Upgrading regular boiler to a combi - initial advice

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Note that if we all get forced down the heat pump route a hot water cylinder will be required, there is no (efficient) heat pump equivalent of a combi boiler. By all means fit a combi now, but don’t re-use the old cylinder space in a way that prevents fitting another one in the future.

Bendy_House

victor2 said:

We had our very old (last century!) boiler and hot water tank replaced by a combi a year ago. Floor standing boiler in the utility room was taken out and the new combi went on the attached garage wall, immediately behind the utility room. It wasn't an outside wall, but close enough to one for the new flue to run horizontally out to the wall. Existing water, gas and drainage were all nearby. We did add a radiator to the utility room as we used it to dry clothes in, and it was always warm thanks to the old inefficient boiler!

We had (still have) an electric shower and the hot water tank was basically an expensive airing cupboard. Plans are now to remove that cupboard and have a dedicated shower running off the combi in its place.

Nice to get rid of the cold water tank(s) in the loft plus the central heating header tank. Existing radiators were flushed and no problem found, so no changes apart from the additional radiator. Combi was initially set to heating temperature of 70, which I've dropped to 60 now and the house is getting heated just fine, but it's been pretty mild so far. Rolling year annual gas usage has dropped by over 30%, mainly due to not heating a hot water tank. We were more comfortable last winter with the new system than before, as it has a programmable thermostat, so we have better control.

The boiler has a 10 year manufacturer's warranty, so long as it is serviced annually. I don't suppose it will last as long as the old one, but the warranty makes us think we'll get at least 10 years out of it.

Only irritation is it takes forever to get hot water through the kitchen tap, which I assume is because the water flows through the old pipe system from the boiler in the garage upstairs to where the hot water tank was and back down again to the kitchen.

That's a very useful real-life example.

If you can keep running the boiler at no more than 60oC for as along as possible, then that will be more efficient. However, if your radiators are the same 'old' ones (ie, sized for a higher water operating temp), and if you haven't upgraded your house insulation, then you can expect to have to raise the boiler output temp to a less-efficient ~70oC (or even more) during properly cold spells.

If your boiler has the provision for adding a 'weather compensator' - an outside temp sensor - then that 'should' do this output temp controlling for you.

The delay in getting hot water to your tap is likely to be down to one of two reasons - the first being that the pipe run is as you suspect, as it has to come from the much further away boiler rather than the old hot cylinder, and the possible reason other is that the combi hasn't been set to 'pre-heat' the water, so every time you ask for hot water it has to fire up from cold. The first would need slight replumbing (is the kitchen next to the utility room? And does the ute get hot water quickly?), and the latter for the P-H to be turned on. Depends on the cause.

I always had the pre-heat on - basically because I didn't know how to turn it off... - but did disable it a few weeks back as the boiler fired up seemingly every few minutes. Each time it does is not only a waste of gas, but wear on all the parts. Yes, hot water delivery now takes very noticeably longer - and the kitchen is next to the boiler - but it's hardly an issue as it's only needed for a daily dish-wash, and showers. I rarely expect a hot water delivery to the bathrooms for hand-washing, as liquid soap cleans just fine with cold.

BUFF

Bendy_House said:

victor2 said:

We had our very old (last century!) boiler and hot water tank replaced by a combi a year ago. Floor standing boiler in the utility room was taken out and the new combi went on the attached garage wall, immediately behind the utility room. It wasn't an outside wall, but close enough to one for the new flue to run horizontally out to the wall. Existing water, gas and drainage were all nearby. We did add a radiator to the utility room as we used it to dry clothes in, and it was always warm thanks to the old inefficient boiler!

We had (still have) an electric shower and the hot water tank was basically an expensive airing cupboard. Plans are now to remove that cupboard and have a dedicated shower running off the combi in its place.

Nice to get rid of the cold water tank(s) in the loft plus the central heating header tank. Existing radiators were flushed and no problem found, so no changes apart from the additional radiator. Combi was initially set to heating temperature of 70, which I've dropped to 60 now and the house is getting heated just fine, but it's been pretty mild so far. Rolling year annual gas usage has dropped by over 30%, mainly due to not heating a hot water tank. We were more comfortable last winter with the new system than before, as it has a programmable thermostat, so we have better control.

The boiler has a 10 year manufacturer's warranty, so long as it is serviced annually. I don't suppose it will last as long as the old one, but the warranty makes us think we'll get at least 10 years out of it.

Only irritation is it takes forever to get hot water through the kitchen tap, which I assume is because the water flows through the old pipe system from the boiler in the garage upstairs to where the hot water tank was and back down again to the kitchen.

That's a very useful real-life example.

If you can keep running the boiler at no more than 60oC for as along as possible, then that will be more efficient. However, if your radiators are the same 'old' ones (ie, sized for a higher water operating temp), and if you haven't upgraded your house insulation, then you can expect to have to raise the boiler output temp to a less-efficient ~70oC (or even more) during properly cold spells.

If your boiler has the provision for adding a 'weather compensator' - an outside temp sensor - then that 'should' do this output temp controlling for you.

The delay in getting hot water to your tap is likely to be down to one of two reasons - the first being that the pipe run is as you suspect, as it has to come from the much further away boiler rather than the old hot cylinder, and the possible reason other is that the combi hasn't been set to 'pre-heat' the water, so every time you ask for hot water it has to fire up from cold. The first would need slight replumbing (is the kitchen next to the utility room? And does the ute get hot water quickly?), and the latter for the P-H to be turned on. Depends on the cause.

I always had the pre-heat on - basically because I didn't know how to turn it off... - but did disable it a few weeks back as the boiler fired up seemingly every few minutes. Each time it does is not only a waste of gas, but wear on all the parts. Yes, hot water delivery now takes very noticeably longer - and the kitchen is next to the boiler - but it's hardly an issue as it's only needed for a daily dish-wash, and showers. I rarely expect a hot water delivery to the bathrooms for hand-washing, as liquid soap cleans just fine with cold.

Or you run the heating system for longer.
Weather compensation can also be done by controllers that take weather information from the internet (probably not as specific for your own building as having an actual physical sensor).

When I had my regular boiler/dhw tank replaced by a combi (WB Highflow in my case) I had the system repiped which shortened the pipe runs from boiler to outlets. I have kept the former tank space free though.
Worcester Bosch (& possibly others) now have at least 1 boiler with an interesting trick for the pre-heat off/wait for hot situation. "Quick Tap"
https://www.worcester-bosch.co.uk/products/boilers/directory/greenstar-4000

Bendy_House

BUFF said:

Or you run the heating system for longer.

If the water temp isn't high enough to provide adequate heat output from the normal-sized rads to warm the house, it doesn't really matter how long its run for :-(

BUFF

Only in the highly unlikely case that heat loss from the property is higher than the heat output of the boiler/radiators - it is unlikely that the installer got it that far wrong. Any net gain in heat will increase the internal temp it's just a matter of how long it takes to reach what you consider a comfortable temp.
In my case ( in a 1905 terrace with solid, uninsulated walls & single glazing to the front)  I typically run my flow temp (so return much lower) at 55C but it does run noticeably longer than if I was using e.g. a 70C  flow. Once we start getting down to 0C I will probably move the flow up to 60C. If we get down to -10C it is more likely that I will switch the boiler to constant from timed rather than increase the flow temp any further.
This works for me but if you are a person who wants to lift the house temp by several C in 30 minutes it won't.

macman

Adding CH to a property certainly adds value and probably will more than recoup the outlay at sale time. But changing a conventional boiler to a combi adds absolutely nothing, unless the existing boiler is unserviceable.
Half the people likely to view the property probably don't even know the difference between a combi and a conventional boiler, and most would see a hot water tank and airing cupboard as a bonus.
The only downside to a 1990's boiler is that it will be pre-condensing, so less economic to run.

littlemoney

elsmandino said:

3. Where do people tend to put combis, when they do a straight swap - i.e. is it less work to put the combi where the current boiler is or where the water tank is, or does it literally make no real difference?

Ebe_Scrooge said:

3.  It makes little difference really.  It needs to be on an outside wall for the flue to vent, but other than that it can go anywhere.  Probably makes sense to stick it where the old one is as the gas and water pipes will already be there.  But it shouldn't be a huge job to put it somewhere else if that would be more convenient.  Obviously there would be more work involved as the installer would have to move the gas pipe and cut a new hole in the wall for the flue, and block up the old one - but in principle it can go anywhere.

Been reading this thread with interest as I am considering replacing my boiler and have similar thoughts. Howeer  my floor standing boiler was installed in 1983 replacing a Rayburn oven  which also provided hot water. The chimney used by the rayburn is now used by my gas boiler. Can a new boiler vent up a chimney or does it have to go on an outside wall?

BUFF

littlemoney said:

Been reading this thread with interest as I am considering replacing my boiler and have similar thoughts. Howeer  my floor standing boiler was installed in 1983 replacing a Rayburn oven  which also provided hot water. The chimney used by the rayburn is now used by my gas boiler. Can a new boiler vent up a chimney or does it have to go on an outside wall?

Depends upon the boiler. Most will have vertical flue options but some may have a length limitation.

Is it a vertical chimney or does it have a kink (some but not all boilers have flexible flue options)? Also flues are meant to be accessible for inspection & servicing.

littlemoney

BUFF said:

littlemoney said:

Been reading this thread with interest as I am considering replacing my boiler and have similar thoughts. Howeer  my floor standing boiler was installed in 1983 replacing a Rayburn oven  which also provided hot water. The chimney used by the rayburn is now used by my gas boiler. Can a new boiler vent up a chimney or does it have to go on an outside wall?

Depends upon the boiler. Most will have vertical flue options but some may have a length limitation.

Is it a vertical chimney or does it have a kink (some but not all boilers have flexible flue options)? Also flues are meant to be accessible for inspection & servicing.

The chimney is straight but the pipe from the boiler to the chimney has a two bends in it.

amateur_house

littlemoney said:

elsmandino said:

3. Where do people tend to put combis, when they do a straight swap - i.e. is it less work to put the combi where the current boiler is or where the water tank is, or does it literally make no real difference?

Ebe_Scrooge said:

3.  It makes little difference really.  It needs to be on an outside wall for the flue to vent, but other than that it can go anywhere.  Probably makes sense to stick it where the old one is as the gas and water pipes will already be there.  But it shouldn't be a huge job to put it somewhere else if that would be more convenient.  Obviously there would be more work involved as the installer would have to move the gas pipe and cut a new hole in the wall for the flue, and block up the old one - but in principle it can go anywhere.

Been reading this thread with interest as I am considering replacing my boiler and have similar thoughts. Howeer  my floor standing boiler was installed in 1983 replacing a Rayburn oven  which also provided hot water. The chimney used by the rayburn is now used by my gas boiler. Can a new boiler vent up a chimney or does it have to go on an outside wall?

@littlemoney - is your current boiler a back boiler? My boiler hasn't worked since May last year, and I've been thinking of replacing it. Mine is a back boiler and behind a gas fire. I've not been in a rush to get it repaired or replaced as it's expensive to run, so I've been using electric heaters and just heating the room I'm in rather than the whole house. Paying the gas standing charge and pilot light is a waste of money for no heating, so I'm not sure what to do - either get the boiler repaired or replace it.