Help understanding KW rating for boilers

ThisIsWeird

jennifernil said:

Why are vented boilers a "long term pita" ?
We replaced our old Potterton Neataheat  18 months back, after  33 years, it had never failed us but was now inefficient.  We went for a heat only  Veissmann Vitodens.    Shower flows are fine, even excellent in the 2 downstairs bathrooms.    We would never consider a combi, we need to know we can always have a HW supply.

'Pita' may be overkill, but I would personally suggest swapping from vented to 'un' if/when the opportunity arises. This doesn't affect how it performs, and has no bearing on DHW water flow. It's purely a long-term reliability issue.

Yes, an unvented system requires an expansion vessel, and yes this is one extra component to go wrong. However, a vented heat-only boiler needs a cold water storage tank, and that comes with a series of potential hangups;

If a slow leak develops, the F&E tank will keep the system topped up and everything working. There will be nothing to indicate that a leak is taking place - other than noticing the actual leak. Meanwhile, the leak can cause unseen damage, and the system water will become increasingly diluted with it being constantly topped up, coupled with dissolved air being constantly introduced to the system water. That equals corrosion and sludge.

If a large leak develops inside your house, the F&E tank will helpfully continue to keep it supplied until you discover the mess and turn off the mains supply. Ie, it can easily flood your house if it occurs whilst you are out.

A stuck ballvalve coupled with an inadequate overflow pipe can cause your ceiling to come down.

A fresh clean unvented system, sealed and inhibited, should cause very few issues long term, and create less sludge. Any leaks are indicated immediately by a drop in pressure, and the boiler shutting down. Should a pipe actually burst, the amount of water released will usually be small and limited; once the pressure drops to zero - around only a kettle full's worth - the majority of leaks will simply stop, or reduce to a trickle.

Cons - the increase in system pressure might expose some dodgy joints. Other than that, it strikes me as an obvious improvement to go unvented when the opportunity arises.

kittennose

I would agree it's a bit of a PITA - for us our gravity fed system is very much at the end of its life. 

Plumbers keep saying it's a much simpler system, but realistically we need to replace

1) 2 X 50L Cold Water Storage Tanks (these are very stretched over the years and the lids don't fit it. A leak would be devastating so we would replace. They are also absolutely full of silt and gunk and the ball keeps breaking.
2) Replace F&E Tank - very gunked up same issues as above
3) Replace CH Pump (30 years old, very very noisy)
4) Replace 5x Gate Valves (all are either stuck or snapped off)
5) Replace Boiler
6) Invest in a shower pump (3L/Min doesn't cut it)
 Replacing the cycling (furred up and takes forever to heat, immersion has failed).

I know combi's are hated by heating experts but I can see the attraction of solving all of this in 1 go with a single device, doubling the attic space and airing cupboard space at the same time.

ThisIsWeird

As long as you have enough flow and pressure, and as long as you understand a combi's limitations (one hot outlet at a time), and as long as you understand that, overall, it's likely to be less reliable over time, then combis do have their place, and can make a lot of sense. I'm happy with mine.

ComicGeek

Personally I'm not a fan of system boilers, as they generally have integral expansion vessels and integral circulation pumps - these are normally the first things to fail in a system, so means being locked into a particular manufacturer and installers. Manufacturers are quick to discontinue replacement parts, and make slight amendments to new boiler models, in order to make existing boilers redundant as quickly as possible. 

Using an open vent boiler in a pressurised system with external expansion vessel and circulation pump is the sensible choice IMO. If you look at an identical Vaillant system boiler (600 reference) and the open vent boiler (400 reference) there is absolutely no difference in performance or efficiency - you're just paying more to be locked into their supply chain for replacement parts. Personally I want to choose my own expansion vessel and pump as well.

A lot of plumbers/people use 'system' to describe all pressurised systems, which is not the case. I've got the Vaillant ecofit pure 418 open vent boiler with a pressurised system. We have a 180 litre hot water cylinder, but it's very quickly heated up by the boiler and have never run out of hot water. Our mains water pressure is awful here, only about 1.5 bar, but sufficient to run 1 shower at a time adequately.

Rather than accumulators you could install a 'coffin' tank in the loft with a pump set to feed the balanced hot/cold water feed to an unvented hot water cylinder, from which you pipe balanced hot and cold water to each shower/bath - that will then give you balanced higher pressure hot and cold water. The downside is the potential noise when the pump is running, but that could be solved with careful selection.

High flow combis might be an option, but personally I like having the immersion on the hot water cylinder as a back-up if the boiler breaks, and I also use PV for free heating during the summer.

kittennose

ThisIsWeird said:

As long as you have enough flow and pressure, and as long as you understand a combi's limitations (one hot outlet at a time), and as long as you understand that, overall, it's likely to be less reliable over time, then combis do have their place, and can make a lot of sense. I'm happy with mine.

I've never had one so I wouldn't know, but doesn't a combi split the HW flow to some degree? I.e take 15 L/Min into the boiler - Less 3L/Min through the boiler etc = 12L/min Out. Perhaps 8 of that is hot and 4 is cold.

If another shower or tap is turned on, does it not weaken this to something like 6 L/min - 4 of which is hot?

For me I think it's a choice of which is the lesser of 2 evils.

Cylinder: Possibility of running out of hot water, but being able to use multiple hot taps.
Combi: Constant supply of hot water, but only one tap at a time

Albermarle

6) Invest in a shower pump (3L/Min doesn't cut it)

In the past we had a traditional vented system with cisterns in the loft like you. We had a shower pump fitted which gave a nice power shower. Later we converted to an unvented system ( due to a loft conversion) which is under mains pressure. I was told that you can not have a shower pump in this type of system, and have to rely on the mains pressure. I believe there are strict restrictions on trying to 'pump up' the mains water flow, from the water companies.

As our mains pressure/flow is not great ( similar to yours) the shower flow was worse than before, when it was pumped. However it was better than before without a pump. So reasonable but not that strong/high flow.

kittennose

FreeBear said:

kittennose said: We could get a Worcester Bosch Greenstar 18Ri but I couldn't find any info on modulation etc. Website is terrible. And everyone keeps saying how they are the worst manufacturer (I always thought they were known to be decent).

Or....we go back to the original plan of a combi, but our flow and pressure may not be enough at 14l per min / 2.0 bar. Seems to be 50/50 who you ask!

Greenstar 18R1 has a rated heat output of 19.28KW. All I can find on minimum is the rated heat input which is not the same, although it can be used as an indicator.. 5.54KW to 18.48KW is what is listed in the installation manual - Like the Vaillant, not brilliant. WB do have a very good reputation for not just reliability, but also for warranty claims in the event of a breakdown. Just about all the local gas engineers that I contacted were pushing WB over any other make.

With a flow rate of 14l/min and a pressure of 2 bar, you are way ahead of me (10l/min @ 1.5 bar), so a combi would work for you depending on the number of bathrooms/showers. As I only have a single bath (and an electric shower), the flow rate & pressure is adequate, and I went for a combi.

What thermostat are you using with your combi FreeBear?

FreeBear

One of these little sensors in the hallway (got more dotted around the house), and an OpenTherm shield hooked up to Home Assistant (via an ESP32) - It does much more than just acting like a thermostat. Times and temperature are all controlled by HA, and I get to log all the data in great detail. Unless you are technically minded, probably not for you.

A Hive or Nest thermostat would probably be cheaper, and much easier to set up.

Veteransaver

kittennose said:

ThisIsWeird said:

As long as you have enough flow and pressure, and as long as you understand a combi's limitations (one hot outlet at a time), and as long as you understand that, overall, it's likely to be less reliable over time, then combis do have their place, and can make a lot of sense. I'm happy with mine.

I've never had one so I wouldn't know, but doesn't a combi split the HW flow to some degree? I.e take 15 L/Min into the boiler - Less 3L/Min through the boiler etc = 12L/min Out. Perhaps 8 of that is hot and 4 is cold.

If another shower or tap is turned on, does it not weaken this to something like 6 L/min - 4 of which is hot?

For me I think it's a choice of which is the lesser of 2 evils.

Cylinder: Possibility of running out of hot water, but being able to use multiple hot taps.
Combi: Constant supply of hot water, but only one tap at a time

Afaik all Combis will cease all heating to radiators when supplying the hot water at the taps. So maximum power goes to heating the hot water.
It's not really an issue unless you are trying to heat the house up and the whole family is taking 15 minute showers one after the other!
The other "advantage" with a combi is that they are very overpowered, so will heat the house up very quickly. Normal Combis are usually 30kW output upwards, whereas an average house probably only needs around a 12kW boiler.
Showers are generally fine with a combi, running a bath can be quite slow though compared to if you have a hot water cylinder and 22mm pipes to the taps.
Down side of combi is if it breaks you have no hot water at all

Also, with the move to heat pumps people who have repurposed their airing cupboards now have nowhere to put the hot water tank that a heat pump needs!

kittennose

FreeBear said:

One of these little sensors in the hallway (got more dotted around the house), and an OpenTherm shield hooked up to Home Assistant (via an ESP32) - It does much more than just acting like a thermostat. Times and temperature are all controlled by HA, and I get to log all the data in great detail. Unless you are technically minded, probably not for you.

A Hive or Nest thermostat would probably be cheaper, and much easier to set up.

We have a Hive but it isn't open therm. If I went combi and Veissmann I was wondering if it's worth swapping to one that supports it such as Nest.