Trying to decide as and when our boilers finally die (both are at the "care-home" stage of their lives) whether to try and find space for a pressurised HW tank rather than stick with the inherent fragility of combi's.
Obviously the downside of stored HW is reduced flue heat recovery from condensing (that can't make much of a difference though can it?) but I feel the advantages outweigh the disadvantages.
Adding the potential of a solar-thermal HW in the summer and looking at my mothers "Ideal" system boiler that is only the second boiler ever fitted to the house in 55 years has me thinking.
And... depending on whatever other "green agenda" nonsense is forced upon us it may well be more suitable for utilising solar p.v. if my backwards-running electricity meter ever has to be replaced.
I stuck with a cold water header tank and stored hot water when I changed boilers not that long ago. Means things can go on pretty well as normal for a day or so if the water is cut off. Much longer if I was careful. And like my high flow shower, and the ability to fill a bath quickly.
But your choice if you could put the room these things take up to better use.
System boilers these days share much of the design and parts list of a combi - just lack a diversion valve and secondary plate heat exchanger.
With a modern fast recovery cylinder (which all unvented ones will be) they can swallow the full output of a modest sized boiler. (my system boiler is 24kW, and the cylinder coil will take up to 22kW). So the boiler will be in condensing mode for much of the reheat.
Yup additional coils can be specced for a cylinder for solar thermal input. (and immersion heaters can be used to mop up solar PV).
About the only heat source you can't easily integrate with an unvented cylinder is solid fuel.
Chapter and verse here:
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Specifically when compared to a combi (of the "normal" type rather than ones with a large internal store of water):
You get high pressure water to all taps from both. You get higher flow rate from the cylinder (mains supply being good enough), and that can also be at a higher temp to allow mixing down to final use temp. Neither need header tanks
Both need reliable cold mains supply.
Combi will be simpler and cheaper and take less space. Heating performance ought to be similar. Hot water performance will be significantly poorer, but can still be adequate for most users if the boiler is sized large enough.
System boilers at the posh end of the market can do more advanced tricks somewhat more easily like use weather compensation or multiple heating zones.
Unvented hot water cylinders need servicing, but that is an easily DIYable job (check strainer is clear, check emergency valves operate and reseal, check pressure in expansion vessel).
Is another possible factor now the ease of conversion to heat pump? (A cylinder seems a plus for that but I've not looked at the likelihood of that making much difference in the overall scale of the buggeration involved.)
OOI how are you extracting the DHW from it, with a coil or an external plate heat exchanger and a pump?
Yup thermal store is another good way to go. Easy to get heat into it, and can take solid fuel as well as heat from a heat pump, and can be direct rather than indirect so full boiler output and condensing efficiency as well. Slightly more complicated getting the heat out, but has the advantage you could run rads of UFH from it as well if its big enough.
I did look at doing one for here, but in the end it worked out a bit more expensive than going unvented if buying a COTS solution, and more hassle if going homebuilt.
Having had various types of CH and HW systems in different houses I would rate them (least preferable first, most preferable last)
- cylinder/tank-fed
- cylinder/mains-fed
- combi/mains-fed
I'm not sure whether combi/tank-fed is an option, but I'd put it second-to-least-preferable if it did.
Combi has the advantage over cylinder that the supply of HW is endless: the HW doesn't run cold so you have to wait a long time for the cylinder to heat up before it can run warm again.
Mains-fed gives a *much* higher flow rate and is easier to balance hot and cold in a shower, because any other tap running in the house will affect the flow of both HW and CW equally.
Our present house *used* to have cylinder/tank-fed and the flow of hot water was puny (so it took ages for the water to run warm given the long pipe runs in our house) because some so-called qualified CH engineer had used 15 mm rather than 22 mm for the low-pressure HW pipes. Normally with tank fed, the HW pipes are larger diameter to allow a higher flow rate even though the pressure is a lot lower.
When that cylinder started to leak and needed replacing, the CH engineer looked at our boiler and said that based on its age it was due to be replaced, and when he removed it he confirmed this: the heat exchanger was badly corroded.
We had the boiler replaced with a combi, the header-tank decommissioned (it's too big to get out through the loft hatch) and the cylinder removed and the pipework replumbed for combi heat-on-demand.
Taps run warm a lot quicker. The length of pipe runs to the kitchen could have been shortened a bit (reducing the run-warm time even more) by not feeding them via where the cylinder had been, but we decided not to pay this extra expense. The shower no longer goes scalding hot every time the washing machine needs more water etc.
The only downside was a trivial one: one of our showers had an ancient copper head like you get on camp sites, in school dormitories etc. And when the engineer tested that shower after finishing the work, the extra pressure/flow blew the perforated disc off! No great problem. It looked naff and we were planning to replace it anyway, so I chose a head that was rated for our flow rate.
My own experience is that the unvented cylinder trounces the combi in every way... Massively faster delivery of water, at higher temperatures, and people can have long showers without turning off the house heating.
One option less commonly considered is using a combi *with* a conventional cylinder - main fed hot water to showers, and perhaps kitchen taps, and then cylinder for rapid bath fills.
You could always go for the Dr Drivel solution, two combis!
(other backup solutions exist like an inline electric hot water heater)
Mine has a 2" thick covering of foam so I expect it to retain heat. I have an immersion heater controlled by a timer which I use to provide hot water if the boilr is not in use for some reason.
On my last overhaul, I decided to stick with a vented cylinder and replaced the Combi with a system boiler for simplicity and convenience (slightly odd loft geometry in 1780's cottage). To get decent flow, I already had a two impeller Stuart Turner for the shower and added a single impeller Stuart Turner for the rest of the DHW. Just an observation.
You'd need a cylinder with a coil designed for a heat pump, due to the lower flow temperature needing a larger coil. Not to say you couldn't get a heat pump compatible cylinder and plumb it into a gas boiler if you wanted - not a problem to have a higher flow temp.
There are times we have had "all day" power cuts. I have a camping style dual gas ring portable cooker + grill that runs from a cylinder of LPG that is quite handy in those cases for making hot drinks at least.
I'd have thought it costs more to run the HW side of things with the heat losses from the cylinder.
But in other respects I tend to agree that HW using a combi is a mixed experience. Takes a good 20s for hot water to come through, although when it does it's a decent amount and plenty hot enough. I don't use the preheat option - it's pretty quick if I do. Multiple showers etc. are not going to happen, which would be more of a problem for larger households. As it is, on balance, a combi suits me better overall.
Since the cost of DHW is minuscule compared with heating a house, who henstly cares?
Takes longer than that for hot water to get to some of my taps with a pressurised tank. So there is always a slight waste, but would be just the same with a combi. a pipe fill of cold water is a pipe full of cold water.
At some level the size of combi needed to heat a decent water flow exceeds the cost of a system boiler and pressurised tank.
I'd only contemplate a combi in a small 1 or two person house.
Waste, to you, is a binary concept. I care about MASSIVE waste, like renewable energy, long before I care about filling the kettle up beyond halfway...
Its a substandard solution with a price that reflects that.
People who add heatbanks and pumps and so on would be better off simply nuying a system boiler and a pressurised tank.
My cylinder hardly loses any heat at all. It an insulated unit and well insulated at that.
That's been the other issue with both Combi's when it breaks which both have done numerous times of the last 15 to 20 years there is no showering until it's fixed. Fortunately one combi is in the flat above our shop and the other at home so it wasn't quite as bad. I still have the bathroom at home to put together as it's been gutted since 2012 so was going to put an electric shower in the shower room and the new bathroom fed from the boiler just in case, but a HW tank negates this requirement.
The diversion valve and actuator on my German boiler at home was the cause of many problems but so far since the updated version was fitted it's not been a problem.
Plate H/Ex on the work boiler AND diverter valve have been problematic but we don't need hotwater in the shop so it wasn't such a problem.
I wasn't aware of that. Thanks John.
I've got this other slightly hare-braned scheme on the cards... using the 4m x 8m space under the balcony as a thermal store. Initially the idea was to duct hot air from a south facing solar furnace just to create a semi warm "greenhouse/garden room" for delicate plants but then went on to thinking of digging deeper and insulating etc to super heat in summer and bleed off in winter... No doubt I'll never live long enough to see that one through though. LOL
Excellent. Thankyou
We're pretty good on that front.
Combi at home is weather compensated which once I finally found the perfect heating/temp. curve works really well so that's a must-have.
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