Radian Posted August 26, 2022 Share Posted August 26, 2022 I have a quote for a gas safe engineer to supply and fit a 24kW combi to my detached garage (to run a 100m UFH loop and single radiator that I've installed). Worryingly, he warned me that he couldn't guarantee sign-off due to the sharing of the gas supply with the 30kW system boiler already in the house. Back in 1998 when we built the house, we had a mdpe gas pipe run from the house to the garage but I don't recall the size of the pipe other than it was the same as the supply pipe coming up to the house. Here's the end where it comes out of the ground and T's into the supply after the meter (22mm copper T): The black sleeve hides the plastic pipe so I can't easily measure it but the outside diameter of both sleeves is 40mm. At the garage end it comes up outside a wall to the workshop next to the water supply pipe: The length of the gas pipe from house to garage is around 17m and runs parallel to the supply pipe until it enters the meter box : Unlike electric where you can be pretty sure you're going to be able to get 100A/240V/24kW out of the incoming supply, I'm not so familiar with gas. I guess the pressure will go down when the house boiler is on and gas hob is lit (the only two house loads) but what about the neighbours and the utility supply? I may be asking daft questions but this is uncharted territory for me. I was hoping the engineer might be a bit more forthcoming about the issue but he was very non-committal. I asked if there was any objective test we could perform before going ahead but he seemed to think we had to install it first and then find out. Pretty crazy if that's the best we can do. Link to comment Share on other sites More sharing options...
Temp Posted August 26, 2022 Share Posted August 26, 2022 Not my field but found this calculator.. https://www.gb-gas.co.uk/clc/pipesize/GAS PIPE SIZING 2016 WEB.htm 1 Link to comment Share on other sites More sharing options...
Marvin Posted August 26, 2022 Share Posted August 26, 2022 (edited) The regulator on top of your gas meter reduces the pressure of the supply to your house. If I remember correctly the regulator reduces the pressure of the gas coming into your home down to about 25 mbar from about 75mbar. It's very unlikely that this is anything to do with the problem. More likely is the resistance to the flow of the gas due to the length and internal diameter of the pipe to the garage. You could easily find out the pressure of the gas but not the flow rate. You could use @Temps suggested calculator, but sadly the only way to be sure is to run an appliance on the other end. Can't really open a gas pipe and measure the flow with nothing on the end and expect to live very long.... Edited August 26, 2022 by Marvin 1 Link to comment Share on other sites More sharing options...
Radian Posted August 27, 2022 Author Share Posted August 27, 2022 8 hours ago, Temp said: Not my field but found this calculator.. Thanks. That calculator seems like it could be useful but I don't get how it comes up with a flow rate at all. Does it assume a standard 25mbar after the regulator as @Marvin describes and then do some guesswork? What I didn't quite get from the engineer is what could be done to work round the issue. I got the impression that adding a separate metered connection to the supply line might solve it. Not that I would want to dig up the patio I just laid to tap into the supply! Link to comment Share on other sites More sharing options...
ProDave Posted August 27, 2022 Share Posted August 27, 2022 I would start at square one, and ask the installer just why he thinks it will need a 24kW boiler to feed one UFH loop and one radiator. 2 1 Link to comment Share on other sites More sharing options...
Mr Punter Posted August 27, 2022 Share Posted August 27, 2022 I agree with @ProDave. A 12kW boiler should be able to do that easily. Link to comment Share on other sites More sharing options...
Radian Posted August 27, 2022 Author Share Posted August 27, 2022 52 minutes ago, ProDave said: I would start at square one, and ask the installer just why he thinks it will need a 24kW boiler to feed one UFH loop and one radiator. Very good point. He started off at 30kW and I beat him down! Strange how they always seem to overspec these appliances. Even at 24kW he thought we might regret it. My heat loss calcs suggest around 2kW at inside/outside delta 10oC for space heating and HW is only required to supply a single basin. I think the problem is that it appears to be only system boilers that go down to 12kW. I couldn't google up any combis. The extremely infrequent demand for HW means a system boiler isn't really the right choice. The right choice would be an ASHP if it wasn't for the infrequent HW and the small matter of a factor of five in the installation cost. Also, not having much space for the outdoor unit. On the subject of alternatives, I have also considered Air to Air but having got the UFH loop in the ground floor I'd like to use it. I did measure up for an A2A outside unit and there is just about space behind the garage as they're smaller than the ASHP. I could still go with one for upstairs as it could really use some cooling in the summer months, powered by PV on the roof. Link to comment Share on other sites More sharing options...
Mr Punter Posted August 27, 2022 Share Posted August 27, 2022 Maybe an instant electric water heater? You could use the 12kW system boiler for heat only. Link to comment Share on other sites More sharing options...
Radian Posted August 27, 2022 Author Share Posted August 27, 2022 26 minutes ago, Mr Punter said: Maybe an instant electric water heater? You could use the 12kW system boiler for heat only. Really trying to avoid resistive heating if at all possible, but it could be a solution. The other reason for getting a small boiler is being able to run without a buffer tank to prevent short cycling. I wonder what the smallest gas boiler actually is? Link to comment Share on other sites More sharing options...
Spreadsheetman Posted August 27, 2022 Share Posted August 27, 2022 As an aside I follow this YouTube plumbing channel https://youtube.com/c/UrbanPlumbers. The guy is pretty clever and he has a bit of a thing about people fitting over-sized boilers. He’s done several boiler installs where they modulate down to quite low outputs for heating, but ramp up to high outputs for hot water. Worth a look to get some ideas. 1 Link to comment Share on other sites More sharing options...
TonyT Posted August 27, 2022 Share Posted August 27, 2022 Yes, I like his channel, does a good job, actually designs heating. Systems to suit properties rather than buying heating packs from the merchants! Link to comment Share on other sites More sharing options...
TonyT Posted August 27, 2022 Share Posted August 27, 2022 Think the Worcester Bosch 4000 can do a 10:1 modulation so go down to 3kw…. must be others too 1 Link to comment Share on other sites More sharing options...
Radian Posted August 27, 2022 Author Share Posted August 27, 2022 6 minutes ago, TonyT said: Think the Worcester Bosch 4000 can do a 10:1 modulation so go down to 3kw…. must be others too Smallest one is 25kW. While most boilers can be rate limited I suspect the engineer will insist on signing-off on the highest setting. Link to comment Share on other sites More sharing options...
Temp Posted August 27, 2022 Share Posted August 27, 2022 (edited) 3 hours ago, Radian said: Thanks. That calculator seems like it could be useful but I don't get how it comes up with a flow rate at all. Does it assume a standard 25mbar after the regulator as @Marvin describes and then do some guesswork? What I didn't quite get from the engineer is what could be done to work round the issue. I got the impression that adding a separate metered connection to the supply line might solve it. Not that I would want to dig up the patio I just laid to tap into the supply! It probably calculates the required flow rate from the power of the appliances and energy density of the gas. Edited August 27, 2022 by Temp Link to comment Share on other sites More sharing options...
Nickfromwales Posted August 27, 2022 Share Posted August 27, 2022 In terms of sign off, the GSR chap will look for 18.5mbar or above ( check the MIs of the appliance you select ) and that’s with the hot water tap fully open and the boiler going at 100% burner rate. The supply from the street is fine for a bigger meter, that’s all you’d need tbh. Combi kW ratings have zilch to do with heating, as that usually takes <12kW even in a poor dwelling, and specifically relates to the devices capability to produce DHW. Link to comment Share on other sites More sharing options...
Radian Posted August 27, 2022 Author Share Posted August 27, 2022 27 minutes ago, Nickfromwales said: The supply from the street is fine for a bigger meter, that’s all you’d need tbh. That's interesting. Didn't realise the meter could be the bottleneck! Link to comment Share on other sites More sharing options...
SimonD Posted August 27, 2022 Share Posted August 27, 2022 5 hours ago, Radian said: Thanks. That calculator seems like it could be useful but I don't get how it comes up with a flow rate at all. Does it assume a standard 25mbar after the regulator as @Marvin describes and then do some guesswork? What I didn't quite get from the engineer is what could be done to work round the issue. I got the impression that adding a separate metered connection to the supply line might solve it. Not that I would want to dig up the patio I just laid to tap into the supply! 10 minutes ago, Radian said: That's interesting. Didn't realise the meter could be the bottleneck! Your typical domestic meter and regulator will be able to supply up to 6m3/h which is equal to a gross heat input of about 64kW so no problem dealing with your 54kW demand which would be about 60kW gross. You can check the rating of your meter easily on the data badge or looking it up to confirm this. The only thing then required is to do a proper pipe sizing calc so that you don't have more than a 1mbar drop from meter to any appliance when all running at max rate. Your heating engineer should be able to do this in his sleep. The one issue I can foresee is if he's only certified for domestic work and the total installed volume of gas takes him into commercial territory so largely depends on the pipe length and required diameter from your meter to the new boiler. What's the proposed pipe length to your new boiler? You're going to struggle getting a combi that will modulate down to as little as 2kW on the heating side. Most system and heat only boilers struggle with that. Link to comment Share on other sites More sharing options...
Radian Posted August 27, 2022 Author Share Posted August 27, 2022 7 hours ago, SimonD said: The one issue I can foresee is if he's only certified for domestic work and the total installed volume of gas takes him into commercial territory so largely depends on the pipe length and required diameter from your meter to the new boiler. What's the proposed pipe length to your new boiler? By "installed volume of gas" do you mean total max gas rate in m3/h for the two boilers because I'm assuming that flow restriction must not be such as to reduce this amount. The below-ground plastic pipe is 17m long then will run for 7m in 22mm or 28mm copper above ground to the boiler. Link to comment Share on other sites More sharing options...
SimonD Posted August 28, 2022 Share Posted August 28, 2022 9 hours ago, Radian said: By "installed volume of gas" do you mean total max gas rate in m3/h for the two boilers because I'm assuming that flow restriction must not be such as to reduce this amount. The below-ground plastic pipe is 17m long then will run for 7m in 22mm or 28mm copper above ground to the boiler. Installed volume is the static volume of the whole installation including the meter. Domestic is max pipe diameter of 35mm and max volume of 0.035m3 above this the gas engineer needs to be Commercial. If your buried pipe is PE then assuming a 32mm diameter pipe you would ideally be looking at 28mm copper as 22mm copper brings the installation close to max permissible pressure loss even without many fittings or bends. In my calc I assumed 4 elbows and a single exiting Tee for the PE but none for the copper at 22mm and this came in at 0.93mbar. With 28mm copper (no fittings) the calc was 0.66mbar which gives room for a few fittings in the copper pipework. Looking at your photos I can see that you've got a 6m3/h meter which looks like u6/g4 type so approximate installed volume of gas is 0.22m3 based on the pipe lengths you've given me, assuming 32mm PE and 28mm copper. On this basis and assumptions it looks like you should be fine for supplying both boilers with sufficient gas. I only have PE SDR11 32mm figures to hand, not anything for 40mm which you'd need to check with whoever installed the buried pipe if you can. I think 40mm PE pipework could tip the installation into needing a commercial gas safe engineer to do the installation and commissioning of your new boiler. 1 Link to comment Share on other sites More sharing options...
Radian Posted August 28, 2022 Author Share Posted August 28, 2022 5 hours ago, SimonD said: I think 40mm PE pipework could tip the installation into needing a commercial gas safe engineer to do the installation and commissioning of your new boiler. You're a star! The loosely fitting protective sheath is 40mm OD so probably a 32mm pipe inside. Link to comment Share on other sites More sharing options...
martamartizi Posted November 7, 2022 Share Posted November 7, 2022 Can you explain how the boiler even works? Link to comment Share on other sites More sharing options...
Carrerahill Posted November 7, 2022 Share Posted November 7, 2022 (edited) On 27/08/2022 at 10:19, Radian said: Very good point. He started off at 30kW and I beat him down! Strange how they always seem to overspec these appliances. Even at 24kW he thought we might regret it. My heat loss calcs suggest around 2kW at inside/outside delta 10oC for space heating and HW is only required to supply a single basin. I think the problem is that it appears to be only system boilers that go down to 12kW. I couldn't google up any combis. The extremely infrequent demand for HW means a system boiler isn't really the right choice. The right choice would be an ASHP if it wasn't for the infrequent HW and the small matter of a factor of five in the installation cost. Also, not having much space for the outdoor unit. On the subject of alternatives, I have also considered Air to Air but having got the UFH loop in the ground floor I'd like to use it. I did measure up for an A2A outside unit and there is just about space behind the garage as they're smaller than the ASHP. I could still go with one for upstairs as it could really use some cooling in the summer months, powered by PV on the roof. 30kW! What is the heat-loss of the area? What is the insulation level like? I helped with a garage conversion for a friend last year and their intention was to have 6kW of electric interior space heating, I did a fag packet calc based on 75W per m³ (rough heating requirement based on a medium insulation level) of area and it came in at about 2.9kW - I talked them into a single 3kW panel with provision for a second and promised them they would never need it. So unless your planning on throwing heat into a totally uninsulated, draughty building, or its a massive garage, I cannot see the heating requirements being anything like even 8kW let along 24 or 30! This is the issue a lot of gas fitters, they just think bigger is better and end up with boilers that modulate to lower output all the time, or in days gone by constant short-cycling. Try a 3kW Willis heater and a pump! Edited November 7, 2022 by Carrerahill Link to comment Share on other sites More sharing options...
Carrerahill Posted November 7, 2022 Share Posted November 7, 2022 On 27/08/2022 at 11:21, Spreadsheetman said: As an aside I follow this YouTube plumbing channel https://youtube.com/c/UrbanPlumbers. The guy is pretty clever and he has a bit of a thing about people fitting over-sized boilers. He’s done several boiler installs where they modulate down to quite low outputs for heating, but ramp up to high outputs for hot water. Worth a look to get some ideas. He is brilliant, recently saw a video where he took a house on twin 30kW boilers down to a single and reconfigured the system, which now comfortably runs on 1 and the second is for backup or DHW only or something. I would be raging if I put in a twin system only to have someone come along and make it work on a single unit! Smart guy - should ditch the tools and become a consultant engineer! Link to comment Share on other sites More sharing options...
Radian Posted November 7, 2022 Author Share Posted November 7, 2022 5 hours ago, Carrerahill said: 30kW! What is the heat-loss of the area? What is the insulation level like? Crazy ain't it! The heat loss is in the order of a couple of kW but difficult to pin down as, while the insulation is reasonably good, the airtightness isn't. In the end I've gone for a couple of mini splits (3.5kW Daikin Emura 3rd gen) and so far they're more than capable of keeping things toasty. The clincher was getting PV on the roof and the scorching summer we just had. The need for cooling coinciding perfectly with PV generation is one of those rare times PV works out the way you'd like it to. Link to comment Share on other sites More sharing options...
SimonD Posted November 8, 2022 Share Posted November 8, 2022 18 hours ago, Carrerahill said: Smart guy - should ditch the tools and become a consultant engineer! Yes, he's very good. Hope he doesn't go the consulting route, too good on the tools and it seems like he enjoys it. Link to comment Share on other sites More sharing options...
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