After a series of issues with a Baxi combi I am ready to throw in the towel.
This combi has a water leak, that despite a new (plastic) valve block still leaks. It also has a E78 failure that can either be a pressure sensor failure of a gas valve issue. Neither seem relevant when the fault occurs.
By way of example, a new valve body is >£300.
Anyway, I was thinking of a very simple heat only boiler, external pump, diverter and a flat plate heat exchanger to imitate a combi, without all the other issues and the cost where everything is off the shelf.
So a very simple boiler, but the remaining causes for concern is:
1) Target water temperature, do systems allow for different DHW and central heating water temperatures?
2) If not are there standards for temperature compensation I can fudge?
2) Ramping up of temperature. The current boiler has an ignition fan speed of 50% and goes up or down from here. I am aware some boilers like to ramp the water temperature up gently to reduce room thermostat cycling. Is there a way of overriding this?
Finally, any heat only boilers that could be recommended?
Sometimes a supplier of second hand / recon parts can make sense.
Vented or unvented?
Yes some do (many don't) - they have separate "call for heat" inputs for each.
Again some support and some don't. You will likely find the ones that do weather compensation are likely to also support split temperature operation.
Pass - you would have to study the details of the boiler in question. Again it will tend to be those with more sophisticated controls that allow tweaks of that type.
Not really got experience with enough heat only boilers to comment. The only one I have used was a Vaillant 400 series in an open vented configuration. That does support split temperature operation. (and weather compensation with the appropriate Vaillant controls)
Personally I would go with a system boiler rather than a combi. But if going that far it seems a shame to go to extra effort to simulate the biggest failing of combis (poor DHW performance) if you have the option not to.
On 18/07/2023 23:43, Fredxx wrote: > After a series of issues with a Baxi combi I am ready to throw in the > towel. <snip>
Heat only boiler and a storage tank is one way (the one I chose last year) but you need to make space for tank etc which is why a conbi. was typically fitted in the first instance, and a bit of jiggery-pokery to reconfigure stuff.
My next bestm or even a better solution is the Intergas DHW/Heat Combi-boiler.
There are no valves or plate heat exchangers to clog up, just 2 copper pipes snaking through a big slab of aluminium which is the Main (only) heat exchanger, the fan, burner, and a flow sensor as main components (off the top of my head) The only thing in contact with the aluminium is the dry heat from the burner, and condensate.
Two things to consider are that the central heating temp. ideally shouldn't be set higher than 60 degrees because the DHW if drawn off while heating is on will pull heat from the heat exchanger pre-heated to C/H temps. That said, unless your radiators are particularly under-sized and your house particularly un-insulated and draughty then CH running at 60 degrees C should be plenty hot enough and reduces the risk of overshoot. Obviously 55 degrees or lower would be a better C/H temp. (bring in the weather comp.)
What else? I believe Intergas boiler has weather compensation in-built but you just have to buy the external sensor.
In a hard water area because you're heating the incoming water directly without any form of inhibitor it is strongly advised to fit a "combimate" phosphate dosing system to prevent limescale forming in the DHW line within the boiler. This is only because the cold water is heated directly so I understand although I can't really see the difference as the big slab of Aluminium would distribute heat very evenly to the incoming water.
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I bought my house 20 years ago, the very antiquated combi-boiler fitted had a phosphate dosing contraption. I'd never seen one before but it was still called a combimate back then and looked the same as they do today so it's been a thing for a very long time is seems.
When I replace the pre-condensing Valiant boiler at work I'll be fitting an intergas combi (with weather comp.) + combimate. No hesitation because every other combi-boiler out there inherently has a diverter valve P.O.S. and heat exchangers that are prone to blocking up on C/H and DHW sludge and scale.
Perhaps bear in mind that you chose a less reliable brand of Combi. Our daughter had a Baxi installed back in the day and it failed after a few years.
Our Worcester Bosch has been running for more than 10 years and is still (as of today) going strong.
Having said that, I regret not installing a system boiler, but only because of low water flow rates which we didn't have at the last house with a Combi.
The primary system can normally be pressurised or fed with a header tank. I presume the former to lose a header tank and in my case there's nowhere to put it.
There is no need for cold water storage as the flat-plate heat-exchanger would be fed with mains.
Except in this case I assume the replaced part has the same issue!
Unvented, I was thinking of using a flat-plate heat exchanger but outside of the boiler.
Ah, I wasn't sure, any names and models I can look up? Is there some phrase I can lookup for this feature?
That sounds good.
That's good to known.
I can't afford the space for a HW cylinder, hence this approach, and my dislike for combi boilers! It's only me and if there's enough heat for a power shower, then that is sufficient.
BTW, what is the difference between a 'system' boiler and a 'regular' heat only one?
It wasn't my choice of install and was here before me. It's a Baxi with an all plastic water/hydraulic body. I accept there are more reliable makes and you intimate.
I don't think the Intergas unit should be classed as a "combi boiler" as such as it's brilliantly simple, but I suppose same scenario, if the fan, burner or flow switch failed you're left without C/H or DHW.
I fitted an ion-exchange water softner at home when I replaced the original combi. boiler with a new (and expensive) German M.A.N. boiler so no concerns about scale (hard water). Managed to keep that boiler going for about 16 years. 3 diverter valves and a couple of pumps was all I had to fit. No "servicing" through its life. In the end the back plate which formed the water passage for condensing rotted out.
Far happier with the current solution of a heat store, heat only Intergas boiler and wood burner boiler even if it meant losing 2 kitchen cupboards and a sink, although the un-used sink and cupboards made space for a washing machine so a far better solution in the end.
But you're still incorporating a diverter valve and that is by far the greatest point of failure in my experience, so all you're achieving is slightly better access to the same components that you'd have in a regular combi. surely?
The phrase "split temperature operation" seems to be one common expression. Vaillant & Bosch do it - plus others - I don' have a mental exhaustive list.
System boilers typically contain the key bits of a system in one box. So for an unvented system it will include the expansion vessel plus the over temp / pressure relief valve. They also usually include the pump. Zone valves, room and cylinder stats are still external.
In most respects you can think of a system boiler as being like a combi but without the diversion valve, plate heat exchanger, and the DHW flow sensing.
It does appear so. My last Ideal boiler was still going strongish after 12 years. Admittedly it had needed about £300 worth of parts at the 10 year mark. The Vaillant combi in my current house is at least as old. It too has needed parts recently - but well under £100 and easy to fit. I'll probably replace it with another combi come the time, although I will be looking at air source heat pumps.
That said, I am looking to install an electric shower over the bath as a backup.
That was a good start. Other terms I have no come across ate "Hot Water Priority" and "PDHW" (Priority Domestic Hot Water).
Hmm, I see your point. But why put expensive valve gubbins in the boiler when it could be outside leading to a smaller and less complex boiler. I saw a Youtube clip where the guy was installing an Intergas boiler, and it seemed the valving was external but the pump still internal, presumably to ensure run-on.
The only extra valve in the combi is the DHW diverter... In many combi installations with only a single heating zone (by far the most I would expect) there are no external valves - it is basically W plan with the zone valve in the boiler. So it makes the whole system very quick and easy to install, and quite neat and self contained for the end user. Hence the popularity.
For a system boiler they will be external, but it will be far more common to have multiple heating zones, and that would be difficult (or at least not cost effective) to provide for those in the boiler itself anyway.
Many heating only boilers with external pumps support run on. The power to the pump is provided by the boiler and not the zoning wiring loom.
Where the bits are is in some ways less important than how serviceable they are. A well laid out combi (see the Vaillant piccie) is relatively easy to work on - the major components can be got at and swapped out without needing to dismantle to much else to get there. Alas some boiler designs seem designed to make simple jobs as much of a ball ache as possible! (For example, it used to be common to stick the expansion vessel right at the back behind everything else!). Same goes for external valves etc. In a cramped space, wedged between a HW cylinder and a wall at floor level is far less nice to work on than one surrounded by free space.
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