New boiler

[DP]

Was invited to look at a leaking Ideal Mexico. HE had sprung a leak. Boiler was fitted to a sealed system comprising three heat zones and UV cylinder. Option was to split the HE and carry out repairs or quote for a new boiler.

New boiler it is as old workhorse is deamed to be a lost cause. Present boiler is 30Kw beastie with a Grundfos light commercial heart

I am suggesting a Vaillant Ecotec 630 while customer is leaning towards 637 or the Viessman 35kw boiler with 5 year warranty (which I cannot offer as my normal work is repairs and breakdowns and boiler fit by special request only).

With Vaillant I am mindful of the circulation issues as one of the zones is distant from the boiler and could cause the boiler to throw the circulation fault code at which point low loss headers becoming a requirement.

Perhaps two boilers might be a safer bet?

Client would like WC on the boiler.

Can the regulars with knowledge of these boiler help client to selector a boiler that will allow the four zones to run together or to choice with linkup to WC controls

TIA

Dan

PS. if a 630 or 637 is selected, will the light commercial pump become redundant or is it better to fit troublesome series 4 Ecotec to make use of the higher capacity pump?

 

[ALEC1]

there is no benefit to zoning other than adding to complexity and cost.

If the client needs to zone he can do so with TRVs on the unused rooms.

There is no benefit to over sizing boilers either....it just eats into your profits!

the 600 series boilers is not the same as the 400 series boilers, which doesn't work well with compensation controls...so you can use the 600 series and expect it to work properly.

 

[DP]

Alec, thanks for reply. He already has the said zones, so no issue with additional costs.

What about the pump. Will the tinsy winsy Grundfos in series 6 be able to do what the light commercial is doing at present? Will internal pump not lead to circulation issues?

 

[ALEC1]

assuming that the pipe size is correct the vaillant and viessman pumps will be fine..

zoning and wc is not really possible... as there is no way to "interlock" the two..

if he is happy to have zones that don't turn on the boiler thats fine..

use something like cm zone from honeywell on some but not all the valves..

 

[DeltaT2]

It's a bit overkill having a LLH on a 30KW system Danny?? I mean would something like a OV HE boiler, on a sealed system, not give you enough through flow rate for that? Remeha springs to mind, where you can WC the boiler only & keep it simple.
Just a thought.

 

[DP]

Thanks for replying Delta. Purpose of the post is to go in and complete the task without any subsequent surprises. Present OV boiler fitted as sealed system ( bad idea) coupled to light commercial pump versus new system boiler with internal pump. Long primaries begs the question if the Vaillant 630 or 637 is going to start throwing fault codes when return thermistor get tired of waiting to see a temperature rise when controls fire the boiler.

Have fitted four series 4 bunnies with 15/60 circulators in the past. Despite the universal call on these pages demanding a pump with guts, I have on my fits balanced the system and had no callbacks.

Viessman looks good with 5 year warranty, dedicated temperature for circulation to cylinder and WC for CH but what is the after sale service like? Client proposing 35kw boiler. If I remember correctly modulates to a very low figure, which will work well here. ideal was struggling to do it's job as it took forever to heat the house, but that could have been due to large lump of a metal HE and possible shunting effect the UV cylinder might have on the boiler

A small bell is ringing at the back my head when I think of Mexico fitted to a sealed system. Did not look too closely for existence of mandatory overheat cut out on the boiler ( fail to see how that could have been done as MI clearly says boiler can only be used on OV systems). Glad to see no plastic plumbing here

 

[DeltaT2]

Oh I can understand your thoughts regarding a LLH Danny, when you're considering these appliances. My advice, for what it's worth, would be; always keep it as simple as possible.

A full property heat loss calculation, may be best, just to establish the heat requirements. Also the home owners lifestyle, as you know, is a factor. Even a simple thing like teenage kids that spend their lives in their bedroom, can be a factor in the zoning. Perhaps I'm wrongly considering design more than appliance?
HTH

 

[mickyg]

WC + Zones and a reliable unit...easy..ATAG Q. Comes with a 20-70 pump built in also.

 

[DeltaT2]

That's a good shout Micky, never fitted one but they certainly look the business.

 

[D_Hailsham]

Present boiler is 30Kw beastie. ... I am suggesting a Vaillant Ecotec 630 while customer is leaning towards 637 or the Viessman 35kw

So you are assuming that the present boiler was (a)correctly sized and (b)is still the right size. Both dangerous assumptions. When CH was first installed and gas was cheap it was normal to oversize boilers to ensure that the "gas board" (who had a monopoly on installing anything gas) could offer "guaranteed temperatures". Also the level of insulation may have changed since the boiler was originally installed.

Use the Whole House Boiler Size Calculator to find out what size boiler is required.

As you are suggesting a system boiler, have you checked that the existing valves, pipes etc will withstand the increased pressure of a sealed system?

With Vaillant I am mindful of the circulation issues as one of the zones is distant from the boiler and could cause the boiler to throw the circulation fault code at which point low loss headers becoming a requirement.

The problem is caused by installers trying to balance the system to an 11°C differential between flow and return. Vaillant and Viessman boilers are designed to run with a 20°C differential. If you try to achieve an 11°C differential, the pressure loss across the heat exchanger is so high that you need a very large pump to provide the necessary head for the rad circuit.

You should balance for a 20°C across boiler and rads. However this does mean the rads will give off about 15% less heat than the manufacturer states. The rads are probably oversized in any case ("guaranteed temperatures") or the insulation has been improved, so this shouldn't be a problem. If necessary a rads which is 20% larger than the calculated size can be installed.

Client would like WC on the boiler.

Boilers with WC are configured for HW priority, similar to a combi boiler.

 

[DP]

DH thank you for the input. This boiler change I have to say does not follow the well trodden path. The owner is a very learned man who has done a fair bit of research and is making suggestions that suit his pocket and needs. I have only cast eyes on the existing boiler for perhaps ten minutes and that was to repair the said boiler. Talk of replacement boiler was conducted at the kerbside when I was about to depart.

While present boiler is indeed suitable only for open vent, in this case it has been unstalled as a sealed system perhaps the reason it is weeping and on it's knees.

While it will most likely be possible to balance the system for 20 degrees C, this system will not operate at that differential in reality. It may not even work to 11 degrees differential due to the nature of installation mentioned earlier. Three heating zones will be the spanner in the works. Run just one zone instead of all four, ideal differential settings fly straight out of the window

 

[gaswizzard]

You should balance for a 20°C across boiler and rads. However this does mean the rads will give off about 15% less heat than the manufacturer states..

More like 30% , this is the issue when a replacement boiler (HE) is fitted to an existing system , the existing radiators would of been sized for a 60c MTD (85c/75c/20c) if we take the new MTD to be 45c (75c/55c/20c) it will make for a coooooold house come the winter months.

Correction factor of 0.7 applies....

2000w X 0.7 = 1400watts.

The radiators would be 30% undersized or in other words heat output would have to be increased an additional 43%. dependant on salesman tactics.

 

[D_Hailsham]

You should balance for a 20°C across boiler and rads. However this does mean the rads will give off about 15% less heat than the manufacturer states.

More like 30% , this is the issue when a replacement boiler (HE) is fitted to an existing system , the existing radiators would of been sized for a 60c MTD (85c/75c/20c)

I was assuming the old system had been sized for 75°C/65°C. If it was installed prior to 1997 then your figures are correct.

The correction factors you find in radiator catalogues all assume that the differential across the rad will be maintained at 10°C, whereas there is in fact a slight difference if the differential is 20°C. A 1kW boiler, (measured with 75/65) will give off 987 watts when running at 80/70. This is probably not important as most rads are oversized by about 10% in any case.

If you need to maintain a 20°C differential across the boiler while, at the same time having a 10°C differential across the rads, then the only solutions are a low-loss header or closely spaced tees.

 

[D_Hailsham]

While present boiler is indeed suitable only for open vent, in this case it has been installed as a sealed system.

If the customer is insisting on a sealed system then a low loss header or closely spaced tees is the way to go. The boiler pump would manage the primary flow and the existing pump (assuming that size is still needed) would control the rad circuit flow. But see 2 below.

While it will most likely be possible to balance the system for 20 degrees C, this system will not operate at that differential in reality. It may not even work to 11 degrees differential due to the nature of installation mentioned earlier. Three heating zones will be the spanner in the works. Run just one zone instead of all four, ideal differential settings fly straight out of the window

1. If you have weather compensation you can forget about the HW zone. This is because weather comp systems always give the HW zone priority, so it gets the full output of the boiler - nothing going to the heating zones while HW is required - exactly the same as a combi boiler. If it didn't do this, the boiler could be trying to heat cold water to 60C using boiler water at 40C.

2. Installing a variable speed pump in the secondary circuit would alleviate the multiple zone problem.

 

[ALEC1]

with weather compensation delta t accross the radiators is irrelevant...as the boiler looks after the amount of heat released into the system...this is NOT to be confused to balancing the heating system where design criteria will require certain flow rates through the radiators...