How much work to change from conventional boilet to combi boiler

[muggles]

Bear in mind that once fossil-fueled boilers are banned even on replacement installs you will need a HW tank, somewhere, to go with the heat pump CH "boiler" you have to install.

So it may be better to invest in a good HW tank now and keep the existing system boiler if it is only the tank that is a problem.

(Oops. Wrong forum. This isn't the money saving one )

It'll be 20 years or more before fossil fuel boilers are banned on replacement installs. Not worth planning that far ahead for most people

 

[ericmark]

I did look at idea of a combi with mother house, old house and new house, with the old house we did move from two boilers one for DHW and one for CH to a combi when one boiler needed changing, as leaking. It seems to have made not difference combining the two boilers, mother house found a problem with water storage and flow rates, the Bosch combi fitted had two options, eco and normal, with the normal there was a small water store, so even if taps only turned on with a trickle you got hot water, but if one used that option the water in shower would go from cold to hot then back to cold then hot again and also shower water pressure was a lot lower once combi fitted as the power shower had to be removed. But both those homes used gas, and gas boilers can turn up/down, so as long as the flow is more than the minimum it can adjust output.

This house uses oil, and it is either on or off, and so most so called combi oil boilers all it does is move the hot water store inside the boiler, it means you can get rid of the tanks, but you lose the capability to heat water should the oil run out, we moved to instant water heating in the last house around 1985 mainly as we had a third child and we could turn the airing cupboard into a bedroom, it did not give better hot water in any way, but did mean we could use the space. This house three stories and 5 bedrooms so not really short of space.

Mother house combi fitted as the water tanks were leaking, and dad could get a combi fitted free of charge or at very low cost due to government scheme.

But once a combi is fitted, wave good by to any idea of using solar panels to heat the water.

 

[gas112]

Combis are a one box solution to providing both CH and HW, much more complex internally than a heat only boiler, so much more to go wrong, where a gas safe engineer is needed to fix it and a more complex fix, than a heat only where more of the parts liable to fail, are external and become potentially a DIY fix.

Much more complex ..you mean it has a couple of extra parts within the boiler .fixing a combi is as basic as fixing any boiler/heating system

 

[vulcancontinental]

The BRE put this to bed years ago, system boilers are marginally more efficient but that is lost between a boiler and cylinder.

If you get an unvented cylinder that’s two services a year, if you plan ahead for a heat pump you’ll need a heat pump ready cylinder and that’s not cheap. You can tie a combi with a heat pump if need be.

System boilers are generally combis with the plate and diverter valve removed, same electronics, different configuration.

If you have a lot of baths get a cylinder, if showering mostly then combi. Determine mains flow rate then get a bigger combi than necessary if you like hotter water in the winter or don’t have a dish washer.

 

[SimonH2]

If there's a cylinder already there which is starting to give problems, consider a thermal store. It'll need some plumbing changes (especially to get the advantage of running the heating from it), but you get the benefit of stored hot water (so baths fill quickly) and mains pressure hot water. Unlike an unvented cylinder, you don't have problems with annual services needed to keep it safe. And like the current cylinder, you can have an immersion heater as a backup for when the boiler breaks down (which it will do far less often without all the combi bits in it).

Of course, what you will find is a lot of plumbers who will do that "sucks hard through teeth" thing before rattling off lots of reasons why they are the spawn of the devil.

As to efficiency, yes they have standing losses. But these can be reduced with insulation, and as long as the cupboard is part of the heated space, then the heat lost isn't wasted (except for a short period in summer when it's hot outside).

As to people being able to diagnose problems with combis as easily as traditional systems, well perhaps my viewpoint is coloured, but I've found plenty of supposedly qualified and competent "engineers" who I wouldn't trust to diagnose a dripping tap without adult supervision. At least with heat-only open vent boilers, most of the system is outside of the boiler and you can diagnose/fix that yourself.

 

[denso13]

As to people being able to diagnose problems with combis as easily as traditional systems, well perhaps my viewpoint is coloured, but I've found plenty of supposedly qualified and competent "engineers" who I wouldn't trust to diagnose a dripping tap without adult supervision. At least with heat-only open vent boilers, most of the system is outside of the boiler and you can diagnose/fix that yourself.

Are you saying combi's shouldn't be fitted at all?

 

[gas112]

Get a Thermal store now we are getting into comedy gold

 

[SimonH2]

Are you saying combi's shouldn't be fitted at all?

Not at all.
But they are more complicated and it has to be said that some in the industry are "not very good" at diagnostics.
But combis are almost always a suboptimal compromise - if the property is small enough for a combi to manage DHW (at a poor flow rate) then it most likely has a heating load well under the minimum range setting of the boiler. Case in point, our rental flat : heating load during the very cold spell in Dec 2010, average of 2kW; min range of boiler, just under 10kW; DHW flow with 30kW available, send it a postcard giving notice you'd like a bath.

Get a Thermal store now we are getting into comedy gold

Why not ? I take it you are in the "are never a suitable option" camp.

It's over 12 years since I put one in the flat and it's worked very nicely. The heating is virtually silent as it uses a modulating pump since there's no need to match any minimum flow rate requirements for the boiler. The DHW is only limited by the mains supply - and you don't need to choose between economy mode (and waste water while you wait for it to warm up) or "instant" mode (and waste gas keeping it hot). The boiler has lasted much longer (I have a thread about the problem I have with a replacement due to flue limitations) without all the combi bits breaking if you so much as look at it. And when the boiler has broken down, there's the immersion heater as a backup. All the tenants have been happy.

I've had one in at home now for a couple of years, and I'm very happy with it.

Oh yes, at the same time as I took the heating load readings I also took the opportunity to measure standing losses. The thermal store had HALF the standing losses (80W) as the then nearly new WB combi next door in it's default mode (160W keeping the HE warm).

Of course where combis really win is new-build shoe boxes where there isn't room for anything let alone a cylinder.

 

[gas112]

Having worked on thermal store for over 30 years and how shyte they are compared to conventional and combi systems i bow to your superior knowledge regards them after all you have experience of two of them . Thermal store and economy, yeah let me heat up a big tank of hot water before I can get my rads to Come on instead of them being up to temp within minutes with a combi and by the way modulating pumps on boilers is a basic thing now .

 

[denso13]

Not at all.

I've been saying horses for courses all along. Some say the same then produce lists of why not to have one.

But combis are almost always a suboptimal compromise

See what I mean.

 

[SimonH2]

Thermal store and economy, yeah let me heat up a big tank of hot water before I can get my rads to Come on instead of them being up to temp within minutes with a combi

Err, the water is already hot, so instant heat.

and by the way modulating pumps on boilers is a basic thing now .

Ah, so boiler manufacturer's allow an arbitrarily low flow rate do they ? Care to cite an example as all the manuals I've looked at (and I've looked at a few over the last few years) have a minimum flow requirement for a max delta-T of 20˚C. So for something like out flat, still a massive mismatch between required flow rate for rads and minimum flow rate through boiler.

See what I mean.

It's a statement of fact. If a boiler is big enough to produce a decent DHW flow rate, then it's usually going to be very over-sized for the heating load. If it's sized to match the heating, then it'll only produce a dribble of hot water - not that they make them that small for that reason.
That's not the same thing as saying they don't have their place, but their place does not include a lot of places they get put.

I too can come up with reasons you might not want a thermal store - but I still have them.

 

[gas112]

Oh deary deary me the instant heat comes from the fact you have to constantly have your hot water on . You can't have your heating on by itself .
As for modulating pumps in boilers you need to brush up on what has been pretty standard for years . Perhaps you have been listening to YOUR guys who think you can run a flue through neighbouring properties

 

[SimonH2]

Oh deary deary me the instant heat comes from the fact you have to constantly have your hot water on .

The idea is that you have hot water on demand But if you don't draw water off, then you don't "constantly have your hot water on" in the sense that you imply means wasting lots of heat. The days of "putting the water on" an hour before you go for a bath are long gone.

You can't have your heating on by itself .
As for modulating pumps in boilers you need to brush up on what has been pretty standard for years . Perhaps you have been listening to YOUR guys who think you can run a flue through neighbouring properties

Err, I've been reading the manuals - the ones produced by the manufacturers. OK, I've not read them for all makes, but so far not one boiler supports an arbitrarily low flow rate through it - it'll exceed it's delta-T limit and it seems most modern boilers will trip on this (whereas older ones would start doing interesting things like kettling).
So, can you cite me a make/model that will allow a fully TRV rad setup, with a modulating pump, and without a bypass ?

 

[gas112]

It's simple you do not have either hot water or heating on demand you have to have a full cylinder of water kept at a constant temp that will result in your boiler firing up throughout the day what bit of that are you struggling with..
As for a fully trv,d system without a bypass would you like to tell me when this is actually allowed and what component is needed within the boiler.
S for modulating pump Google modulating pump and put all the major boiler manufacturers name in front and see what it brings up .

 

[SimonH2]

It's simple you do not have either hot water or heating on demand you have to have a full cylinder of water kept at a constant temp that will result in your boiler firing up throughout the day what bit of that are you struggling with..

I don't struggle with anything. From observation, if the tank is reasonably insulated then the boiler fires up quite infrequently - obviously if you are daft enough to have an uninsulated tank, uninsulated pipework, and not enclosed in a cupboard then you'll have have a radiator heating the space and that will need topping up - but anyone with any sense won't have any of those When drawing on the DHW and/or heating, the boiler will fire up as required, burn for a while, then shut down - this can be improved if you have a twin stat setup or a single stat with higher than normal hysteresis.
Contrast with a combi. Every time you turn on the hot tap - it fires up, often for a very short time. And because it will almost always be grossly oversized, it'll keep firing intermittently in short bursts all the time the heating is on. I'm sure you'll be familiar with the industry term for this - short cycling.

As for a fully trv,d system without a bypass would you like to tell me when this is actually allowed and what component is needed within the boiler.

Exactly. You have implied it's allowed :

As for modulating pumps in boilers you need to brush up on what has been pretty standard for years.

In the context of what I wrote - having a near silent heating system because I don't need to match requirements of the boiler - that's what I take from that.

S for modulating pump Google modulating pump and put all the major boiler manufacturers name in front and see what it brings up .

Not what I asked, cite me an example where the boiler can deal with a rad loop with all TRV and no efficiency killing* bypass. I know they have used modulating pumps for a long time, but that's not the same thing as allowing an arbitrarily low flow rate - they don't.

* Unless you're on a new install (so not a retrofit designed for higher temps) with decent sized rads (so probably excludes a lot of new builds !), and can keep the flow temp below 54˚ then a bypass will kill condensing over a significant part of the heating operating range - but you knew that right ?