3-bed semi central heating: combi vs. conventional boiler?

I am planning a new central heating for my house which is a 3-bed semi. There is currently only myself living here, but I may want to rent the place out to a family at some stage. I need to decide between a combi (sealed) system vs a conventional system with expansion tank.
Sited in my loft, is a Fortic cylinder with integral expansion tank. It appears to have an internal coil (not in use), as there are two unused pipe connections on it. I'm currently using the Fortic cylinder as an immersion heater to heat my tap water.
I gather I'd consume less gas if I choose a combi, rather than a conventional boiler. However, my experience of combis is that they are expensive to maintain. I always envied people with older systems that seem to require virtually no maintenance. Even slight leaks can be ignored in a not-sealed conventional system, I gather.
Has anyone got any views on the overall cost (money + trouble) of installing and running a sealed combi system vs. a conventional system?
Thanks for any advice...
Al
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On 06/09/2012 16:03, AL_n wrote:

Modern or old house? Large or small? I use a small holiday place (modern semi) where a combi works absolutely fine, but all the pipe runs are very short: it was designed around the services. I have a rambling 18th century house (originally two cottages) where the pipe runs are a nightmare. I've had two inadequate combis over 20-odd years and have just replaced the boiler with a conventional one with a hot tank in the loft. Lots of lovely hot water thanks to a Stuart Turner pump!
With a combi, the issue is not so much leaks, but there is more to go wrong with the secondary heat exchanger and the control system.
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It's half ancient and half modern. Half of it is solid stone, and the other half is cavity blockwork. Overall, it's not a big place, and the longest pipe run would be about 10 mtrs.

Yes; I had those sort of problems in my last house, where I installed my own combi susytem. Come to think of it, leaks weren't too much of an issue. Fter finding I had to replace the secondary heat exchanger about once every two years, I installed a phosphate doser, hoping it would help, but I sold the place shortly after that, so I never found out.
Thanks for the input,
Al
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Those are not either/or options - they are two unrelated things.
You can have a combi or a system boiler. The combi has extra guts inside to do instant water heating, which the system boiler is missing. If a system boiler is going to heat the hot water, it needs a separate hot water storage tank/cylinder.
In theory, either of these could be sealed systems or vented with a header tank in the loft. In practice, I suspect you may find that just about all boilers only specify sealed operation today. It was heading that way 10 years ago when I did my system, and no one was installing new vented systems even back then - you only got a vented system when retrofitting a new boiler into an existing one, and even them, many plumbers would convert it to a sealed system at that point.

If you want to heat that with the boiler, you would use a system boiler. You would probably also want a sealed system, as you would struggle to get a vented tank high enough to drive the coil, yet lower than the header tank on the Fortic.
How good is the fortic, in terms of wear/corrosion, and insulation? Unless it's good on both counts, designing your system around reusing it might not be a good idea.

Why? That depends on a number of things. Could be true for a single person, but probably less likely with a family and a well insulated tank.

They shouldn't be - they can turn into dry rot, etc.

For a rented property, if you have the space, I would use system boiler and hot water storage tank, with back-up electric immersion, because this gives you some redundancy, which means a boiler failure is not quite such an emergency, and a system boiler has less to go wrong in it than a combi. (Another option would be an instant in-line electric heater backup for a combi.)
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snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote in

It does look pretty old and decrepit, although I've never had any problems with it, using it as an immersion heater. I must say, I'm not all that keen on having part of the system in the loft, because the older I get, the less excited I get about crawling around up there to sort out some problem.

Thanks for pointing that out. It does make sense.

Some helpful insights there; many thanks.. I guess I can keep the existing immersion heater in service, as a backup tap-water heater, if I install a combi, yes?
Al
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Some more questions:
1) Is it easier to regulate the temperature of the tap water, using a combi system? (I'm particularly thinking of the shower, which needs to always be at the same temperature.) If I were to use a system boiler, some of the DHW pipework would be running through a draughty loft, unless I relocate the Fortic tank somewhere on the landing.
2) If I install a combi, the heated water will reach the taps quicker, than it would from the Fortic tank in my loft. Would this difference tend to be noticeable, in terms of how long one has to wait for the water to run hot, after turning on a tap?
3) My gas pressure is borderline, as I am at the end of a long run of gas main. Is that likely to pose a problem with a combi as opposed to a system boiler?
4) Someone once told me to always site the boiler in a warm, place like a broom cuboard, because if it's in a cold room (e.g., a garage), it will keep firing up all the time and wasting a lot of gas. Am I right in thinking that this problem could be overcome by simply by connecting the boiler to a thermstat that is in a warm part of the house?
Thanks, A
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On Sep 6, 6:13pm, "AL_n" wrote:

No, as it's related to flow. Stored hot water should be more consistent. In any event, an appropriate thermostatic mixer should be used on the shower.

Possibly. On the other hand, the combi boiler will be firing up for every silly little HW draw-off.

Could do, combis usually have a higher gas load because they need to provide instantaneous HW.

No, because the boiler should have its own internal stat for frost protection, separate from the room thermostat.
Owain
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AL_n wrote:

No. Combi systems unless they are Ginormous always run cold or show pathetic throughputs of water flow.
You need some form of hotwater storage and a pump or mains pressure to get a decent shower.
Coupling heat banks to combis is a bodge solution = once you have a hot water storage system there is little point in a combi.
They are cheap low physical fopotprint solutions to domestic heating and a patheic DHW supply.

No. Unless you are inches away from the combi.

Cant say. Might do.

Its good to site boilers in the house but not for the bollocks reasons you have been given.
They generate a lot of heat that doesn't go into the water. Might as well have it IN the house.
Also in the event that there is a hard frost and you aren't there, amd leave the heating off, water pipes connected to outside boilers can freeze.
That's why outside boilers in sheds have frost stats to fire em up a little in the cold

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On 06/09/2012 18:50, The Natural Philosopher wrote:

Can't see why you think that... Many modern showers are designed to run on around 7 to 10 lpm - well within the reaches of even the most feeble combi.
If you want giant soaker heads and multiple body jets, then sure, go for stored hot water.
In the previous house the 35kW combi managed two reasonable showers at once (i.e. both about twice to three times as good as an electric shower), or one very good one with a bit of capacity to spare.
I am not suggesting that a combi is a panacea for all ills, but one ought not let your dogma cloud the issue. Showers are playing to their strengths.

I can think of applications where a combi and stored hot water would work quite well... in fact its a solution I considered for this house. (Cylinder in the middle of the house with all the bathrooms clustered in close proximity, and boiler at the far corner of the house adjacent to the util and kitchen. DHW from the combi would be an effective way of getting potable hot water in the kitchen without the long dead leg to run off from the cylinder, or the need for secondary circulation loops).

Quick and easy to install so popular with builders etc. The boilers themselves are no cheaper than system boiler though (not surprising since most system boilers and combis are built on the same platform)

I would have said it is actually a valid reason. Modern boilers have frost protection built in, and so in cold environments will cycle to keep themselves warm even if there is no external demand. However the amount of cycling required is only minimal - a brief fire every few hours probably.

Modern HE boilers generate very little heat from the boiler case itself.
In the past it was common to not fit a rad in the room with the boiler since its waste heat would keep the room warm. With a boiler capable of achieving over 95% efficiency, there is not going to be a couple of kW slopping out from the casework.
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John.

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Yes, the design of my Keston is that the heat that does leak out the sides of the heat exchanger is picked up by the air intake, and carried back into the heat exchanger as pre-warmed air for the burner.

When designing my system, I looked for the figure for heat loss from boiler casing and couldn't find it, so I assumed 200W. This turns out to be wrong, due to the above. If it's been on a very long time, there's a warm patch in the middle of the front just near the heat exchanger, but it probably amounts to only a few tens of watts max.
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Andrew Gabriel
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snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote in

Unfortunately, the kitchen where my boiler will be located doesn't have any room for radiators. I wish I could pay less and buy an inefficient boiler, so that the boiler itself would keep the room warm!
Al
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On Sep 8, 2:05pm, "AL_n" wrote:

A plinth mounting fan-assisted heater? http://www.discountedheating.co.uk/shop/acatalog/myson-kickspace-500g-fan-convector-heater.html
Owain
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Fantastic! I didn't know such things existed. Thanks for that...
A
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On 08/09/2012 18:13, AL_n wrote:

We have one in our kitchen and it's very good, undersized a bit but it works OK on boost; make sure you size it so it can run on low (and quiet) fan speed.
--
David


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On 08/09/2012 14:04, AL_n wrote:

Have you thought of options like kickspace heaters that go under kitchen units (fed from the CH but usually with electric fan assist)?
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Until I read Owain's post (and yours) I didn't know they existed. They look like a good possible solition... Thank you....
A
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No, it's easier with a storage tank, as the temperature remains constant irrespective of draw-off rate. With a combi, it depends on the quality of the combi, but a change in flow rate (e.g. opening a tap somewhere else) is very likely to change the temperature and/or the hot water flowrate into the shower. Ideally, for a shower running from an instant water heater, you want a fast-acting (bi-metalic) thermostatic shower valve to try and maintain the temperature in the face of sudden changes in hot water temperature and/or pressure/flow rate. For a storage tank system, you can use a cheaper slower acting wax pellet shower mixer, or even a manual non-thermostatic mixer, because the water temperature and pressure is not going to change suddenly. OTOH, you might need a shower pump if you don't have much head of water.

It is a good idea to consider the location of the stored hot water or the combi, with a view of minimising pipe runs, particularly to sinks and wash basins where you only want hot water for a moment, and it's a waste to have to run off loads of cold before the hot comes through. Not important for a bath or shower where you're going to run lots of water anyway. Being in the loft means you also have to protect against frost.

Yes.
There is another option, you could use a combi and a stored water system, each supplying outlets near them, but that's probably the worst option unless you have a compelling use case that fits it well, such as multiple simultaneous baths and showers.

Could be - combis tend to be higher power for the instant water heating. Often a house needs significantly less for the heating, and as the hot water cylinder heats over time, you can normally ignore the extra load that might need. Do you know what your max gas rate is, and what other gas loads you currently have? A normal domestic meter can go up to around 55kW. If there's a flow problem into your gas governor, they may be able to turn up the street supply pressure to compensate, if it's not already at max. Otherwise, I don't know what their responsibility is for fixing their supply/pipework network if you can't get enough gas.

He's probably thinking of built-in frost protection. How that works will depend on the boiler. Some combis also keep a little hot water ready to run instantly, which means they have to keep it heated up. Sometimes you can turn this preheating function off.
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snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote in

All I can remember is that when Transco installed a new gas meter a few months ago, he commented that my gas pressure was "only just within the permissible range" or words to that effect. I can't remember the figure he mentioned.
Apart from a potential boiler, I have a gas cooker and one medium-sized radiant gas fire.
Thanks to everyone for the very helpful input.
A
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On 06/09/2012 19:16, AL_n wrote:

Don't quite understand that. The main should be at a significantly higher pressure than the house has, and the regulator in the meter should give you the right pressure in the house. Unless you are on very old supply, and/or a long way from any other customers.
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On 06/09/2012 19:16, AL_n wrote:

The static pressure coming out of your gas governor will normally be in the upper 20's millibars. When actually flowing it should regulate it to 21 mbar at the meter. That then leaves you 1 mbar to play with as losses in the internal pipework to deliver gas at the 20 mbar most appliances are nominally designed for.
Having said that, many do have wider operating limits then the theoretical limits.

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John.

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