Choice of system boiler: Worcester Bosch or Glow-worm?

Hi
We need a new system boiler for our house. Our local installer is likely to recommend a Glow-worm 30sxi but I have heard good things of the Worcester Bosch Greenstar 28HE system. Would anyone like to comment on these two choices? Is one clearly better than the other?
I also want to enhance the heating controls. We have a simple, digital, two settings per day, Potterton programmer for hot water and heating. I want to move the room thermostat from the hall (which is large and cold) into the lounge where we spend most of our time. To make installation easy, I propose to use a wireless thermostat. I can see two options:
1) Replace the existing thermostat by a Honeywell CM61/67 NG RF and set the heating on the existing programmer to 'Continuous'. This leaves two settings per day for hot water.
2) If we choose the 28HE, we could discard the existing programmer and fit a Worcester Bosch DT20RF Programmer / Wireless thermostat in the boiler. This will give greater flexibility for hot water settings than (1).
Any comments please on these options? I guess (2) is more expensive than (1), because there will be more wiring changes, but is the extra flexibility worth it?
Does having the programmer internal to the boiler better suit a modulating burner (i.e. does the programmer provide more variable control - not just on/off - in this circumstance) ?
Thanks in advance.
David
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On Sun, 5 Mar 2006 20:58:00 -0000, "Aldrich"

Glow Worm used to have a really bad reputation in terms of their build quality. I threw out an old one that at the time was 18 years old (Fuelsaver) about three years ago and vowed never to have another.
I looked at others, including Worcester Bosch, Vaillant, Keston that have been regarded as good quality condensing system boilers.
For reasons of the build quality, design, specifications and controls, I eventually settled on a MAN Micromat. http://www.mhg.de/en/products/gas_units/micromat_ec.html
This one has wide modulation range, weather compensation, analogue sensing of room and water temperatures etc. It has been a good choice, and I'd go for one again. However, the price tag is about twice that of the ones that you are considering.
One of the reasons that I was put off by the Bosch originally was the aluminium-silicon heat exchanger. I couldn't find many reliable references to the suitability of this material for condensing boilers, whereas stainless steel, of the correct grade is a well understood technology. Having said that, people in this NG do have Greenstars and are happy with them as I understand it.
I've just been looking through the installation manual of the Glow Worm and it's very clear that the influence of their ultimate parent, Vaillant, is in the design. For example, the heat exchanger and burner are of a similar design to that of the Micromat - basically a radial burner with stainless steel tube heat exchanger.
The same concept is used in a larger installation/commercial version of the Micromat - the Strata 1.
This data sheet has some pictures of the internals such as the heat exchanger and burner and hook ups with multiple units for commercial use.
http://mhsboilers.com/pdf/boiler/strata_1_RC-0206-5624.pdf
Most condensing boilers in the 28-30kW range, modulate down to something in the 7-11kW range. I was looking for something better than that and the Micromat goes down to 3.5kW at the bottom end. This basically eliminates cycling of the burner on and off for most of the year since it will drop the flow temperature as low as 40 degrees on occasions.
The Greenstar goes down to 7.5kW and the Glow Worm to 5kW. Not a huge difference, but you might find that attractive.

I looked through the documentation for the Bosch DT20RF. Basically it's a Drayton Digistat. There seems to be nothing in the documentation that I could see that suggests that it is anything other than a 2 channel programmer timer. It doesn't say that it does anything with the boiler modulation and doesn't mention optimised starting.
In that sense, you could pick either of these boilers and use the CM67RF and you would get night set back and optimised starting as well as proportional control. This means more stable maintenance of room temperature than is achieved with a simple on/off thermostat.
Therefore, I am not sure that the built in timer on the Greenstar really buys very much.
It is possible to have more sophisticated control. For example, some of the Vaillant models have outside weather compensation. Basically this is an external temperature sensor which provides the boiler electronics with the means to know outside temperature. This means that the boiler can take that into account when running the radiators and provide more or less heat output according to the weather. In effect it provides a more accurate match to the building heat loss.
You can go a step further and have a room thermostat which passes the room temperature rather than just an on/off signal back to the boiler. THe Micromat and Strata 1 have this using the RE2132 controller (made by Siemens for this boiler). It works really well and maintains the room temperature at a very constant level with the boiler modulating as required. I believe Viessmann, another German manufacturer in this part of the market has it as well.
However, you don't have to go with the complexity that I have. I don't think you would be disappointed with either of the products you mentioned. However, I would probably be looking at the Glow Worm and Vaillant in that kind of range.

--

.andy


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Hi Andy

Thanks very much for your comprehensive reply. It was really helpful. Consequently, my 'working assumption' is that we will go with the Glow Worm and add a Honeywell CM67RF.
Thanks again,
David
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On Tue, 7 Mar 2006 18:23:56 -0000, "Aldrich"

If that is the set of things you are selecting from, then it would be my choice as well, but I would look wider as well.
--

.andy


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The Glow Worm is excellent. It is a Vaillant underneath; Glow Worm are owned by Vaillant, but it is cheaper.
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wrote:

I like the 38, which is two boilers in one case. The inards are shown. Although using two separate Glow Worms or Vaillants would do the same thing, and "maybe" cheaper. Eco-Hometec rebage these boilers.
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to
Sorry, in a bit late on this one.
I have a Bosch Greenstar 28HE. It is very good. It just works.
The (relatively) new Glowworm range, which includes the 30sxi is also well regarded, unlike their previous condensing range, which was not good.
I would be very happy to have either these boilers.

I have a feeling that the Bosch solution uses an analogue temperature sensor so could conceivably apply variable control. I am not sure of this, or whether the algorithms used are significantly better than a on/off control. My system is substantially subzoned, so would not have been able to use it.
Christian.
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Hi
I would just like to thank everyone who has replied to my question. I would like to check my choice of RF thermostat but will start another thread for that.
David
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Hi
I would just like to thank everyone who has replied to my question. I would like to check my choice of RF thermostat but will start another thread for that.
David
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