Replacing existing Combi (NOT condensing)

[Nick Holmes]

This is sort of a followup to an earlier thread.

I have a floorstanding 20 y.o. gas Worcester Combi boiler (inherited from previous owner). It has a heatbank internally and quite a high output (130,000 btu I think). It feeds our aqualisa shower at fairly high pressure/flow as long as nothing else is drawing a lot of presssure off (loo cisterns are Ok, taps or WM nix). Incidentally it does have a motorised valve (I had to replace the motor a few years back).

I was thinking about replacing it this year. It sounds like I have to do this PDQ if I dont want a condenser?

Having read your FAQ, I am not at all sure I want a condensing boiler. All our rads (19) have thermostatic valves. We like them this way as we have no room thermostats and anyway we like to be able to change temperature of individual rooms. Also I am not sure we can drain (see below).

I think I would go for a like-for-like as, although the combi has some disadvantages, we have no desire to install roof tanks and cylinders.

My main concern is flueing. I really dont want to move the boiler* and it currently uses a vertical flue through an adjacent chimney stack (liner/cap etc.); problem is flue length is about 7.5m (3 story edwardian townhouse). Many of the boilers I have looked up cant be given this length of flue.

The other thing is we would like it to be room sealed (ie draw air direct from outside, but it doesnt have easy access to either outside wall or any drain (solid floor) and internal door between boiler and ext wall.

Where does the drain on a condenser actually fit? I supposed within the boiler itself? but maybe on flue??

Oh I should say I aim to get employ a pro to do all the work.

Excuse my ignorance and any good advice really appreciated.

Best wishes

Nick

• dont have anywhere alse nearly as suitable as current 'dead' floor space between door and chimneybreast in utility room and dont want scary trouble and expense of moving all the pipework (we have 3 rad zones plus HW)

-- Nick Holmes mailto: [e-mail]

[John Rumm]

Unless you can score enough points on the assessment that would allow you to fit a conventional boiler...

See here:

Having read your FAQ, I am not at all sure I want a condensing boiler. All

Other than a room thermostat which ought to be fitted for any type of boiler (and is a building regs requirement), none of the above preclude the use of a condensing boiler, and having a condenser would not force you to change the way in which you use the system (other than paying less for gas of course)

Some will allow flue extensions to be run in plastic drainpipe (you can do this with a condenser because the exhaust gas temperature is far lower)

Doubt you will get any choice in that, pretty much all the current boilers have balanced (fan assisted) flues.

The flue air intake can be run with the exhaust (in fact most boilers use a concentric pipe arrangement to achieve this)

It normally connects to the boiler along with the other pipes. The only difference is that it is usually piped in plastic rather than copper. With the addition of a pump the drain pipe can be routed pretty much anywhere, so it can follow the existing HW or Heating pipe run if that is convenient.

So why ask on a DIY group then ;-))

[John Stumbles]

Because quite a few pros hang out here? :-)

[Nick Holmes]

7.pdf

Thank you very much for this reference; and for the rest of your very helpful reply.

I am not sure if we would qualify. Most alternative locations for the boiler or flue do not need to be considered under their exemptions. However it is probably possible to resite it into the kitchen without either a 2m+ flue or a sump drain (940pts) - we might be able exclude this option under one of the flue exclusions (if the flue plume would be less than 2.1m above ground). This would however be v. expensive (major relocation of plumbing and gas, destruction of custom kitchen units) and I havent got my head around whether this would qualify. If we keep the location we dont exceed the 1000 pts (890, the scores are cleverly constructed aren't they).

From what you say it sounds like I have misunderstood.

It seemed to me that a requirement of condensing boilers was that one maintained an adequate temperature difference (e.g. 15-20 C) between the output and input. If the rad valves close this wont be the case surely? At present in our existing combi, the CH thermostat then cuts the gas so we get quite a bit of cycling (unless weather is especially cold)

That's good then.

Yes I knew this, I think some are even flexible - which we would need to get thro the chimney) - . In some cases there seems to an option for twin pipes, which can run to different places which can allow longer fluing lengths. If you had a separate air intake what are the rules on siting and termination?

Yes, it looks like a pump will be our only option.

Well, because there seem to be so many expert contributors. I did ask a local engineer for advice and he proposed to fit a boiler which turned out to have max concentric flue length of less than 4M. It could be that I am wrong, but our site and requirements seem to be particularly tricky.

If anyone has recommendations for a really good heating engineer in Cambs....

Best wishes

Nick

[Christian McArdle]

You appear to be confused. A condensing boiler has no relevance to the TRVs on the radiator. However, any modern boiler installation must bring the controls up to modern standard. This, in particular, requires a "boiler interlock", which is their way of saying that the boiler must entirely shut down if the house is warm and not keep the pipes warm unnecessarily.

The two suggested methods are either a room thermostat, or some sort of flow sensing on the circuit, with all radiators having a TRV. This is nothing to do with condensing boilers. All oil or gas fired boilers must now have this when installed and have done for some time.

Many can. However, you are unlikely to get a modern boiler that will reuse your old flue liner. There may be a few ancient inefficient and dangerous open flued types you can get if you're quick.

There are some models of condensing boiler that can use cheap mPVC drainpipes as their flueing system. You can poke both the exhaust and inlet tubes up the existing chimney and attach the relevent terminals at the top.

This is only possible with condensing boilers, as conventional boilers have hot exhausts that would melt the pipework. It helps if the existing flue is straight, as the pipes can be dropped down the chimney pot without any hassle.

Always on the boiler, but some flue routings require separate drainage as well. This would not normally apply with an "up the chimney" routing. The drain would be your biggest problem to solve. You may just be able to channel a drain into the floor. You might even be able to pump the condensate to the floor above and drain into the bathroom or something.

Christian.

[John Rumm]

you noticed that! ;-) Other than the issue of the condensate drain, there is not much to be afraid of from having a condensing boiler. Many of the usual objections people raise (increased complexity etc) apply to most modern boilers regardless of whether they are condensers or not.

With traditional boiler designs, condensation was a "bad thing", and one way to help ensure it did not happen, was to keep the return temperature high enough. The condensing however does not have this limitation and hence return temps _can_ be lower. However they do not *have* to be lower - it is just that efficiency improves a bit more if they are.

Also modern condensers will have modulating burners, so instead of running flat out until the demand is met, and then shutting off. They will monitor the return temperature, and as they see it rise they will reduce the gas rate to the burner. This way they can maintain their preferred temperature drop without needing to cycle (unless the load falls so low as to be under the minimum value it can modulate to). So your rad valves won't have any negative impact when they shutdown because the boiler will modulate to compensate for the reduced load.

(It is a bit like the argument that condensers run more efficiently at lower flow temps than conventional designs, and hence when retrofitted to an existing system will result in the radiators being undersized. This is also a bit of a red herring because for most of the year it won't be so cold for this to matter, and when it does, you can turn the temperature up. You will lose some efficiency, but it will still be better than the equivalent non condenser).

I don't think there are any special limitations on where the air intake is sited... perhaps Ed or John would be able to confirm?

Sorry, my question was more intended as a rhetorical one, and not to be taken quite that seriously ;-)

(Quite a number of recent posters seem to dive in with a pre-emptive statement that they will "get a man in" to do it. Which is a little ironic given that this group is one of the few places where most folks are not going to raise an eyelid should you say you were doing all yourself!)

ISTR that Kestron have been mentioned in the past for their particularly flexible fluing arrangements. Perhaps that is one worth a look.

[Christian McArdle]

There is no such requirement. The condensing boiler will be fine on any two pipe radiator system whatsoever. There are various methods of optimising the design of the radiator system for condensing operation, but this is not a requirement and the condensing boiler will still be considerably more efficient than the traditional boiler however it was designed.

termination?

It varies. Typically, they can be independently run anywhere, but with a few minor restrictions, like not terminating on opposite sides of the building. Presumably they are worried that forced flow in a storm might not be overcome by the boiler fan.

Christian.

[Mark]

Have a look at the Worcester-Bosch Greenstar Hiflow 440. It is washing machine sized and floor standing. It is condensing but fluing options are pretty flexible. It has a condensate pump so the outlet for this can be above the boiler.

Mark.

[Nick Holmes]

Thank you very much John and Christian and Mark for you kind and helpful replies.

You have allayed many of my concerns about condensing boilers.

I still believe that I may have a problem with draining, because of my site.

Incidentally, the Worcester HighFlow 440 that Mark mentioned does come with an integral condensate drain pump but its max lift is 1.5m. It may also just underpowered for our needs.

Keston (if this is right) dont make a combi but you are right about thier fluing (can be 50mm twin, separate air-in; exhaust of long run).

Incidentally, I do some DIY e.g. have replaced both the pump and diverter valve motor on my existing boiler but I couldnt truly call myself competent for gas regs purposes.

Best wishes

Nick

[John Rumm]

Yup, sorry no "r"... I have a feeling there have been threads on combis that can drive a long flue in the past. Google ought to find them.

It seems to me there is a distinction to being a competent "gas fitter" in the general sense, and being competent for one specific job. Assuming you are not the former, it does not IMHO preclude you deciding you are competent in the latter.