Dual boiler setup - pro & cons

[Smacky]

Hello,
I have recently bought a house and found that there are two boilers working in tandem for CH and hot water, ie. both boilers fire up when heating or hot water is scheduled.

What are the advantages of a dual system setup?

Would switching off one of the boilers reduce my gas bill?

Thanks
Smacky

 

[SJB060685]

How big is your house? Any idea of house heat loss? What outputs are these boilers? How many hot water cylinders?

It’s not uncommon to have two boilers. Usually both are sized correctly for the demand and with sequencing control the boilers will switch on and off at set times in sequence. An advantage of this you would have a back up boiler should one breakdown.

It’s also possible that the property was extended over the years and an additional boiler was installed to cope with demand.

Any chance of a picture of the two boilers and associated pipe work?

 

[Smacky]

The house is 6 bedrooms with about 22 rads over 3 floors. The two boilers are Worcester Greenstar 30CDi.

It seems both boilers fire simultaneously. If this is the case what is the benefit? Would be hot water tank heat quicker? What impact would it have on central heating?

I am asking as I wondering if a dual boiler setup is wasting gas. I seem to use huge amounts of gas. Approx 70,000 kWh.

 

[SJB060685]

70,000KWH annually is on average 190KWH a day. That is a massive amount of fuel consumption throughout the year when you consider in the warmer months you barely require the heating on.
How old is your house? Level of insulation like cavity, loft, double glazing etc? Any idea of the actual heat loss at design temperature?

The cylinder heat exchanger or coil is what governs the hot water heat up times. How many cylinders are in the property and what volume?

Without knowing a bit more information it’s hard to say but I suspect your property doesn’t need half the boiler output you have.

 

[Smacky]

Yes, 70,000 KwH is an enormous amount. For a while I thought I was subsidising my neighbours
The house is a 1996 construction, well insulated, and doesn't have noticeable draughty areas. All windows are double glazed and less than 6 years old. New sliding doors were installed this year and have very good u values.

There is only one hot water cylinder. Approx. 300l. and similar to my old house (4 bed semi) where a singled 37kW boiler did the trick nicely.

 

[SJB060685]

I would get a reputable engineer/firm in to do a thorough heat loss calculation. Once you have an accurate idea of the load then you can start talking about improving/updating the system. Installing the right size boiler isn’t the only thing that needs questioning. You need to take several things into consideration when deciding how you’re then going to control the system and when to heat if you want a really good efficient system.

 

[ShaunCorbs]

Loft insulation 400mm ?

 

[SJB060685]

Loft insulation 400mm ?

If we knew the floor area of the building we could get an estimated idea of the heat loss based on the description above. I’m willing to bet that property doesn’t even need half the current output.

 

[Smacky]

Floor area ~ 4000 sqft
I don't know the depth of loft insulations but it's the standards loft insulation bags.

I've had a heating engineer look at the system and he believes the wiring/setup is wrong. Given I have two system boilers he wants to split the system and add a commercial pump (grandfoss magna1). Perhaps a boiler to the hot water and other for heating. It will cost a bit to get this done. I am trying to work out the longer term benefits.

In terms of heat loss, what would be the average loss for my size floor plate? Ie. 1 degree in 4 hrs with outside temp of 6c?

 

[SJB060685]

4000sqft is roughly 371m2. Using a quick guide I get a heat loss of around 18-19kw. I believe this quick guide is an outside temperature of -2°c and a target room temperature of 21°c.
If your property is anywhere close to these descriptions then you don’t even need one of those boilers at those outputs.