New Vaillant 428 and pump flow rate?

[alyne]

Hi - first post here. sorry its so long!

Firstly my system - installed 10 years ago when we bought a run-down cottage and renovated and extended - effectively a 'new-build'
4 bed semi - UFH upstairs and down (no rads at all) all running off of a DPS Heat-Bank direct thermal store. Hot water also runs off the thermal store through a plate heat exchanger.
There is a plan this summer to make the control system a whole lot more elegant, but for the time being the UFH pump is controlled from room stats, the boiler's "heat-demand" signal comes from an immersion-stat in the store - about 1/3 of the way up. currently set to mid / high 60's (its set lower in the summer when the incoming mains water is warmer and needs less delta-T added to it). The system worked pretty well for 10 years with a Baxi-Barcelona boiler, except that the boiler was a piece of **** and kept going wrong again and again and again. The boiler finally kept going into lockout and only resetting after multiple attempts a few weeks back and I've just had a new boiler installed.

The new boiler (Vaillant 428) has been somewhat problematic and I think the problem is down to flow rate, but I can't understand why. The boiler pump is attached to the store, and was a grundfos 15/50 on speed 3.
The baxi seemed to do what a boiler on a store is supposed to do and run at full tilt, until the store was almost up to temp, and then start to modulate down (I guess as the flow from the boiler reached the boilers max temp and the return started to catch up with it) and then get shut off by the store stat. I've got some very primative pipe stats near the store and delta-T was around 20 degrees, maybe a touch less with the baxi. There was some cycling, but that was due to the hysterisis on the stat being insufficient I think. (part of the reason for the new control system being fitted later this year).

The new boiler however (set to 85 degree flow on the dial) reaches 85 very very quickly and often shuts down in its anti cycle then starts up again. When the store is just topping off the boiler seems unable to modulate down quick enough and the temp shoots through 85 degrees and it shuts off. Using the D-codes (D-40 and D-41) on the boiler to get the temps I can see almost 30 degrees Delta -T across the boiler when its running full chat (30kw). I did a bit of research and reckon that means insufficient flow rate. But I can't understand that. Same power boiler, Same pipework, Same pump. So the only thing is the flow restiance of the boiler itself. Can the vaillant really be so much more restrictive than the baxi?
I reckon I'm getting around 850-900 litres/ hour flow (30kw at 30 degrees) and i need around 1200-1250 for a 20 deg delta-T at 30kw, so today I fitted a 15/60 pump, which seems signifiantly more powerful in the specs. Its a bit better, but not much. I've got about 25 degree delta-T. The boiler behaves better, but still not 100% ideal. with 20 degree delta-T it would be about perfect as the hotest return the boiler is going to see is going to be around 65-70 and the boiler should have time to modulate down as the temp won't shoot up to 85 too fast for the boiler to deal with. And 20 degree is what the boiler is designed for so its logic is probably having a bit of a paddy with high delta-T's

Anway - any ideas? Is there a more powerful direct fit replacement pump for the 15-60? The boiler feed / return is in 22mm with 28mm near the boiler. As part of the summer strip down and refurb I could replumb in 28mm with less sharp bends, which could improve the flow rate with the 15/60. I'm surprised that I can't get the flow through, even with 15/60. Does anyone know what the system version of this boiler has pumpwise?
Is it possible to calculate what 'power' pump I need given the above flow rates and the required flow rate?

 

[swbjackson]

How is the pump wired? I had problems with a Vaillant where the pump wasn't wired through the boiler even though the pump was running continuously. (temp wired up to test)

 

[alyne]

The pump is wired to the boiler - fixed on / off signal from the boiler - live / neutral / earth. Its all doing exactly what Its expected to do in terms of when the pump runs - pump overrun, etc all works fine.

 

[gaswizzard]

Can't see an issue with the 15/60 , the TS is basicly a big low loss header with little resistance , from memory the 428 has a resistance of around 2.8 metre head , this leaves roughly 3 metres head to overcome resistances to primary pipework/fittings , maybe a blockage in primary pipework to and from boiler to TS? , maybe the sludge bucket is full of ****e and contaminated primary pipework. , possibly wasn't an issue with the existing boiler due to the HEX design.

 

[alyne]

pipework should be pretty clean. The whole deal was powerflushed,cleaned and flushed again when the boiler was installed. It was all pretty clean anyway. There is a thermostatic mixer in the feed from the store to the boiler which is supposed to keep the boiler in condensing mode when heating the store from cold - but I think its fairly redundant as the return is almost always above this temp anyway. Maybe this is too restrictive - there are a fair few tight 90 deg turns in the pipe work in 22mm, so maybe its all a bit too much.

The system is workable, if not ideal at the moment. The plan was to have a comprehensive strip down and rebuild in the summer anyway - different control system, changes to the underfloor system, some changes to the hot water plumbing, new bathroom fitted, alsorts. I may as well redo the primary circuit in 28mm with gentle bends and remove the mixer valve - see what that does.

 

[Agile]

Sorry but there is no logic in your setting the store temp lower during the summer!

28 kW is way more than any four bed property should ever need.

The 400 series is known for a high internal resistance and when 28 kW output is required many say a 15-60 is not adequate.

Why set the boiler flow to 85 C when the max efficient flow will be 75 C. That is still 5+ C more that your store stat is set for.

My suggestion is to turn the boiler down to 75 C and reduce d0 to 20 kW which is still going to be way more than the heat loss of the house.

What was the power of the old boiler when it was working?

Tony

 

[gaswizzard]

A 15/60 is fine , problems can arise when circulator is used to overcome index circuit and boiler , this 15/60 is only used to overcome the resistance of boiler/primary circuit and TS.

 

[alyne]

the old boiler was set at 30kw. The heat loss to the underfloor system is negligable, but the high flow rates for hot water flow - eg a bath and a shower simultaneously will drain the store temp very quickly and every watt of that 30kw is needed to try and keep up. the heat exchanger for the hot water is 100kw - so the store will get cold very quickly unless the boiler works away to keep up. The store is smaller volume that we really would have needed for the hot water requirements, but space constraints dictated the maximum height of the store.

I think there is logic to dropping the store temp in the summer. the incoming mains cold water is almost freezing (I think it comes from a water tower round here - so the water gets close to outside air temps) - say 5 degrees - so the store has to heat that water up to 50 odd degrees in the winter - thats 45 degrees increase. In the summer the incoming mains water is much warmer so the store is probably only increasing it by 30 or 35 degrees - much less energy is required.

 

[Dan Robinson]

The GX unit (blending valve on the return is not so much to keep the boiler condensing, but to improve reheat times for hot water from a cool start. It is mixing warm water from the top of the store with the cold return water from the bottom. I wonder if that is causing the problem?

I wouldn't bother dropping the store temp in the summer though.

Out of interest is your store OV or unvented?

There does appear to be an issue with modern boilers sitting on thermal stores. They just don't expect such huge deltas. I don't know if actually having a 30 degree_ delta is actually harmful to Vaillant HEX's but it sure seems that it doesn't like it.

To get around the problem -at least as a test. Shut the KW's down 20% but leave the flow temp up. If the cycling stops, then gradually notch up the kW's until you reach the peak.

Bunging a bigger pump on is not worth it.

For others looking into thermal store systems, you need to pick a boiler that uses a pressure sensor rather than Delta to prove circulation. It saves a whole lot of heart ache.

 

[Agile]

I think there is logic to dropping the store temp in the summer. the incoming mains cold water is almost freezing (I think it comes from a water tower round here - so the water gets close to outside air temps) - say 5 degrees - so the store has to heat that water up to 50 odd degrees in the winter

Reducing store temp in summer reduces the storage capacity.

You have already said that its not large enough for your needs so I would say leave it where it is as long as the boiler is operating efficiently.

But you did not comment on reducing the boiler flow temperature which would greatly increase efficiency.

I also seem to remember the boiler is set to turn down if the Δ - T becomes higher than 25 C.

Tony