What controls needed for running two boilers in parallel?

[Ed Sirett]

A typical condensate pump will have an 'alarm' float switch (The Sanicondens has a changecover switch) this can be used to stop the boiler and/or show a fault light or sounds an alarm.

[Ed Sirett]

I'd try a slightly bigger 'split' between the two units in this case.

[Ed Sirett]

If the current system use a sealed primary and you wanted to install 2 x20kW and the flue was straight forward I'd go for the Vaillant. What's more they might have the controls as a package. Check the price of the 20kW system boiler against the 24kW combi (19kW max for heating).

I think the Kestons would be 2x 28kW (do they do smaller?). If the flue is awkward this is the way anyway.

If you are making your own control system, or if the primary is open vented then the W-B is the choice.

Any one who does this job must (Building regs) add TRVs to the system. This job is slightly less work than doing do complete systems (but not by much).

[Ed Sirett]

Have you use one of the web calculators to get another estimate of the actuall heating requirement? If it came out loads less than 41kW it would make me do some more investigations.

What was the duty cycle of the 41kW previous unit?

[dennis]

ITYM must fit proper controls of which TRVs are the worst option you can get away with.

[dennis]

I think he would be better off spending half the cash on some sort of heat recovery and insulation and fitting one boiler. You can fit a filtered air unit with heat exchanger for the sort of money a second boiler and controls cost. That with CWI and loft insulation should reduce the heat loss by a lot.

[Doctor Drivel]

The cost of two cheap 24kW boilers can be cheaper than one 45kW boiler, or not much in it.

He should fit more insulation and then do recalc on heat loss.

[Doctor Drivel]

BTW, they have a 5 year guarantee.

[Ed Sirett]

That's not what I meant. TRVs are _part_ of the control system along with timers and thermostats and maybe other stuff.

[John Stumbles]

And to Pox^H^H^HBaxi-Potterton :-)

[Doctor Drivel]

Who slap it together poorly.

[Phil Addison]

Hi guys,

Thanks for all the advice. However, it now looks as if the problem of controlling 2 boilers is going away, because a re-assement of the required heat output is looking as if the existing boiler is well over-sized. A first estimate from the existing rad sizes indicates in region of 23kw is required. I don't know the exact output of the rads so think a single 24kw WB is marginal, and two of them totally too much, but there is now a 30 and 40 kw WB available. It seems likely a single WB 30CDi Conventional will be a good choice subject to doing a more accurate heat-loss calc, and its not particularly expensive, certainly less than 2 smaller ones.

Just for info, the biggest problem now is access to the roof to fit the flue, being 4 storeys up, and we are looking at putting a Velux in the loft to give access from inside.

Thanks again for the input, hope the discussion of 2-boiler controls proved interesting to others, it certainly was for me. It seems an area where acceptable control laws are not readily available or clearly defined. My Google search wasn't very productive, the best I found was the

site but even that only describes its controller in marketing-speak.

Phil