Hi, I'am deciding on replacing a very old baxi boiler-it uses too much gas. I only have one bathroom and I will also replace rads aswell as the old ones are begining to leak,I will need 11 radiators.For now I just want to know if the Vaillant eco-tec combi will be a good choice or if I should avoid it.And what model will I need,,the 19 kw one or the more powerful type.I'am pretty sure the system wll need to be power flushed as its over 26 years old. Thank you.
It isn't possible to determine the size of boiler required from the number of radiators. They can be small ones of a few hundred watts up to large panel ones of several kW.
Having said that, you can oversize a boiler quite happily within reason, because they automatically modulate down to the required output.
The risk comes in undersizing. Assuming that the house has been heated adequately in the past, you can make a reasonable estimate. Go round and measure each radiator and not whether it is single or double panel and with/without fins. Then look at data sheets on radiator manufacturer web sites for similar products. You can get a good approximation by size and use the heat output figures. Then add them up and allow perhaps 10% more.
The more important point becomes provisioning of hot water. If all of the system is of this age, then it would make sense to replace the HW cylinder as well and to use a fast recovery type. These are able to accept 25kW or more, thus providing a fast re-heat.
Alternatively, if you feel that you would rather dispense with the cylinder and go for a combi, it would make sense to go for a high power one - e.g. 38kW. However, checks into water supply flow and gas provisioning are then needed. A 19kW combi is going to be very disappointing for filling a bath in anything less than geological time in the winter. In general, pipe re-runs may well need to be done in order to get sensible flow of hot water to the right places and to reduce the effect of turning on a tap in one place affecting another. A combi *can* be OK if requirements are modest and an adequate one fitted, but if you are used to the flow and filling time from a cylinder, it will be difficult to match it. Either way, not something to be undertaken without careful checking.
A combi can be an economical replacement for a storage hot water system but there are downsides on hot water. One that has not been mentioned is that a boiler breakdown results in complete loss of hot water as well as heating - no immerser backup. If you are used to a well designed storage system I would put the slow hot water delivery problem differently. Yes baths are slow to fill but they do get there eventually. Just needs planning and the flow won't trouble the overflow. A much bigger problem IMHO is that the hot water takes a long time to arrive at a basin distant from the boiler. Since the boiler is probably diffrently located than the existing cylinder and probably futher away the difference can be very annoying. Watch out for existing 22mm pipework being reused as that makes that problem particularly noticeable.
Then you may need to do the sums on what is needed.
What is the output of the existing boiler? If the heating is currently adequate and there is no short cycling of the boiler (which would indicate it's over powered for the application) then all you need to do is to match or exceed it.
When we bought our house about 20 years ago the boiler was already there,they is a dial on the boiler going from 1 to 4 we use 3 and 4 this heats up our house to our liking but I have no idea of the boilers power. And about the boiler,shall I avoid it or shall I stick to it,my whole family uses Vaillant and are very pleased with it,so is this eco-tec combi good then?
- Look for a rating plate. The boiler is probably range rated though, meaning it can be adjusted for a range of outputs by tweaking the burner pressure.
- Run the boiler and time the meter - i.e. how much gas in 5 minutes with nothing else on and boiler running continuously. Emptying the HW cylinder and reheating it should achieve that.
If you look at a gas bill, you can work out what the relationship is between gas volume and kW.h
From that, you can calculate the power input of the boiler in kW. The output will be in the order of 65-70% of the input.
However, in the winter, the mains water is at 5 - 8 degrees. Shower water temperature is 40 degrees. Combis are rated in terms of flow rate for a 35 degree temperature rise. Therefore, the flow rate at the shower will be that of the combi.
A 30kW combi will manage about 12 litres/min under these circumstances. A worthwhile shower is at 15lpm and above, so a 38kW combi is really needed to achieve that.
All of this assumes that the mains supply can deliver this flow rate.
This puzzles me. I had a 30kW non-thermostatic multipoint. It was too powerful for a shower. Turning the flow rate up to get the water cool enough not to be painful resulted in rather painful high pressure jets, and water squirting out past all the O-ring seals. (It's been replaced by one with some thermostatic control since then, which is also 30kW and works brilliantly.)
The kw is 17.0kw this was on some plate in the boiler,now all I want to know is will the Vaillant eco-tec be a good choice please tell me if the vaillant should be avoided
If you look back through the archives of this group, you will find that Vaillant, as a manufacturer, is helod in good esteem.
Really you have to check and decide on the model.
If you go for the 824 (max 19kW output to CH), the DHW flow rate is 9 litres per minute. The 831 (max 24kW to CH) will do 12lpm The 837 (max 28kW to CH) will do 15lpm
All of them are modulating and even the largest will go down to 12kW on CH which is less than the 17kW you have now.
The decision is really about the DHW flow rate you want.
Just called the plumber yesterday and was surprised by what he told me. He said it's not worth paying extra for the Vaillant when you could get a Biasi or a cheaper one that does the same job,he told me that even the cheapest condensing combis are reliable! Is his advice right?
2.I was wondering if a glow-worm cxi will be good.
I reckon your plumber is probably right. My Worcester has a big Bosch label on the pump, but closer examination shows it's made by Grundfos like all the rest. The heat exchanger *might* be superior but all the other components look pretty generic to me. Cheap boilers probably work as well as cheap cars.
An in-line instant electric water heater can be fitted as backup. Enough for a tap or two and a shower until up gain. Combis are so cheap you can fit two of them. or zone up too which a cheap way to get high flows and back and zoning. Cheap to run: You ca fit two Atmos Intercombis (the most simple and reliable boilers on the market). One doing upstairs CH one down. One doing one bathroom, one the other, combine the outlets for baths to give a flowrate of 24 litres/min.
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