Mr Gasman's May service of our 1984 Potterton Netaheat showed that it burns very well, with zero CO output. I was warned that no replacement will match its long life and have no intention of being proactive about replacement, but one needs to be prepared.
A few months ago I was informed here of the benefits of unvented heating primary circuit. If I was installing from scratch, I would not be having any qualms. However, there is precisely zero access underneath the floor-boards in most of the house, unless I cut through the hard-wood flooring.
So, what is the chance of introducing leaks underneath the floor when swapping from vented to unvented primary in a 25+ year old installation? And what other complications should I expect from the lack of access?
Thanks in advance for any opinions,
Kostas
p.s.: The other thing Mr Gasman suggested was installing a (Vaillant) combi and running it as a heating-only boiler with an unvented secondary, since I am not keen on the HW aspects of the combi. I think the motivation is he can get it cheap, because he buys so many of them. Thoughts?
If it ain't broke don't fix it. The biggest leak will develop in the most in accessable place 5 mins after you have gone away for the weekend and bring down the ceiling and ruin the brand new carpet of the room you have just finished decorating...
also think that with higher pressure you can run it cooler, thus increase condensing, thus increase efficiency; am I right?
Yes, that's exactly my kind of thinking (only I would go for fortnights, rather than weekends). But it doesn't always work to one's favour. I think I should make an informed decision on this one; it's a major cost and inconvenience, you want it done well.
The *big* advantage of a sealed system is that if it leaks, there is only so much water in the system. After it is de-pressurised (10L or so) not much else will leak at high level. Even if it did, it's not being topped up, so the volume of flood is limited to that of the system.
With the normal vented system there is (in this area) the whole of Kielder reservoir available to leak out!
In general, pipes that have not leaked in 25 years are unlikely to start doing so - especially where the water in use is fairly constant and not being continually refreshed. Some people find that bleed valves, and lockshield valves may weep under pressure - however these will tend to be accessible on the pipe stubs etc emerging from the floor.
A combi boiler and a system boiler are very similar in function and design[1], although its not uncommon for the latter to cost slightly more though since they sell fewer of them even though the system version has fewer parts. The only combi failure mode that a system boiler does not have is the failure to produce hot water from the HW side of the boiler (often flow switch, secondary PHE, or diversion valve related). If in your system you are not using that part of the boiler at all, then you probably would not even care if that bit did go wrong.
So in summary, there is little to choose between a combi and a system boiler *if* you are using the combi just/mainly as a system boiler. (this may be less true if you want clever system boiler functions like weather compensation and split temperature operation)
If you do use a combi in place of a system boiler then it does allow the option of (say) taking potable hot water to the kitchen from the combis DHW side though. Depending on location etc that might represent a shorter dead leg than using HW from the cylinder. If you do lose DHW output from the combi then you have only lost HW to the kitchen.
[1] Beware the tales of woe comparing unlike with like - i.e. cast iron heating only lump Vs modern condensing combi.
The pressure in the primary system does not have any influence on condensing efficiency that I am aware of. However the other points about easy filling and freedom from air locks, plus greatly reduced scope for serious leaks and less system corrosion etc still hold.
My own experience (10 years ago) was that a few days after the unvented boiler was installed, a pin-hole leak developed in the back of one of the upstairs radiators - certainly it was a pretty old one, but showed no signs of corrosion damage or otherwise that would have made me think it should have obviously been changed as a precautionary measure. As you've pointed out, at least the amount of leakage was contained since it was a sealed system... however it wasn't pretty.
"Cor, never seen that happen before" said the plumber (although this was the guy who managed to totally screw up half the CH connections - see "Bloody plumbers" thread...)
Three days later exactly the same thing happened to the only other radiator in the house which was of similar vintage.
Rads (being steel and ready to corrode with just so much as a stern glance) are a slightly different category to the embedded copper pipework. They are also easy to get at and change, unlike pipes under wood floors etc.
Um, no. You're missing one of the main advantages of a sealed pressurised system. Once the contents of the pressure vessel have leaked, the system isn't being constantly replenished so flooding should be less severe than with an open system.
That said, I'd go with the "If it ain't broke" approach.
Thanks, John. Judging from the various answers, I may not bother with pressurising, though, depending on the cost of this aspect of the work.
Oh, that's unexpected. Weather compensation may be interesting; split temperature op I don't understand.
That's really nice, the boiler is in the kitchen and the HW flow to it from the cylinder is crap (mea culpa, wrong tap installed).
So, can I confirm that, although I would have a 28kW or even 35kW combi dork, the CH (which only needs ~9kW) will not be burning gas like there's no tomorrow, because that's a lower-rated circuit, that's also modulated. However, the HW leg to the kitchen will (or will it just modulate down to, say, a quarter of that, because the demand will be low?).
Understood. One of the reasons for holding out is improvements in the technology (and hopefully but not expected, really, in reliability).
We had a pressurised system fitted but the radiators were also changed, however majority of the very old (over 25 years) pipework is still in place and has not leaked for near on 3 years now.
A lot of combi boilers don't really modulate very low (less than heating only boilers) if your house needs just 9kw in a worst case scenario, the boiler will porbably cycle for majority of the time.
Cost would be relatively low, changing the boiler and removing (or just disconnecting) the header tank. Possibly adding a filling loop - so fairly trivial for just converting the primary side to a sealed system.
Well when you analyse it, the components in a system boiler are similar to a combi - the combi just has the flow switch, diversion valve, and plate heat exchanger extra. Often they are otherwise the same chassis.
Gains more efficiency from a modern boiler by running an optimised flow temperature, promoting maximum condensing while remaining just high enough to cope with the current weather.
Its where you want a lowish flow temperature for running the rads, but then have it switch up to a higher temp for recharging the cylinder so that you can store more energy in the cylinder. Normally if you have weather compensation then you need the split temperature operation as well. Some brands support this - sometimes generally, or sometimes only when partnered with the same brand of unvented cylinder.
You could replace the cylinder with an unvented one while doig the boiler. That would give you (close to) mains pressure hot water at all the taps as well. (and do away with a storage tank in the loft). This works best where you have a decent incoming cold water supply flow rate.
Yup, chose a reasonable boiler and most will modulate to under 10kW. Some go as low as 3 or 4kW
Combis will normally run flat out when doing hot water. If your kitchen hot tap could shift 15 lpm, then that will swallow most of the output of a 35kW combi.
Many higher power combis (i.e. >30kW) will run the heating at less than maximum power anyway. The one I used to have could feed 35kW to hot water, or from about 8 to 25kW for heating.
Well the main difficulty is that modern boilers are more complex regardless what type they are. Hence there is more to go wrong. One ought to be able to say, stick to a decent brand, and it will last, but that is not always the case. Much depends on how much gas you are using. You have to do the sums and work out if a new one will save its cost in fuel in a reasonable time frame. For many, nursing an old lump until it finally dies may be the most cost effective option. For high users it might be worth ripping and replacing before it dies just to get the lower usage.
Same here last time I converted a system. I don't recall anything leaking. Some of that was fairly old.
Most go down to at least 12kW many a bit lower. How far off the mark is will depend on how well insulated the house is.
The other point to remember is that cycling is not the demon that it once was. On an old system with separate pump with no overrun, once the stat cycled off, it left a big heavy boiler full of several gallons of water, just sat there so that it can waft all the heat out of the flue. Hence there was a significant penalty every time it cycled off and cooled. With a modern boiler, lightweight HE, low water content, and a controlled pump - any residual heat in the boiler will be dumped into the house and not out the flue (the flue will not even be natural draft
- so without the fan running it will lose less than a conventional flue anyway). There was also the issue that short cycling was bad for the boiler since it allowed greater scope for (corrosion inducing) condensation to occur during the re-warming phase as it cycles back on. Needless to say this is not a problem for a condensing boiler.
For the rest, I guess that the boiler needs to know that the new consumer is a voracious tank, rather than a puny, part-satisfied rad. But won't the return temperature tell it enough? We have covered unvented cylinders before; too stingy to fit them.
Vaillant is what I would get if I was getting one now, maybe a 28kW, though I wonder if the 35kW lump has better materials. (Though I am curious about Remeha as well, as recommended by Dr Drivel; I know his reputation, but still... :-) I have also grown up with Buderus oil lumps, one boiler for 10 flats or something, but they don't pop up too frequently in the UK.)
That does not sound great, but I guess it's what they do anyway, at everyone's home.
I do about 20,000 kWh pa with a 65% efficient boiler, so I am not sure the argument is there. If the Netaheat did not suffer complete unavailability of the gas valve part I would have been even calmer about it.
Good point about the cycling, on our system, once the call for heat is over the pump runs for 10 minutes so the a lot of the excess heat is dumped in the radiators as you say. If the system was a regular boiler with a S plan control set up, I take it the heat is dumped in the boilers integral bypass.
With the simplest system, there is no bypass as such. Once the stat stops calling for heat, the boiler and pump stop immediately, and the valve springs shut.
Well say its a mild day, and the external sensor reading combined with the selected response curves for the compensation system decide that a flow temperature of 45 degrees is adequate. That is not going to heat a cylinder to a useful (or Legionella safe) temperature.
It will control the power input to an extent but is unlikely to raise the maximum flow temp as dictated by the compensator.
Depends on if you need to compensate with more expensive wide bore taps, shower pumps etc. It does not take that much to make up the cost difference.
Vaillant seems to be gaining favour over WB. They also do a reasonable amount of kit with weather compensation capability and split temperature operation.
Yup. The actual performance will depend on the inlet water temperature. During the warmer months when it is around 15 degrees, then a 35kW combi will almost keep up with a moderate flow rate main. The poorest results are in the depths of winter with very cold incoming water.
Yup at £600 a year or so, you could probably save £200 or so... so probably only really worth doing anything if you can DIY and keep the installed cost of a new boiler under a grand.
forgot about that yesterday. I have a Honeywell CM-zone system. When the controller stops calling it's because all rads are satisfied and their TRVs are (I presume) screwed shut (it's like every room has a room thermostat). Assuming an open-vent primary, I smell trouble with pump-overrun. I think I should keep one radiator without TRV, as I have it now.
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