Plumbers overspecifying boilers

[frediaz]

I talked to some plumbers about a new installation I need for a 4 bed 2 storey house with two bathrooms, one toilet and one kitchen/utility. The bathrooms have a bath each. One has a mixer shower and the other an electric one (above the baths so they cannot be used at the same time as the baths).

I said to them I was not keen on the traditional vented system. Two came back with a suggestion for a 120,000 BTU Combi boiler, probably because they do not believe it will work so "get the guy to pay for a top of the range boiler to play it safe"

Another suggested a boiler (combi again I think) with a Megaflo. From what I have read I do not need this or the expense that comes with it.

I am surprised none of them suggested a Thermal store with or without integral boiler.

Now I am almost convinced that what I need is the Thermal store, giving mains pressure in a vented system, with or without an integral condensing boiler. Does anyone agree/disagree?

I am looking for economy in the plumber's charge to install the boiler/thermal store but I do not know if there is a great saving in the installation of an integral unit, which is more expensive than the two items bought separately.

 

[JohnD]

have you measured the mains cold water delivery rate in litres per minute? (fill a bucket at the kitchen sink and time it)

 

[Corgigrouch]

Provided that you have enough mains water flow for the size boiler then a big combi would be your cheapest to install.... The large output is not for heating the house but to provide a decent flow of water and a bigger output boiler will give a bigger flow of water, all other things being equal... For heating, most decent combis will modulate their output down to suit the requirements of the house

Do remember though that if your cold water supply doesn't provide you with enough flow, then you will be disappointed with the performance of a combi, unvented or a heat store.

 

[pannierstan]

120000 is only 35Kw and for a 2 bathroom house you would STILL be lucky to run more than one tap!
Why do you presume that these people are feeding you a load of rubbish!!
Whoever suggested a 35 kw combi is probably right on the mark if you have at least 20 litres/min from your cold main
A thermal store or unvented are going to be more expensive and take up alot more room so where did you get the idea that thermal stores where such a good idea?

Stan

 

[frediaz]

120000 is only 35Kw and for a 2 bathroom house you would STILL be lucky to run more than one tap!
Stan

Haven't you just agreed about the poor performance of a Combi here?

 

[frediaz]

The first fix plumbing is not installed yet. I only know that the mains pressure is 3 bar and the supply pipe is 25mm MDPE.

Other than this, there is a contingent of people, headed by the NHBC (my inspectors) who insist that Combi boilers are only good for small properties. Hence my belief that they are overspecifying the boiler to get the maximum flow possible even if the high heat output is for a bigger property.

 

[giblets]

Hence my belief that they are overspecifying the boiler to get the maximum flow possible even if the high heat output is for a bigger property.

Read Corgigrouch's post above.

The space heating requirement on most combi's can be adjusted to suit the installed load. The maximum output is utilised to heat the hot water requirement to ensure the specified flow rate/temperature rise is met. By "specified" this means the boiler manufacturer's specification not the customer's requirements.

 

[Corgigrouch]

How have you determined that the boiler is over specified? If you need that kind of output to heat the water, then you need that size boiler... Decide what your priority is....Price or flow, there is always a compromise.

My view is for a one bath and an en suite used occasionally, then a large combi would do fine, two bathrooms in regular use look at some kind of tank

 

[doitall]

I sort of agree to both sides

Never in a million years would I be talked into fitting an combi in a 2 bath, utility room etc property.

Drivels favorite boiler the ethos 42 ish delivers a fantastic 25 L/min at 40 degs, wooppee, at 5.6 m3/hr your standard U 6 meter is no good so you have to have a gert big U16.

As for sludge buckets sorry thermal stores, Gledhill do a super model that catches fire for a past time, do make sure the insurance is upto date.

I would consider an 28kw combi serving the kitchen/utility and an unvented cylinder on a standard Y plan for the heating and bathrooms.

But who listens to me anyway

Drivel will be along later with his cylinder launch video and three pages of drivel.

 

[frediaz]

How have you determined that the boiler is over specified?

My belief that it is overspecified comes from the calculations for heat output required for the house (maximum figures):

UFH: 8.72 KW/h
Radiators first floor: 4.5 KW/h
Allowance for tap water: 3KW/h
Total 16.22 KW/h (approx 56,140 BTU)
So a 120,000 BTU boiler gives more than double the required heat output and what they are looking at is the higher flow rate they can get from a boiler that has a heat output suitable for a large house, unless I am misinterpreting the figures

 

[frediaz]

two bathrooms in regular use look at some kind of tank

Is a thermal store a tank? for this purpose

So, do thermal stores really collect a lot of rubbish?

 

[petit_pablo]

How have you determined that the boiler is over specified?

My belief that it is overspecified comes from the calculations for heat output required for the house (maximum figures):

UFH: 8.72 KW/h
Radiators first floor: 4.5 KW/h
Allowance for tap water: 3KW/h
Total 16.22 KW/h (approx 56,140 BTU)
So a 120,000 BTU boiler gives more than double the required heat output and what they are looking at is the higher flow rate they can get from a boiler that has a heat output suitable for a large house, unless I am misinterpreting the figures

Your calculations are for specifying a system boiler. A combi needs the high output to acheive the temp rise at max flow rate. The heating can be adjusted to whatever output the property needs

 

[Gasguru]

....the mains pressure is 3 bar and the supply pipe is 25mm MDPE.

Other than this, there is a contingent of people, headed by the NHBC (my inspectors) who insist that Combi boilers are only good for small properties.

The ONLY sensible way to go is an unvented cylinder (not Megalfo) perhaps installed in the roofspace. Flowrate will be excellent (far better than any sensible priced/common combi) and you have the most important feature...an immersion heater backup when the boiler fails. The system will be easy to maintain and almost certainly more reliable than a combi setup.

Forget thermal stores...there is no really point in overcomplicating the setup. There principle selling point is the fact that no licence is needed to install them however you won't necessarily even need a G3 licenced installer for the unvented cylinder...just get the approval through along with the rest of the building work. Annual maintainence on the cylinder is very simple.

A larger house deserves a better setup than a combi.

 

[Balenza]

frediaz wrote

I am surprised none of them suggested a Thermal store with or without integral boiler.

Thats because few know about them. Those that do harp on about them sludging up, scaling up or catching fire.

Heating a cylinder of water with a plate heat exchanger or delivering the hot water via a plate heat exchanger to the taps is alien to most domestic boiler engineers.
Remember they fix things for a living and when it gets to complicated they resort to parts changing. And if all this fails they look up diynot in the hope of finding an answer.
The bulk of their tools can be carried in small suit case.

I have to use a large van.

Have a look at the DPS Heat Bank website. The Pandora would be a good choice especially if you wanted to install it in the roof space.

 

[Doctor Drivel]

120000 is only 35Kw and for a 2 bathroom house you would STILL be lucky to run more than one tap!

35kW will run two taps for sure.