Changing Existing Boiler Type for new Combi System


Postby mikejb » Sat Oct 06, 2018 3:13 pm

That said I have what should be an easy question for those in the gas safe/plumbing trade. I am looking to replace a conventional boiler (fairly new) with a combi system in order to do away with the need for a hot water tank and indeed cold water storage tank. This would also create more space in the bedroom where the HW tank is currently located.
The question is, what is involved with such a replacement and what fitting costs should I expect?

Hope someone can assist. By the way the location for this work is the west london area in case that makes a difference
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Postby ericmark » Fri Oct 12, 2018 10:36 pm

Can't help with costs, I am an electrician, but I can tell you of my mistake so you can avoid it.

I also got rid of the cold water and hot water tanks, in two houses, in one to get more room in the other as tanks leaking.

The Bosch combi boiler I have has two options eco on or off, what this means in real terms is eco off there is a small reserve of hot water, this means faster hot water to taps, and maybe more important hot water even if tap not turned on full, however with shower it means it goes hot cold and hot again. Turn eco on and taps need to have a high flow rate for boiler to fire up, but once shower goes hot it stays hot.

Shower was another problem, we had a power shower fitted, seems these are illegal with combi boilers as they could suck dirt into the main if there is low water pressure, so shower had to be changed for one without a pump, still good shower with mains water pressure, but clearly a cost involved.

Also it means if I fit solar panels, I can't use excess power to heat water, once a combi is fitted you only have one water heater, with standard twin tank you can have multi ways to heat water including wood burners.

The biggest mistake was using a Bosch boiler, it seems Bosch will only allow you to use their own thermostats connected to the boiler bus, other makes allow you to use third party thermostats using the Open Therm protocol.

So at this point I had better describe how a condensating boiler works, does not matter if combi or not, to get extra energy from the fuel it cools the flue gases to below boiling point to use the latent heat. To do this the boiler has to be able to adjust the flame height (modulation) so that the return water is cool enough. This has turned everything upside down as to boiler installation.

So the thermostat radiator valve (TRV) controls room temperature, and as they all close the water pressure increases and the by-pass valve lifts allowing hot water back to boiler, which in turn modulates so flame height adjusts according to demand.

However a point is reached when it can't turn down any more, so at this point the boiler starts to cycle, however there is nothing to actually turn whole boiler off or on, unless some extra control is added.

You could fit a on/off thermostat in a room kept cool with no outside doors or alternative heating, however many homes don't have such a room.

With open plan houses you could use a modulating thermostat like the Bosch wave in the living area, this then not only turns boiler off when not required, but turns the temperature down first, so less heat lost out of the flue, however if not an open plan house a single thermostat does not work.

So next is electronic heads on the TRV which tell a central thermostat (hub) what each room wants, so it in turn tells the boiler what output is required. EvoHome is a good example, however EvoHome, Nest, etc have two methods of working either no/off or modulating the boiler using Open Therm, so unless the boiler is open therm these very cleaver thermostats can't work at their best.

So I decided to upgrade my house a bit at a time, with a pair of electronic heads costing £80 approx I did not want to replace all at once, I read how energenie heads would work with nest, so thought start with two heads then add to it, however then found Bosch is not open therm, so although nest can modulate open therm boilers with Bosch it can only turn it on/off.

The MiHome Energenie heads work well, they keep room as temperature set +/- 0.3 degs C no complaints with the heads, but still need to use on/off type thermostat in the hall to turn off boiler when not required.

I think I had made an error anyway, as nest uses a follow command which means all rooms at same temperature, where EvoHome allows every room to be independent. However I am not trying to sell EvoHome, all I am doing is telling you about open therm, there are many makes which do use open therm and as long as you get one that does, you can add rest latter, it gives you that flexibility.

It does seem strange, we hear digital this and digital that as been good, but with central heating analogue is king, what analogue does is remove the hysteresis so room stays at constant 20 degs C not heating to 21 then cooling to 19 then heating again to 21.

Anyway learn from my mistake, get a boiler with open therm, then you have the option.
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