Converting from Oil to Gas heating - what type of system?

[Kaymo]

Hi,
About a year ago I moved in to this 1905 construction, stone house, its only a 3 large bedroom upstairs but multiple rooms downstairs and sums to 20 rads. The current heating system is an ancient firebird popular with a "British Oil Burners" burner installed next to the hot water storage tank in the garage. I am knowledgable enough about combustion systems that I have been able to keep this going over the last year but it is clearly both inneficeint and on its last legs and as we have mains gas on the street gas would seem to be the sensible option.

However, so far I have had a quote for oil and a quote with two options for gas,
Oil boiler (Greenstar 50kw) £7260
Baxi 40kw gas combi - £4800
Greenstar 40kw gas system - £6940
and for gas connection about £550

Maybe I am being unrealistic but these quotes seem very high and largely seem to be because of the labour charges because everyone seems to want to move me to an unvented system boiler. Looking at these prices its seems uneconomical to change, given our low use I could see it taking about 10 years to recoup the outlay even with a significant saving in fuel.

Can anyone help answer what would be wrong with just switching to a a vented gas boiler sticking with the overflow tank in the loft and the hot water tank that is already there based on the following?

- I dont care about power showers etc as I am quite happy with my 10kw electric shower
- I dont need to free up any space taken up in the loft by the water tanks or in the garage by the boiler/hot water tank
- we dont use a lot of hot water - dishwasher or kettle for dishes and electric shower for washing I even put a bypass on hot water tank so that we wouldnt waste energy heating tanks of hot water when we dont use it!

Is there any benefit to unvented/pressurised systems versus vented other than the space saving?

Is the £600 quoted for powerflush and £230 for filter worth it given that I dont currently have any cold spots - would a chemical flush in the system for say for a month before install be adequate?

Thanks in advance for any help/comments

 

[Boilerman2]

I would pay for a Powerflush of the system (to keep the new boilers warranty conditions satisfied) I would replace the Hot water Cyl with new, rejig the pipework to allow fully pumped operation as required by Blg regs Part L fit TRVs to all but the sitting room rads fit a RF Room Stat in the Sitting room

This would keep your system open vented, (no need to seal) and would retain hot water storage at atmospheric pressure (no change on what you have)

P.S. if you keep with oil don't go for the Greenstar they are utter carp
I would recommend a Grant Vortex 36/46Kw utillity boiler - nice kit
as for gas either of the boilers qutoted are ok - have a look a Vaillant or Remeha as well!

 

[ajrobb]

I keep going on about W-plan with weather compensation and separate temperature controls for DHW and CH. A large Y-plan system tends to inefficient as the CH flow temperature is typically stuck at 70°C, which is not optimal for condensing in mild weather. Work out the output from your existing radiators as well as calculating boiler size directly (if you are happy with your heating, there is little point buying a boiler too big for your radiators).

It might be worth asking for a total cost for supply, installation and 5 years' servicing/maintenance.

I quite like the idea of a Viessman Vitodens 200W boiler with Lambda mixture control. With claimed efficiencies up to 109% Nett (98% gross), it is very efficient. With a 50kW boiler, a combination boiler would give a good flow and you hardly need a HWC.

 

[Kaymo]

Boilerman2, thanks for the reply - One of the 1st things I did after moving in was to fit TRV's to all the rads, and the hot water tank was replaced a few years ago I believe (it has the modern moulded insulation on it), there is already a sitting room wall stat stat and timer controls so this all might not need done if I stay with vented. Thanks for the suggestions on boilers.

Thanks Ajrobb, not sure what you mean by W-plan versus Y-plan but I will look this up and educate myself! I like your comment about sizing the boiler based on existing radiators as this makes sense and I was a bit worried that I was being sold an oversized boiler. Is there a rough calculation that can be done based on number and surface area to calculate this?

 

[ajrobb]

Based on Stelrad radiator brochure:
type height W/m
K1 300 564
K1 400 733
K1 500 890
K1 600 1036
K1 700 1172
K2 300 1115
K2 400 1406
K2 500 1681
K2 600 1943
K2 700 2192
KP 400 1067
KP 500 1270
KP 600 1460
KP 700 1637
P1 300 483
P1 440 665
P1 590 856
P1 740 1042
P2 300 752
P2 440 1042
P2 590 1340
P2 740 1633

 

[ajrobb]

From Excel, reasonable output approximations in W/m (length) are:
P1 = 1.27h + 104
P2 = 2.00h + 157
K1 = 1.52h + 120
KP = 1.90h + 314
K2 = 2.69h + 322

Where h is radiator height/[mm]

 

[Kaymo]

Thanks Ajrobb, i'll give this a try.

 

[ajrobb]

With W-plan, the boiler is either on central heating (CH) or domestic hot water (DHW). With Y-plan there is also a mid-point for both CH and DHW at the same time. I think if the DHW stores more than 15 litres it needs to be heated to above 60°C to prevent the growth of legionella bacteria (maybe this can be done for just an hour a week). With W-plan, it is possible to have separate flow temperatures of CH and DHW. Modern timers allow you to heat DHW before the CH comes on. With weather compensation, the CH flow temperature is lower in mild weather and that improves condensing boiler efficiency. A common form of weather compensation uses an external temperature sensor on a north wall but there are other possibilities.

A problem with W-plan is that you usually have to choose CH or DHW priority. With DHW priority you won't run out of hot water but the house might cool off a bit when someone has a bath. With weather compensation, the CH heating is on most of the time but at a lower level and you could run out of DHW with CH priority. I asked if this was necessary and someone suggested a pair of HWC thermostats, one high-temperature one low down with CH priority and a low-temperature one high up on DHW priority with a solenoid switch to change priority. (Fitting Y-plan with a larger boiler means you don't have to make this choice but you might not get weather compensation and so use a bit more gas for heating.)

A typical hot water cylinder (HWC) has about 25mm of insulation. There are some with 40mm of insulation. If you don't mind losing a warm airing cupboard, consider fitting an extra jacket to the HWC and lagging all warm pipes.

 

[Kaymo]

Thanks for the info Ajrobb, as I only rarely use the hot water tank I dont think this will be too much of an issue for me. I was interested/concerned by your comment though about legionella as I often leave the tank cold for long periods of time so I found below on the internet. From this I think that my practice should be ok based on the fact that if there is any dormat bugs in there, heating it up to a temp above 60 should kill them quickly enough.


* 70 to 80 °C (158 to 176 °F): Disinfection range
* At 66 °C (151 °F): Legionellae die within 2 minutes
* At 60 °C (140 °F): Legionellae die within 32 minutes
* At 55 °C (131 °F): Legionellae die within 5 to 6 hours
* Above 50 °C (122 °F): They can survive but do not multiply
* 35 to 46 °C (95 to 115 °F): Ideal growth range
* 20 to 50 °C (68 to 122 °F): Legionellae growth range
* Below 20 °C (68 °F): Legionellae can survive but are dormant

 

[ajrobb]

I think heating HWC above 60°C is a building regulation. However, new bathrooms must have bath tap limited to 45°C with a thermostatic mixer valve.