi, and your boiler will be running inefficiently on and off all the time.

When we had our condenser installed, it was set to the full 20KWh output, where really it only needs 13KW. I reckon there are a lot of boilers that are just not set correctly for the houses they are in. Probably a lot that have been specced for huge flowrate "just in case" and are then outputting more heat than the radiators need wasting heat or can't be lowered to the correct numbers for the house.
 
I have been researching boilers recently, a Valliant 937 boiler will not give you 20L/min of hot water unless you fit a mixer at the boiler so it can mix hot water with cold water to increase the flow rate.

The 937 like every other combi has a flow restrictor on the hot water feed, without the mixer it only produces 15l/min @ at 35c rise which is the same as other boilers without a store. Valliant are a bit sneaky as the only mention of this in the specs is on the stats page where it says it can supply 20l/min of blended hot water and disclaimer saying figures are dependent on "installation situation" it wasn't until I got a quote to fit one off an Valliant installer that he mentioned the mixer, interestingly another two weren't aware of the requirement but one after speaking to Valliant confirmed one was needed to get the required figures.
 
Also seems to be some confusion in this thread on what a combi and condensing boiler is, most currently available combi boilers are condensing boilers, by law you can't fit a non condensing boiler into a new house.

As mentioned you need fairly low return temperatures for a boiler to start condensing efficiently an ideal system is oversized radiators that you run at a lower temperature. Most modern boilers will automatically reduce it's power output for the demand that is required so a 30kw boiler will range down to 12kw if required some can even drop to 6kw.

The main downside of a combi boiler is reduced hot water flow rate and when producing hot water it can not supply heat to the radiators, an alternative is to have an unvented hot water cylinder and a system condensing boiler this will give you mains pressure hot water to multiple outlets at the same time and can simultaneously heat the hot water and supply the radiators.
 
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In a 4 bed not a combi.

We won't and Local Authorities won't let us put combis in anything above two bedrooms, this is from years of aftercare where combis have failed in higher usage houses, although someone will have a combi in a 20 bedroom house and it will have been fine.

Standard condensing boiler and megaflow
 
Yeah, sounds like condensing is really actually the way to go. The combis are designed only for 17litres of water a minute, to fill a bath that'd take about 15 minutes! Is that right? Doesn't sound much use.

Really just want to get rid of the hot water tank. Have seen that you can put it in the loft, but you need the cold water tank 1.5m above the hot water tank, is that right?

Just moving the tank to the loft would be enough, as it'd mean the space would be gained from the bathroom.

Is it realistic to expect to be able to put the hot water tank in the loft, and put the cold water tank right in the rafters?
 
alexthecheese said:
Yeah, sounds like condensing is really actually the way to go. The combis are designed only for 17litres of water a minute, to fill a bath that'd take about 15 minutes! Is that right? Doesn't sound much use.

Really just want to get rid of the hot water tank. Have seen that you can put it in the loft, but you need the cold water tank 1.5m above the hot water tank, is that right?

Just moving the tank to the loft would be enough, as it'd mean the space would be gained from the bathroom.

Is it realistic to expect to be able to put the hot water tank in the loft, and put the cold water tank right in the rafters?
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I am no expert but we are currently getting our combi boiler replaced with a system boiler and unvented hot water cylinder. If you go for an unvented cylinder you can put this in the attic if you want with no need for a cold water tank assuming your water flow rate and pressure to the house are good.

The benefit of the unvented cylinder over older vented hot water tanks the system is then balanced to provide you mains pressure hot and cold water.

The main reason we didn't go for a new combi is due to the hot water flow rate, we have big baths that are used regularly and our current boiler has a water store and no flow restrictor so the baths fill up pretty quickly the downside is in the winter when the incoming water temperature is lower the boiler can not maintain a constant hot water temperature after the internal store is depleted. (This is the very reason modern combi boilers require a flow restrictor for the hot water)
 
A lot of talk in here about condensing or combi :confused:

The two are not mutually exclusive. You get combi boilers, system boilers, or heat only boilers. Any of the three can be condensing.

Our house has a brand new Glowworm heat only boiler, fitted by the previous owner. I'd much prefer a combi to be honest as the hot water pressure on a vented system is pretty crap all round.
 
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