1 CH/HW system, 2 boilers - possible?

[Andy Kirkland]

Here's the scenario:

At the moment by CH and HW is heated by a gas powered Rayburn (with a thermostatically controlled boiler). It runs on LPG which isn't cheap and isn't helped by having a rather inefficient Rayburn on the other end of the pipe from the tank.

So...

I was wondering of the possiblities of linking in a separate (modern, efficient) gas boiler to provide and alternative heat source whilst still keeping the Rayburn in the system.

I'm aware of the problems of people's attempts to link in solid fuel devices (stoves, Rayburns, Agas etc.) into existing CH systems and the risks and pitfalls (boiling the back boiler, problems in event of power/pump failure etc. etc.). I'm also aware of the Dunsley heat exchanger device thingy.

I am trying to find reasons why a system with dual boilers would not work and also the possible setup (boilers in parallel? boilers in series?). Since both heat sources are controllable thermostatically I cannot see any major issues as per using solid fuel devices.

Anyone seen/installed/experienced a 2 boiler setup?

Thanks in advance

Andy

[Onetap]

Yes, just about every commercial installation and many large domestic ones. 10 or 12 modular boilers (large domestic types) is common, 15 years ago it would have been 2 or 3 commercial boilers handling the same load.

You'd need some custom controller if boiler selection is to be automatic but fairly simple switching with thermostats should be adequate, e.g., the new boiler is the lead boiler, the Rayburn is held off by a pipe thermostat so only operates if the lead boiler fails.

Have a Google for primary/secondary pumping. The heating circuit would have its own pump and the boilers would be on secondary circuits which would only draw water from the primary when the boiler & secondary pump runs.

You need to ensure that hot water isn't circulated through a non- firing boiler unless it has a fanned flue, or else it will be acting as a heat dump. You also need to consider some device to keep an elevted return temperature to a non-condensing boiler.

.

[Doctor Drivel]

Don't bother with Dunsleys, a waste of time. Use a vented thermal store/heat bank to couple the two boilers up directly to the cylinder. The DHW and CH comes from the store. No headers or primary/secondary pumps, etc. Simple. Both can operate independently. Just turn one off if not needed.

Look at

They have good diagrams on how to connect.

[The Natural Philosopher]

Don't bother with drivel, he's a waste of time. I forgot new installation=new killfile.

Out you go drivel!

[Doctor Drivel]

Put me on your killfile. You are a complete snotty uni plantpot.

[Onetap]

Beware of Drivel, he is the resident moron.

For an example of his incompetent ramblings see above.

The point of the Dunsley neutraliser is that it provides hydraulic disconnection between the primary and secondary pumped systems. All flow and return pipes connect to the Dunsley device and so there is no pressure difference between the flow and return pipes causing nuisance flows. Boiler A's pump running does not cause flow through Boiler B or through the heating system since they are hydraulically disconnected.

The heat store does the same job, but with some thermal storage capacity; connecting boilers to it will require primary (usually heat sources) and secondary pumps (usually heat distribution devices).

I wouldn't use the Dunsley device because I can get better for less.

[Andy Kirkland]

Thanks everyone for their input.....

COuld you explain further on "better for less" vs the Dunsley? The Dunsley does seem pricey for a block of steel (albeit researched and engineered)....

Alternatively to the above, can anyone point me in the direction of a good heating engineer who can, paid, give me some good design pointers and/or installation advice and/or actual installation - I am after someone with specific multi-source experience. I live in North Derbyshire (30 mins from Manchester and Sheffield).

Cheers

Andy

[ARWadsworth]

You forgot to tag him. Is your nurse on holiday?

Adam

[Jonah]

"Doctor Drivel" wrote

Good idea. Thanks.

[Onetap]

The Dunsley things are about 6" x 18" diameter. If you need some thermal storage, most cylinder manufacturerers would braze additional tappings onto a direct cylinder. They're usually required for solid fuel (open vented systems); you have to ensure that it is rated for the system pressure and a suitable presure relief valve is fitted. Buffer vessels were used on heating and chilled water systems long before Dunsley marketed their product.

If you do not need thermal storage, you can get a low loss header fabricated from steel or copper pipework. This will provide hydraulic disconnection of the various primary secondary circuits.

[Doctor Drivel]

The Dunsley is a waste of expensive time offering little at all. Thermal store/heat banks offer CH buffering and instant mains pressure DHW, and simplicity which an expensive header will not.

[Doctor Drivel]

His nurse left him.

[Doctor Drivel]

pleasure.

[Doctor Drivel]

You are an incompetent idiotic fool. The fool goes on...

As does a thermal store. But better. He continues to drool drivel...

As does a thermal store.

As does a thermal store.

My God!! He has got it!!!!!! Has he been converted on the road to Damascus?

A great advantage. The secondary pump (heat distribution devices as you call them) can be a Smart pump on the CH circuit. Have TRV's on all rads and then no nusisance central walls stats.

A gravity fed boiler can heat thermal store.

Like a thermal store/heat bank. Which are cheap and offer so much more.

[Onetap]

Yes Drivel, but my I just point out that in your first post you said that vented thermal store/heat bank had " No headers or primary/ secondary pumps,"

It has both primary and secondary pumps, like the Dunsley job. My point was that your post was bollox, as usual.

A low loss header is not expensive if you make them.

[Doctor Drivel]

There is no expensive headers and no carefully designing primary and secondary pump circuits. No complex, expensive, boiler sequencing controls, as two boiler anti-cycle stats on the cylinder will just bring in both boilers and heat up ASAP. No constant click-click of the boiler when cycling when on headers.

Even in summer it doesn't matter, just a rapid re-heat if two boilers switched on. In summer one boiler can be switched off by the user. Two cheap simple boilers can be used. They don't need to modulate or have complex boiler controls. All simple. All easy. Want waether compensation? Well use a Broag "dual temperature" boiler with integral weather compensation and an external 3-way diverter valve controlled from the boiler. When the top DHW section calls for heat the boiler runs up to maximum temperature and reheats ASAP. Then reverts to CH and weather compensation control and heat the bottom CH section to the outside weather dictates. All simple and easy and cheap.

Headers are a direct loop back to a boiler's return reducing condensing efficiency. They can be complex and expensive and have to be sized right. They do not provide CH buffering as a thermal store does. Thermal stores are easy, just two tappings for each boiler.

A thermal stores/heat banks are easy. A flow and return tapping for each boiler onto the cylinder. The same cylinder stat(s) switches in both boilers. Simple. A gravity wood burner can be hitched up the same way. Simple and easy. Even you could understand it - I hope.