Boiler efficiency query

[Terry - Somerset]

I have no detailed knowledge but would expect that the size of boiler is specified to warm the whole house to a standard temperature (20-22C?) on the likely coldest days during the year.

The size/capacity required would have regard for the insulative properties of windows, walls, roof, floor and building orientation.

That a boiler becomes effectively oversized when ambient temperatures are above the minimum specified is inevitable. It would also be effectively oversized if radiators in some rooms were switched of - eg: bedrooms not in use.

I would guess the technically easiest, and cheapest, way to reduce boiler output to match demand is to cycle on and off.

 

[Bingy man]

@Bingy man The house has a modern self throttling boiler, whilst refurbishing the house last year I had all pipes replaced with a minimum 15mm off the main feeds. I have just about 100% UFH, and the house has been fully insulated. The boiler is slightly over sized, I’m comparing it with the size required for replacement air source heat pumps that would be required. I’ve had the system designed such that I can swap the boiler at any time to air source. What I’m finding, is that for most of the time, because the house is set at a constant temperature, which I understand is the best way of running UFH, heat is only required at one time for say 5~10% of the house. So the boiler is being asked to throttle back to 5~10% of its rating. The boiler set at 55C is still not having a flow return sufficiently low to achieve optimal condensing mode.

I have to be honest with you my experience with UFH is limited to say the least . I know for example that it runs at a lower temperature to radiators as is designed more as a background heat but it’s constant rather than the on / off effect with radiators therefore you don’t get the periodic temperature lags that you get with radiators and a room stat . The house I’ve been involved with for almost 2 years has UFH on the ground floor and I’ve had to turn it off as it’s just too hot to work in . My apologies but my previous advice was general Advice on System design and layout and not specific to your home

 

[Spectric]

The boiler has only one heat setting which is used for both the hot water and the heating side, so setting it too low affects hot water temperature / creates opportunity for legionnaires to breed.

This could be something that needs looking at because as you say the hot water needs to reach 60°C to prevent the bugs and is set on the hot water cylinder but your UFHS could run on a much lower boiler temperature. Have you tried lowering the boiler setpoint and seeing how low you can get it before the hotwater fails to reach 60°C ? It might need a longer time period but worth doing some checks.

That Worcester 40 CDi is I believe a discontinued boiler, but a 40Kw boiler seems on the large side for a house heated by underfloor heating.

One thing that has changed when using radiators is that to run on a lower boiler flow temperature the radiators used are sized larger so give out the same heat but with a lower temperature differential so the return flow temperature is lower to put the boiler into condensing mode. With underfloor heating you can use a flow temperature as low as just 45° C and the return is right for the boiler to condense but the hotwater is your problem. Ideally you need a way to control the boiler so it has two setpoints depending if it is heating or hotwater.

 

[John Brown]

Not only am I surprised that your boiler only has the one temperature setting for DHW and CH, but I was also under the impression that modern boilers were programmable to the extent of raising the DHW to a higher temperature once a week/month whatever in order to guard against legionella.

 

[Spectric]

Not only am I surprised that your boiler only has the one temperature setting for DHW and CH

That is because it is a system boiler, with a combi you get the two settings because the boiler provides instant hot water and flow for the heating, it is a self contained unit. A system boiler just provides a single heated output, everything else is external to the boiler such as the zone valves to control flow to different areas, an external pump and a hot water cylinder is needed but they are more efficient, more reliable and cheaper to fix.

 

[John Brown]

That is because it is a system boiler, with a combi you get the two settings because the boiler provides instant hot water and flow for the heating, it is a self contained unit. A system boiler just provides a single heated output, everything else is external to the boiler such as the zone valves to control flow to different areas, an external pump and a hot water cylinder is needed but they are more efficient, more reliable and cheaper to fix.

So how do you guard against legionella??

 

[Spectric]

So how do you guard against legionella??

Well looking into my past technical guides legionella is an airbourne pathogen and more common in aircon systems. But being airbourne then if your hot water cylinder is unvented (sealed / pressurised) then no air present so in theory no legionares concerns. This needs further looking into but maybe it allows unvented cylinders to run cooler without risk.

 

[Bingy man]

Hot water stored below 40 deg c is susceptible to legionaries as far as I recall ..

 

[Spectric]

Hot water stored below 40 deg c is susceptible to legionaries as far as I recall

But it needs to be exposed to air, I think this is why there are more issues with air conditioning systems because of the condensate and they move air through heat exchangers whereas an unvented cylinder has no air within.

 

[Pedronicus]

That is because it is a system boiler, with a combi you get the two settings because the boiler provides instant hot water and flow for the heating, it is a self contained unit. A system boiler just provides a single heated output, everything else is external to the boiler such as the zone valves to control flow to different areas, an external pump and a hot water cylinder is needed but they are more efficient, more reliable and cheaper to fix.

Not strictly correct. There are system boilers available that, when running on primary hot water demand, produce full output at a higher temperature but when running on heating can be adjusted to provide a lower flow temperature to satisfy under floor heating, etc. It would be a question of 'either/or' but not both at the same time. So far as I am aware the OP's boiler does not have this facility.

 

[Bingy man]

But it needs to be exposed to air, I think this is why there are more issues with air conditioning systems because of the condensate and they move air through heat exchangers whereas an unvented cylinder has no air within.

Yes and I’m going back a bit when unvented , thermal store was in its infancy and 90+!% was open vented and the stat was turned down to save money ..

 

[Spectric]

Yes and I’m going back a bit when unvented

Much simpler days, no combi's and well built cast iron boilers, yes just like our woodworking machinery they have gone from big and solid to lightweight sheet metal. There were header and expansion tanks in the loft and people had airing cupboards with a poorly insulated hot water cylinder that kept all the cloths warm. Boilers lasted for decades and we could afford the gas, but they call what we now have as progress !!

 

[Chris70]

It's my understanding that condescending gas boilers do not work most efficiently at maximum output. Are you old enough to have been an apprentice in the pre-condensing age?

I find this question quite condescending.

 

[XTiffy]

I think I just about follow this subject! We built a pair of cottages using ASHPs with one having both floors with UF heating and one with UFH down and special low heat rads for upstairs. Don't recall much difference overall. The system was programmed to fire up the immersion heater every now and again to heat HW to 60 degrees to combat legionela. We have just finished building a house (Daughter & SiL)using ASHP UF htg. downstairs and large rads upstairs and that too has a system of weekly boosts to around that figure. When I can nail the installer down I will interorgate on the overall system design. Our own house has a condensing gas boiler that has a fault (Overheat), which I have posted about recently. We are awaiting Gas Safe engineer. As the house is 100 years old it would be difficult to change to ASHP, without digging up the floor etc. to achieve the required level of insulation.

 

[Bingy man]

Much simpler days, no combi's and well built cast iron boilers, yes just like our woodworking machinery they have gone from big and solid to lightweight sheet metal. There were header and expansion tanks in the loft and people had airing cupboards with a poorly insulated hot water cylinder that kept all the cloths warm. Boilers lasted for decades and we could afford the gas, but they call what we now have as progress !!

What amazes me is the technological advances in boiler design but it’s pretty pointless when you consider the cost of the gas and electricity to run it . You have to pay £1200 + for a decent efficient boiler but the cost to run it is out of control. .so people turned back to solid fuel to off set the cost and to supplement the heating in the form of log burners but low and behold these get regulated to the point the cost to buy and install one is beyond reasonable. I did hear that they too were being banned but I could be mistaken. You will never win with uk government imo ..

 

[Spectric]

There are system boilers available that, when running on primary hot water demand, produce full output at a higher temperature but when running on heating can be adjusted to provide a lower flow temperature to satisfy under floor heating, etc.

Which brand and model as this would be a potential solution for @deema if he chooses to replace the existing boiler ?

 

[Pedronicus]

Which brand and model as this would be a potential solution for @deema if he chooses to replace the existing boiler ?

The ones that I am particularly familiar with is the Intergas HRE OV and HRE SB range having installed a fair number for clients and also one in our previous house.

 

[Spectric]

Must say I have never heard of them, will look into them though.

 

[deema]

I decided to get in an expert to have a look at the heating system, and he’s turned everything I thought I knew on the subject on its head. I now have the boiler running efficiently and not short cycling.

I had been turning the flow temperature down to 55C. However, the boiler is designed to be optimised for its condensing at a return temperature of 55C and not 1 degree over. So the flow needs to be c75C! Well, after adjusting the pump flow, checking all the UFH manifolds are set at the correct low temp. and setting the boiler to 72C to ensure we don’t go over 55C return flow temperature the boiler is now in fully condensing mode and not short cycling.
I’d misunderstood that to optimise the boiler efficiency I needed a low flow and even lower return temperature. What I’d clearly missed out considering is WHAT the boiler is designed to operate at. I bet I’m not the only one!!

 

[deema]

The easy test I was suggested I try to ensure the boiler is working in condensing mode is for the flue to be generating a nice white cloud immediately it comes out of the flue and for water to form on your hand if you stick it in front of it. If there is no flume or the cloud forms away from the flue it’s not condensing mode