Hi. You may be running with your preheat on which means that the boiler is keeping it's self permantly warm. Some Boiler manufacturers have fitted this function due to the problem of the delay of delivering warm water. This function costs you around £75-£150 per annum in gas even when your out. The combisave delivers a similar performance but without the cost.
The low flow rate is 2.5litres per minute is for only around 0-15 seconds, with an average saving of 5-6 litres and around 15 seconds quicker in getting to temperature. The bucket test of a before and after will prove this when one is fitted.
Please wait for the reports from the plumbers/Heating engineers on this Furum.
Cheers
David Furlong
Thanks for your reply to my post, #26, I am not a plumber but I have a science degree, a masters degree, and a Maths A level.
I had read the study on your website and the results of your tests do not support buying this product, but I may have missed something and I am happy to be convinced.
FYI I have a Rehema Advanta plus in my house. We normally fit Worcester Bosch combis. I have lived in houses previously with Vaillant, WB and Baxi combis with no complaint about any of them.
A plumber of heating engineer is not the right person to answer the simple question of whether your product is worth it.
Your website should be able to do that.
Is your product worth it? I see it as a basic maths question and on your website you have a long and detailed Report by Knowles & Green dated 2010 which describes a study based in a test house.
I doubt if many heating engineers are going to read it the report:
1. unnecessarily complicated
2. Does not address the simple question
3. And my reading of it does NOT support buying your product.
The Knowles/Green test had a tap at the end of 7.3 mtr & sjower at the end of 15.5 mtr of 15mm copper pipe. Obviously the water in this pipe is not hot and could be anywhere between hot and room temperature.
There is about 2.5 ltr of cold water standing in the pipe to the shower and just over 1 ltr going to tap. The test was done at various flow rates, note at 6 ltr pm to tap the cold water would take 10 seconds to clear the tap before we get water from the boiler.
In my experience this standing water can be a big inconvenience to people but can be a bigger problem for stored hot water when that store of water is a long way from kitchen tap and only a small amount of hot water might be required for washing up but the tap has to run for a minute before hot water arrives.
Section 3.2 of the report says that in real life hot water is probably useful after a rise of 30 degrees and you are basing your claims for the Combisave on flow rates of just over 9 lpm at the tap and the shower, this seems reasonable to me.
In the Knowles Study (I wish I could cut and past it, but I can’t):
(3.3)
at temperatures below 35C combi save restricts to 2.5-2.8 lpm and combisave is ineffective.
And it is quicker to reach a 15C rise without comb save than with it.
Figure 3 seems to show that combisave is making very little difference at low flow rate of 3.7 ltr p m
Figure 4 flow rate 6 lpm 15C rise faster without combisave and at 6 lpm at combisave is not worth it even when you compare 30 degree rise.
When we get to Fig 6 with 9+ ltr pm the report claims Combisave is worth having. A 30 degree rise will be reached about 20 seconds fast with combisave at this flow rate but surely all the other data already given shows that the tap should not be turned on full until required temperature is reached.
Furthermore, quite a lot of the data in this study suggests it would be BETTER not to have combisave and just to turn the tap or shower on low flow rate of between 3 and 6 ltr per minute.
One further point: what is the flow rate of taps & shower in a house? I know typical low flow shower rate is about 9 ltr p m but it can be lower. I don’t know what flow rate is typical for taps but of course if the house has low flow taps throughout then combisave would be useless.
I believe WB & Vaillant dominate the combi market and I expect somewhere in their technical data they can tell us how long it will take any boiler to reach given water temperature rise at a specified flow rate. I think we are talking about seconds. So as I said this a solution for a problem which doesn’t exist. The problem is all the cold water sitting in 15 mtrs of pipe.