Thanks so as an engineer - Whats the correct way to size up a circulating pump? Is it volume of water when you drain down the heating sytem? Height of FandE tank or some other way?
Two pumps on a heat only boiler quotation
- Thread starter JackK
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Large bore one pipe... Can be a PITA!
Mass flow rate and other factors.
I am not an engineer. But I think most engineers just use experience and a bit of logic and guesswork. Do you run your current pump at Speed 3? Below is the only chart I can find for a pump with the same name as yours. If you look at the dark black lines, the top one is for Speed 3. Your current boiler needs a flow of at least 30 litres per minute (but probably substantially more). But taking that best scenario, at 30l/min the pump is producing just under 4m of head. If we say that 1.5m of head is used up passing the water through your boiler. That means the remaining 2.5m of head is successfully circulating the water through your radiators. Bearing in mind that in the quote a separate pump has been specified purely to send the water through the new boiler, I would say that a basic 15-60 (6m head) pump costing £150 is all that is needed for the radiators etc. But your installer is recommending a £700, 10m head pump to achieve the same thing. That is why I was a bit surprised at that choice. Perhaps the explanation is that your installer determined your current pump isn't producing sufficient flow?
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Thanks that is really helpful - I think the installer like many others are confused and think one pipe system means a PITA - The potters buy guy said that just earlier
The pipework I have is wide bore so 3 inch pipework so I thought thats why the pump was specced to be so powerful
When I ask about the way to double check the right size I get told "other factors" which dont help - So is it really a guessing exercise or is there some sceince to tell you if you need a 6m / 8/ 10m pump
EDIT - I think my pump runs on one speed only so there is no way to change it - At leat thats what the manual says https://www.dabpumps.com/sites/default/files/2016-03/VA_VB_VD_TS_ENG.pdf
On that particular link, it seems to show three speeds. Maybe your pump is an older version?
Grundfos have a pump sizing calculator on their website.
What you really need, though, is an engineer who can advise whether there is any reason why your large bore 1 pipe needs lots more pump power than a modern set up. I can only use the logic of what you currently have works! Do you know what temperature difference you are getting between flow and return at the boiler? That may give some basic information about your current flow rate.
Also, you mentioned earlier that you really wanted a smaller Intergas boiler. Have you made an assessment of what you actually need? A basic starting point would be square footage and insulation levels.
Very good points and am grateful - I have not checked flow and retun temps. I guess you need a thermometer for that yes?
I have an ideal mexico 2 BED 125 which warms the house nicely - Was actually unsure whether to get a 24Kw or 30Kw. The installer wanted to fit a 438 valiant which I think is oversized but what do i Know
Its a 4 bed detached 200 square metres 17 rad setup with no cavity walls if that helps
Do you mean that you have solid external walls? Are they insulated at all, either internally or externally? Do you have loft insulation; how much?
Hi i have solid walls no insulation at all
In the loft I have 100mm loft insulation - Thats all
That link to your boiler provided some very useful info, you have a 36.6kw (125k BTU/hr) boiler, with 3 stage "modulation" , 29.3kw,32.8kw & 36.6kw, the HEX, as we thought has a very low head loss, the Vaillant has a head loss of 4.05M at its rated flowrate of 27.2LPM at its rated output the Mexico's is only ~ 0.55M, this at a rad dT of 20C, the old circ pump appears to give a flowrate of ~ 30LPM at a 4M head so probably looks like ~ 3.5M available with the mexico and this pump to circulate through the rads, you would require a pump head of (4+3.5), 7.5M with the vaillant, so looks like a 8M pump required.
Wow - Thank you - This is the level of scientific knowledge that this forum beats hands down to installers who just guess - Really grateful indeed!
If I went for a 30KW or 24KW Intregas HRE OV as this forum seems to suggest for simplicitly and one pipe systems do I still need a 8M pump?! And I suppose a 30KW is required?!
I think thats my last question on this topic
If I went for a 30KW or 24KW Intregas HRE OV as this forum seems to suggest for simplicitly and one pipe systems do I still need a 8M pump?! And I suppose a 30KW is required?!
I think thats my last question on this topic
Definitely a 8M pump and probably a 30kw boiler IMO.
The Intergas 30 OV doesn't actually seem to give out 30KW. Only about 26.5KW.
Your house may have a very high heat loss with those type of walls. I would be tempted to carry out a more detailed assessment of what you might need.
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