Boiler change consideration

[rick]

After another 6 days with no heating ..... boiler repaired under maintenance cover (burner, burner door, seal & electrodes), but thinking it's time to look at replacing .... becoming pretty unreliable. Its now 22 years old. Its a 30kW Heat only wall mounted gas condensing boiler - connected to a pressurised thermal store.

I need a standard/Heat only ..... can't use as system boiler (as thermal store control need to manage pump)

Want to replace with 'similar' .... I did like idea of Vaillant boilers with SS heat exchangers - but only offer these in System boilers.

Couple of heatings guys advised that current range of Baxi is now vastly improved ..... so could use Baxi Heat 800 Though only with aluminium heat exchanger (but with 10 yr warranty)

I did ask on another forum .. the answer there is to use the Dutch boilers by Intergas, but there are no local Intergas approved installers When I have asked British Gas/SSE and independent GasSafe guys they have never come across one.

Thoughts on the Baxi Heat ? or other Heat only models to consider

[Tricky Dicky]

You can get system boilers without a pump, we had a Bosch Greenstar at the last house and the pump was separate, in fact sited some distance from the boiler. It is Combi boilers that usually come with an integrated pump. A friend of mine who had a thermal store had a very similar arrangement to yours with a Bosch boiler.

Richard

[Dave Plowman (News]

Is that the case today? No thermal store can be used with a system boiler? When I did a self install quite a few years ago, Viessmann had an incredibly helpful technical advice guy in the UK. Seemed to know everything.

Just that system boilers being the norm, may be far better value due to economy of scale.

[John Rumm]

They do the 400 series heating only models (mostly open vented, but some also suitable for sealed system).

The newer ones seem to be Ali HX though.

No experience of them, so can't comment.

Worcester Bosh have a few as well like the Greenstar 24Ri features

[rick]

OK ... I thought all System boilers had expansion vessel & pump built in .... my misunderstanding.

[rick]

Not tried Viessman .. if you have a name to contact be happy to try them.

The reason I can't see how to use System boiler is that thermal store operates the pump to 'stir' the store when you are drawing off DHW ..... otherwise heat exchanger gets cool quickly and you lose hot water.

Works really well ..... but with a System boiler the Boiler controls the pump, and therefore the Thermal Store controls can't operate a 'stir'

When I put in the system teh company specified a HEAT only boiler for this reason. The thermal store installed is not changing, and heating company state I should still use a HEAT only boiler.

I had thought of installing Vaillant but they advise only their system boilers can be connected to a pressurised Thermal store not Heat only. When I asked them how do I get the pump to stir the store - that was beyond their experience and could offer no solution.

[rick]

I contacted Vaillant support they were very clear that Heat only boilers cannot be connected to a pressurised thermal store, they have to be used open vented. Pity as they are supposed to be god boilers.

[Dave Plowman (News]

It was many years ago. The number I have is 01952 675000

[AnthonyL]

I thought god's boilers were that big bright things in the sky?

[John Rumm]

Be careful with your terminology here though. Since cylinders can be vented and unvented, and they can be connected to primary systems that are also vented and unvented.

So there is nothing stopping you having an unvented cylinder (i.e. mains pressure cold feed), heated via an indirect coil connected to an unvented primary heating circuit.

Much as the (more common) sealed system primary heating a vented cylinder is also ok.

The time you may have a problem is with a sealed system thermal store[1] connected in a "direct cylinder" setup connected directly to a boiler (i.e. the CH primary water also fills and mixes with that in the store, and it uses a coil or external plate Hx to extract the energy from the water stored water).

For the latter case you will need a boiler that can be used with a sealed primary system. Vaillant claim[1] that some of the 400 series

*can* be used in sealed systems (although obviously you would need to provide an external expansion vessel, filling loop and over pressure relief valve) [1] and to be fair, many traditional vented boilers can be used in a sealed system with the addition of the right external bits. [2] And even then it not uncommon for people to build heatbanks and thermal stores using unvented cylinders - but they don't have to be used that way.

[rick]

You able to find the name of the guy in any old emails ? finding someone at a Boiler manufacturer who understands needs of Under Floor Heating is not easy. Their standard answer is go talk to Heat Safe engineer - never come across one who has any clue about such systems, and had a great many from SSE, Baxi & British Gas at my place over the years.

I had a right numpty last week ... (boiler broken again) repair contract company sent a guy, who decided pressure gauge of thermal store were too low and started pumping them up with water from filling loops - luckily I walked in as he wa doing this .... Explained basic science to him, and then he agreed to release the pressure .... watched him for several minutes trying to drop pressure in underfloor heating circuits by tripping Thermal store pressure ..... explained again the basics, he could not comprehend. declared boiler had PCB fault .... would report it to service company.

Luckily they engaged 'Baxi' to do the re-visit as they do some form of fixed price repair ..... in only few minutes the guy identified boiler overheating inside casing ... found burner box door seal disintegrated in corner, and burner inside all broken. Replaced both - boiler fine

[Dave Plowman (News]

Did it all by phone. He was the one I got put through to when asking for technical help. Of course it's likely just a robot now.

He was an expert on Viessmann boiler related stuff. So would likely know about using a system boiler with a heat store. Even my now quite old Viessmann can communicate with other devices. So it's possible they do a pump controller for a heat store. But just musing.

[rick]

The issue is I can't use an external controller .. I have a full controller for Underfloor Heating (17 zones) so that needs to stay. It needs to control the pump ..... Its mains pressure HW through a heat exchanger in the Thermal store ... once you start running HW after a couple of minutes the water around heat exchanger will cool The control start the boiler pump - so it 'stirs' the store without powering up boiler, meaning you can get loads of DHW before it needs to start boiler and replenish the Thermal store. Added advantage is that I have my Solar panels on roof dumping any excess electricity into the Thermal store .. in Summer boiler never fires up at all.

I found Tech Help email address for Viessman so will ping them a mail

[rick]

In this instance the boiler is connected via primaries to Thermal Store

- it circulates the contents of the store around the boiler. There is a pump, expansion vessel and 2 port anti heat-syphon valve on the prinmaries No direct connection to DHW or heating The store is pressurised. Inside the store is a horizontal baffle .... below which there is a heat transfer coil for the Heating circuits Above the baffle is a multi-plate heat exchanger through which is fed mains water = providing the DHW for the house.

Vaillant advise that only their system boilers can be connected to a pressurised thermal store. I explained set up to them .......

They were clear that their OV boilers cannot be connected to a thermal store.

[John Rumm]

OK fair enough - not sure why they say that when they also describe how to run the OV boilers in sealed systems... Perhaps its just a safety "catch all" statement since going unvented requires the addition of external components that are not part of the boiler (i.e. Over temp/pressure relif valve, expansion vessel, filling loop and pressure gauge etc).

(One of the difficulties I find with the Vaillant kit is getting "big picture" documentation or information. There is tons of low level docs describing many features, but not always so much covering whay you want them or when to use them).