Which heat only replacement boiler?

[BigBurner]

Thanks for all the posts - all very informative but I thought I would try to clarify what my objectives were and also answer some questions raised and see if there is any consensus.

We have a Gloworm Ultimate 120FF with 28mm flow and return reducing after the pump and cylinder branch. The cylinder is a 300 litre Ariston (I put this in 10 years ago and was new to pressurised cylinders and this was an economical buy. It works well and is only a pain because the sacrificial anodes have to be changed occassionally - I now specify stainless cylinders) and we have a pumped return.

You may want to look at for replacement now, or in the future, to a stainless steel thermal store/heat bank - main pressure DHW and no explosions taking the block down. Water heater blasts:
http://au.youtube.com/watch?v=pu3FwgIHsQA
http://au.youtube.com/watch?v=GF_Wrm-Ns0I

BTW, running your heating off the store means a Smart pump can be fitted on the CH, which mean TRVs on all rads and no wall stat and no boiler cycling when two cylinder stats are used.

http://www.advanceappliances.co.uk/gas_or_oil_thermal_store_systems.html

DPS have a DHW only vented main pressure DHW heat bank called the Pandora. This requires no large bore metal discharge pipes or overflows to atmosphere (which can play havoc with heat loss of a house). It can be fitted centrally in a house. Great for architects as no external penetration of the building fabric. Look down for Pandora. They also make square units for better use of space, to order.
http://www.heatweb.com/

So large bore metal discharge pipes can be eliminated. Note: Using a Hepworth HepVo trap and heat resistant plastic drain piping a large bore discharge pipe can be eliminated on a normal unvented cylinder by taking it to the internal drains. But! The boiler discharge pipe still has to go to atmosphere, not into the internal drain. But again! Atmos have one of their boilers having the boiler system discharge into a tundish/HepVo trap and to the internal drain.
http://www.atmos.uk.com/core_files/productDoc(135).pdf
Look at: "Combined Safety Discharge and Condensate Drain"

Atmos say:
Subject to the approval of the Local Building Inspector, the Safety discharge pipe and condensate discharge pipe may be combined into a single common discharge pipe. Atmos have provisional approval for such an arrangement (copy of letter available on request).

Combi boilers in blocks in London are a pain as they leak all over the outside of the buildings and all the external pipes and flues make a block look very ugly and messy.

We have 21 rads and towel rails and the underfloor heating circuit so the pressure vessel is big. We have a Horstmann programmer and Honeywell CM 67 programmable stats dealing with the underfloor and rads and the towel rads run on the wild side - I would change this given time again. The pipework around the boiler is pretty neatly set out and although we have 28mm F & R the boiler short cycles far too much for my liking.

The reason I posted for advice was because the boiler has had a few faults and I would like to fit an economical replacement. I spent a lot of time looking at Viessmann and these are used so much more in mainland Europe and I know they are very good. I used to specify Kestons but after a lot of problems would never go back to these. I know that the trade likes Vaillant and these seem to be good but the Viessmanns seem to be better and they modulate so should be much more economical. I said earlier that we need 35kw but should have clarified that this is because the 120FF is this size I would not want anything smaller. When it's cold we run this flat out.

Most boilers modulate, however they tend to have different algorithms and how and why they do it tends to a mystery with some makes as the their customer facing technical people tend not to know how it operates in detail.

I had thought that system boilers would be unsuitable because of the vessel size and pump (we have some long circuits) and I had not wanted to rebuild the whole system and had hoped to find a boiler which would relatively easily fit the present space hence the early choice of Ultracom HXI 38. Most of you think this is a heap and some of you think I should go into system boiler. I know nothing about Atag (except that they make good kitchen equipment) but now there is a big range from your various recommendations incliding the 400 series Vaillant and the original HXI - see the list below. I agree, probably, that it would be better to rebuilt the whole system but if money were no object I'd probably change all the rads, pipework as well. Cost is important and a heat only (if possible modulating) boiler would be the easiest (probably cheapest) choice and if not I don't know which to choose. Is there any consensus from you?

What is a Magnaclean? Should I fit one? I should also add that the system is pretty clean, power flushed about 4 years ago and well topped with inhibitor. Rads all work well and with good output.

A maganclean is essential as it grabs iron particulates (magentite), which causes sludge and can ruin a heat exchanger and complete system in time.

Many makers use the Gianonni heat exchanger, or similar: Vaillant, Broag, Glow Worm, etc. It is highly efficient. It needs to be free of solids otherwise problems. Gas News had an interesting article about this heat exchanger, raising some questions about its potential reliability:
http://www.gas-news.co.uk/archive/storage/pre-10/2007/comment/1006.htm

Atag E32
600 series Vaillant (Vaillant 637)
400 series Vaillant
Broag Avantaplus 39C combi
Viessmann 200W
Gloworm HXI38
Ecotec plus 438

All good boilers. Vaillant are expensive for what they are. If you just want a straight boiler swap then HXI38 looks good...but at £1,300.

You may want to fit two Broag 18v open vent boilers then backup....and integrated weather compensation and a control system that will ramp up to maximum temperature when DHW is called for fast re-heat. The weather compensation will run it up to maximum automatically on CH when zero C outside. It also keeps the return temp as low as possible for condensing economy. The compensator modulates the burner and keep the return temp as low as possible.

If you go to a sealed system the Broag 39C combi without the DHW section connected, or connected and isolated with a 1/4 turn valve for backup (which I would do), would do and the added option of a good control system and integrated weather compensation. An extra expansion vessel can be added if going system boiler. Or you may want to stay open vented in preparation to adding heat bank thermals store in future.

Two Broag 18v boilers cost less than one HX38.
http://www.tradingdepot.co.uk/DEF/c.../Central Heating Boilers/Broag Remeha Boilers

Look at control options:
http://www.avantaplus.com/docs/Issue 5 -Avanta Schematics Booklet.pdf