Linking two Combi Boilers

[Agile]

Most combis have an opposite flow through the plate so the cold water inlet is at the cooler return end.

The typical temperature difference from primary to secondary of the plate is 15-20 C which will easily enable condensing to occur.

If the user does not wind up the boiler DHW temp they will be very efficient.

Unfortunately its a British trait to want the DHW at 55 C and add cold water at the point of use thereby preventing the boiler from operating most efficiently.

Tony

 

[sturharv]

Is there any danger that if I link the HW pipes from both boilers together and only one basin tap is turned on, the HW flow won't be activated.....?

Although I would prefer to common the HW together, however would it be better to split the HW?

 

[Doctor Drivel]

Is there any danger that if I link the HW pipes from both boilers together and only one basin tap is turned on, the HW flow won't be activated.....?

It depends on the combis. If they activate on very low flows (don't get pressure operated) then all is fine.

Although I would prefer to common the HW together, however would it be better to split the HW?

Splitting divides and rules. You could fit a secondary DHW circulation loop on the combis. I am nor sure if I explained how they are fitted further up the thread. I would insulate the DHW draw-off pipes to maintain heat. If there is no DHW at the taps because the dead-leg lag is too long, most people will only be washing theirs hands in cold water.

 

[sturharv]

We are proposing to use 2 no. Vaillant ecoTEC 37kW condensing combis, are they pressure operated?

The distance between the majority of the HW outlets and the plant room is about 7-8 metres, they have had no problems with the dead leg lengths on the current system so I think it will be ok without the secondary HW loop.

 

[ChrisR]

Would have though by now that somebody would have asked what the mains pressure/flow/supply sizes are!
If it's 1.2 bar....

 

[sturharv]

The mains pressure is sufficient, the existing system was run off combis.

 

[sturharv]

I have seen two 24 kW system boilers (what combis are) coupled up by having a 28mm pipe for the flow and both went into this. The system dictated a 28mm pipe to carry the 48 kW. The same for the return. Each system boiler had an auto by pass to protect each boiler and a non-return valve on the flow of each. So, each boiler pumped into the flow pipe. Simple, cheap, effective, fast to fit and worked. They were both modulating boilers, so wound down reducing cycling when the place was up to temperature. No boiler sequencing controls were used. In Spring and Autumn only one boiler was on, winter the two

Doctor Drivel, so on the 2no. 24kW system boiler set-up you described, there was no additional pump?, the integral pumps in the system boilers (same for combis) were used for the full system? May increase to 35mm. for combined potential output of 74kW.

 

[Doctor Drivel]

I have seen two 24 kW system boilers (what combis are) coupled up by having a 28mm pipe for the flow and both went into this. The system dictated a 28mm pipe to carry the 48 kW. The same for the return. Each system boiler had an auto by pass to protect each boiler and a non-return valve on the flow of each. So, each boiler pumped into the flow pipe. Simple, cheap, effective, fast to fit and worked. They were both modulating boilers, so wound down reducing cycling when the place was up to temperature. No boiler sequencing controls were used. In Spring and Autumn only one boiler was on, winter the two.

Doctor Drivel, so on the 2no. 24kW system boiler set-up you described, there was no additional pump?, the integral pumps in the system boilers (same for combis) were used for the full system? May increase to 35mm. for combined potential output of 74kW.

A U16 gas meter is needed for 74kW.

No additional pumps. If the DHW is split not combined. When one is on DHW the flow will reduce in the CH on the other as only one pump is operating. However this will only be for a short time serving wash basins. If combined then both combis will be switched over to DHW and no CH flow. Then again only for a short time.

The beauty of splitting the DHW and teeing both into a 35mm flow pipe is that standard off-the-shelf components are being used. The only complexity is inside the combi cases.

Two domestic pumps on the combis should give the circulation you need. Check the suitabliity of the two pressure vessels too. There again they should do it, but check.

 

[sturharv]

Doctor Drivel,
Yes, a U16 gas meter is fitted.
So I’m proposing to tee into a 35mm. common flow and return pipe (sufficient for 74kW ? although I doubt the boilers will ever run at FULL capacity)… not 42mm.?

Just on the subject of by-pass valves. I am aware the Vaillant ecoTEC boilers have integral by-pass valves fitted, with this in mind would you still suggest putting an additional bypass on the common flow and return??

 

[gas4you]

Vaillant do a special kit that mounts and connects the Ecotec Plus boilers together, so all you basically have to do is connect the ends

 

[sturharv]

Thanks gas4you,

do you know where I would find some info on this?

is it a generic item i.e. suitable for the 24kw, 30kw and 37kw models?

 

[Doctor Drivel]

Doctor Drivel,
Yes, a U16 gas meter is fitted.
So I’m proposing to tee into a 35mm. common flow and return pipe (sufficient for 74kW ? although I doubt the boilers will ever run at FULL capacity)… not 42mm.?

35mm is fine for 74kW, more than enough. With a condensing boiler with a wider delta T across flow and return more heat is delivered, so reducing pipe sizes.

Just on the subject of by-pass valves. I am aware the Vaillant ecoTEC boilers have integral by-pass valves fitted, with this in mind would you still suggest putting an additional bypass on the common flow and return??

No. The integral valves are fine.

Does the system have TRVs on all rads?

 

[sturharv]

There are some rads without TRV's

On the existing system which is 2 no. Baxi Bahama combis, there is a bypass valve between the flow and return, however Im not sure if they have integral valves… some manufacturers say that you should fit an additional bypass??

 

[gas4you]

My rep gave me the info a while back now when I needed it for a quote.

As usual I cannot find it when I need it. I would suggest you contact your rep or Vaillant sales at Rochester.

 

[sturharv]

Dave,

Did the kit work well?

Im having no joy finding anyhting about it?